# Fibre-rich ingredients differing in physicochemical
properties modulate digesta transit and digestion
kinetics in pigs

C. Lannuzel, R.J. Veersma, G. van Erven, M.A. Kabel, W.J.J. Gerrits, S. de Vries

► To cite this version:

C. Lannuzel, R.J. Veersma, G. van Erven, M.A. Kabel, W.J.J. Gerrits, et al. Fibre-rich ingredients differing in physicochemical properties modulate digesta transit and digestion kinetics in pigs. Animal, 2025, 19 (4), pp.101473. 10.1016/j.animal.2025.101473. hal-05515082

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# Fibre-rich ingredients differing in physicochemical properties modulate digesta transit and digestion kinetics in pigs

<figure><img src="image_2.png" /></figure>

C. Lannuzel<sup>a,1,*</sup>, R.J. Veersma<sup>b</sup>, G. van Erven<sup>b,c</sup>, M.A. Kabel<sup>b</sup>, W.J.J. Gerrits<sup>a</sup>, S. de Vries<sup>a</sup>

## ARTICLE INFO

**Article history:**

Received 21 June 2024

Revised 14 February 2025

Accepted 17 February 2025

Available online 24 February 2025

**Keywords:**

Digestibility

High-methylated pectin

Insoluble fibres

Mean retention time

Phase separation

## ABSTRACT

Physicochemical properties of fibres can affect digestive processes such as digesta transit, thereby affecting nutrient digestion kinetics. We evaluated the effects of (1) insoluble fibres differing in cell wall composition and physicochemical properties; wheat straw (WS), softwood flour (WF), and sunflower seed hulls (SF), and (2) addition of pectin to coarse insoluble fibres from straw (WSP), on the mean retention time (MRT) of digesta fractions and nutrient digestion throughout the gastrointestinal tract (GIT) of pigs. Twenty-four boars (49.1 ± 2.96 kg) were allocated to one of four diets, containing either 138 g/kg of WF, 144 g/kg of SF, or 150 g/kg of WS without or with the addition of 100 g/kg high-methylated pectin. The apparent total tract digestibility of nutrients was evaluated by quantitative faecal collection. After feeding diets supplemented with tracers, pigs were dissected following a frequent feeding procedure to approach steady-state passage of digesta, and assess MRT of liquids (Yb-EDTA), fine solids (TiO₂), and insoluble fibrous particles (Chromium mordanted fibres) throughout the GIT. Unlike coarse WS, no differences in pH nor DM were observed between the proximal and distal stomach for WSP, and fine WF and SF (*P* > 0.05). Compared with WS, WF and SF reduced the MRT of fine solids (–01:51 to 01:03 h:min; *P* ≤ 0.05), and fibrous particles (–03:14 to 02:23 h:min; *P* ≤ 0.05) in the proximal and distal stomach, decreasing their separation, especially in the distal stomach (–01:30 to 01:20 h:min; *P* ≤ 0.05). Addition of pectin to WS reduced separation between fine solids and liquids in the distal stomach (–43 min; *P* ≤ 0.05), increased starch digestibility in the mid-small intestine (SI; +9%-units; *P* ≤ 0.05) but not at the end of the SI, and reduced total tract digestibility of nitrogen (–8%-units; *P* ≤ 0.05), reflecting a lower ileal digestibility (–8%-units). In the large intestine (LI), coarse WS reduced the MRT of digesta phases compared with fine WF and SF (–17:38 to 08:08 h:min). Regardless of origin, fermentation of insoluble fibres was poor (≤19%), while complete fermentation of pectin did not impact the fermentation of WS. In conclusion, fine WF and SF accelerated the emptying of solids and suppressed regional differences in pH in the stomach, and delayed digesta transit in the LI, compared with coarse straw. Pectin addition to coarse straw reduced gastric sieving between fine solids and liquids, and increased starch digestibility in the proximal SI.

© 2025 The Author(s). Published by Elsevier B.V. on behalf of The animal Consortium. This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/).

## Implications

Identifying the effects of fibre-rich ingredients on digestion is key to increase their use in monogastric feed. We tested the effects of fibre-rich ingredients differing in composition and physicochemical properties, on digesta passage of liquids, fine and fibrous solids, and nutrient digestion throughout the gastrointestinal tract

of pigs. The type of insoluble fibres impacted gastric pH and emptying, and transit in the large intestine. Pectin addition affected sieving and pH in the stomach, and the site of starch digestion in the small intestine. These results can help in designing dietary strategies to release nutrients at targeted rates and sites in pigs.

## Introduction

Together with hydrolysis and absorption, digesta transit impacts the rate of nutrient digestion (Gerrits et al., 2021; Nadia et al., 2023; Schop et al., 2020). In turn, this may affect the timing of nutrient delivery to the blood, ultimately influencing the metabolic use of

<footnote><sup>a</sup> Wageningen University & Research, Animal Nutrition Group, De Elst 1, 6708 WD Wageningen, the Netherlands</footnote>

<footnote><sup>b</sup> Wageningen University & Research, Laboratory of Food Chemistry, Bornse Weilanden 9, 6708 WG Wageningen, the Netherlands</footnote>

<footnote><sup>c</sup> Wageningen Food & Biobased Research, Bornse Weilanden 9, 6708 WG Wageningen, the Netherlands</footnote>

<footnote>* Corresponding author.</footnote>

<footnote>E-mail address: corentin.lannuzel@inrae.fr (C. Lannuzel).</footnote>

<footnote n="1">¹ Present address: Université Paris-Saclay, AgroParisTech, INRAE, UMR PNCA, 91120 Palaiseau, France.</footnote>
nutrients (Batterham and Bayley, 1989; Boulier et al., 2023; Nadia et al., 2023). The behaviour of digesta transit is complex, and involves both dietary (e.g. feed composition) and animal factors (e.g. BW) (Le Goff et al., 2002; Schop et al., 2019). Among dietary factors, the physicochemical properties of feed ingredients such as particle size, water binding capacity, and solubility are increasingly recognised as key modulators of digesta transit behaviour, particularly in the stomach (Lannuzel et al. 2024a; Nadia et al., 2021; Schop et al., 2019). Within diets, fibre-rich ingredients exhibit a wide range of physicochemical properties (Schop et al., 2020; Slama et al., 2019). Previous studies have shown that the addition of soluble and gelling fibres such as oat β-glucans or citrus low-methylated pectin reduced the separation between solids and liquids in the stomach, and either reduced or unaffected the extent of protein and starch digestion in the ileum (de Vries et al., 2016; Lannuzel et al. 2024b; Schop et al., 2020). Increasing the intake of insoluble fibres from wheat bran did not affect the transit of liquids and solids in the stomach but accelerated transit in both the small intestine (SI) and large intestine (LI), without impairing the extent of starch and protein digestion in the ileum (Wilfart et al., 2007a and 2007b). Not only the inclusion level but also the particle size of insoluble fibres were shown to influence digesta transit. Particle size reduction of wheat straw accelerated gastric emptying of fine and coarse solids, and reduced their separation, without affecting digesta transit in the SI or the extent of protein and starch digestion in the ileum (Lannuzel et al., 2024b). Nevertheless, it is unknown how insoluble fibres from distinct botanical origins, differing in cell wall composition and structure, and physicochemical properties, such as particle size, affect digesta transit behaviour throughout the gastrointestinal tract (GIT), and nutrient digestion along the SI. Although adding low-methylated pectin to wheat bran was shown to influence gastric sieving and emptying (Lannuzel et al. 2024b), it is unclear how adding high-methylated pectin to a diet containing coarse insoluble fibres influences digesta transit behaviour and nutrient digestion in the GIT. This currently impedes the prediction of nutritional consequences associated with greater fibre inclusions in diets fed to pigs, which ultimately limits the use of fibre-rich ingredients by the swine industry (Gerrits et al., 2021; Owusu-Asiedu et al., 2006). Therefore, this study aimed to assess the effects of (1) highly lignified and poorly fermentable insoluble fibres differing in origin, cell wall composition, and physicochemical properties such as particle size, i.e. softwood flour, sunflower hulls, and wheat straw, and (2) addition of high-methylated pectin, rich in gelling soluble fibres to coarse insoluble fibres, i.e. wheat straw, on the mean retention time (MRT) of digesta phases (i.e. liquids, fine solids, and fibrous particles), and digestion of nutrients, not originating from these sources, along the GIT of pigs (Slama et al., 2019; Liang et al., 2022). It was hypothesised that feeding fine insoluble fibres, i.e. softwood flour or sunflower hulls, to pigs would accelerate gastric emptying of digesta phases, thereby reducing gastric digestion and the subsequent rate of digestion along the SI. In the LI, feeding coarse insoluble fibres, i.e. straw, was expected to promote colonic peristalsis and reduce retention time compared with fine insoluble fibre. The addition of gelling pectin to coarse straw was hypothesised to reduce digesta phase separation in the stomach, impair nutrient accessibility and the rate of digestion along the SI, and decrease retention time in the LI by increasing the flow of undigested material to the colon.

## Material and methods

### Animals and housing

The experiment was conducted at the research farm 'Laverdonk' of Agrifirm Innovation Center (Heeswijk-Dinther, the Netherlands). To study the effects of (1) insoluble fibres differing in origin, cell

wall composition, and physicochemical properties, and (2) addition of pectin to coarse insoluble fibres on digesta retention time and nutrient digestion kinetics, twenty-four boars (mean BW ± SD: 49.1 ± 2.96 kg; Tempo × TN70, Topigs Norsvin, Helvoirt, the Netherlands) were allocated to one of four experimental diets based on BW. From d0 to d14 of the experiment, pigs were housed in pairs in three adjacent rooms (8 m² per pen; 2:3 ratio of solid to slatted floor). During feeding, one of the two pigs was moved to an adjacent empty pen so that pigs could be individually fed until meals were fully consumed, or after a maximum of 1 h per meal. Then, pigs were regrouped. From d15 to d19, pigs were individually housed in adjacent pens for quantitative collection of faeces. On d20, pigs were regrouped with their initial pen mates until the end of the experiment.

### Diets and feedings

Four diets were formulated to have comparable levels of dietary fibre, originating from 138 g/kg of softwood flour (WF; Fibre-cell® M1, Agromed Austria GmbH, Kremsmünster, Austria), 144 g/kg of sunflower seed hulls (SF), or 150 g/kg wheat straw (WS), except for the wheat straw plus pectin diet (WSP), where 100 g/kg of starch (maize and potato) was exchanged for 100 g/kg high-methylated citrus pectin (Classic CU 201, Herbstreith & Fox GmbH, Neuenbürg, Germany) in addition to 150 g/kg wheat straw (Table 1). Isolated starch and protein sources were used to ensure the fibrous ingredients tested were the sole source of fibres in the diets. To study the influence of the different fibre-rich ingredients on starch and protein digestion kinetics, a combination of highly and poorly digestible sources was used. Starch originated from a mixture of highly digestible maize starch (60%), and poorly digestible native raw potato starch (40%). Proteins originated from poorly digestible zein (25%) and collagen (25%), and highly digestible fish meal (50%). Experimental diets were formulated to meet or exceed the nutrient requirements of fattening pigs according to CVB (Centraal Veevoeder Bureau, 2018). The experiment lasted 26 days. In the first 3 days of the experiment, pigs were gradually shifted from a commercial diet to their experimental diet. After 11 days of acclimatisation to the experimental diets, a total collection of faeces was performed for 5 consecutive days. Inert tracers were included in the diets to follow digesta liquids (Yb-EDTA; 0.8 g/kg) and fine solids (TiO₂; 4 g/kg), as discussed by de Vries and Gerrits (2018). During the last 5 days of the experiment, pigs were fed the same diets, to which 5 g/kg of Chromium mordanted softwood flour, sunflower seed hulls, or wheat straw was added to represent insoluble fibrous particles, prepared as described by Udén et al. (1980). Wheat straw meals, i.e. WS and WSP, were reconstituted before feeding by separately weighing the straw and the basal diet to prevent particle segregation during storage. Diets were fed as mash and mixed with water (1:1 ratio of feed to water, w/w) before feeding. If present, feed refusals were collected and recorded. Pigs had ad libitum access to water during the entire experiment. From day 0 until 4 days before dissections, animals were fed twice daily, at 0730 and 1530 h, at 2.5 times their metabolisable energy (ME) requirements for maintenance (419 kJ/kg BW<sup>0.75</sup>; Centraal Veevoeder Bureau, 2005). Feed allowance was adjusted weekly, based on initial BW, and expected daily gain (800 g/day). From at least 54 h before dissection onwards, the daily feed allowance of pigs was equally divided over four meals, with an equal meal-interval of 6 h to approach steady-state passage of digesta in the LI. The day pigs were dissected, they were fed once every hour, and received a minimum of 6 meals before dissection to approach steady-state passage of digesta in the stomach and SI (de Vries and Gerrits, 2018). During hourly feeding, each meal equalled 1/24th of the daily allowance. Dissections were performed over 2 consecutive days. For each treatment, the first pig dissected
<caption>**Table 1**</caption>

Ingredient composition and analysed chemical composition of experimental diets fed to finishing pigs containing either softwood flour (WF), sunflower hulls (SF), or wheat straw without (WS) or with the addition of high-methylated pectin (WSP).

<table><thead><tr><th rowspan="2">Item</th><th colspan="4">Diets</th></tr><tr><th>WF</th><th>SF</th><th>WS</th><th>WSP</th></tr></thead><tbody><tr><td>Ingredients (g/kg as fed)</td><td></td><td></td><td></td><td></td></tr><tr><td>Softwood flour<sup>I</sup></td><td>138</td><td>—</td><td>—</td><td>—</td></tr><tr><td>Sunflower seed hulls</td><td>—</td><td>144</td><td>—</td><td>—</td></tr><tr><td>Wheat straw</td><td>—</td><td>—</td><td>150</td><td>150</td></tr><tr><td>Pectin<sup>I</sup></td><td>—</td><td>—</td><td>—</td><td>100</td></tr><tr><td>Native maize starch</td><td>294</td><td>293</td><td>286</td><td>226</td></tr><tr><td>Native potato starch</td><td>196</td><td>195</td><td>191</td><td>151</td></tr><tr><td>Zein</td><td>40</td><td>40</td><td>40</td><td>40</td></tr><tr><td>Collagen</td><td>40</td><td>40</td><td>40</td><td>40</td></tr><tr><td>Fish meal (70% CP)</td><td>80</td><td>80</td><td>80</td><td>80</td></tr><tr><td>Soy oil</td><td>57</td><td>52</td><td>58</td><td>58</td></tr><tr><td>Sugar</td><td>100</td><td>100</td><td>100</td><td>100</td></tr><tr><td>Premix<sup>III</sup></td><td>5</td><td>5</td><td>5</td><td>5</td></tr><tr><td>L-lysine HCl</td><td>6</td><td>6</td><td>6</td><td>6</td></tr><tr><td>DL-methionine</td><td>3</td><td>3</td><td>3</td><td>3</td></tr><tr><td>L-threonine</td><td>3</td><td>3</td><td>3</td><td>3</td></tr><tr><td>L-tryptophan</td><td>2</td><td>2</td><td>2</td><td>2</td></tr><tr><td>L-valine</td><td>2</td><td>2</td><td>2</td><td>2</td></tr><tr><td>L-arginine</td><td>1</td><td>1</td><td>1</td><td>1</td></tr><tr><td>L-isoleucine</td><td>2</td><td>2</td><td>2</td><td>2</td></tr><tr><td>L-leucine</td><td>1</td><td>1</td><td>1</td><td>1</td></tr><tr><td>L-phenylalanine</td><td>0.6</td><td>0.6</td><td>0.6</td><td>0.6</td></tr><tr><td>L-histidine</td><td>2</td><td>2</td><td>2</td><td>2</td></tr><tr><td>Magnesium sulphide</td><td>5</td><td>5</td><td>5</td><td>5</td></tr><tr><td>Limestone</td><td>10</td><td>10</td><td>10</td><td>10</td></tr><tr><td>Monocalcium phosphate</td><td>6</td><td>6</td><td>6</td><td>6</td></tr><tr><td>KCO<sub>3</sub></td><td>2</td><td>2</td><td>2</td><td>2</td></tr><tr><td>TiO<sub>2</sub></td><td>4</td><td>4</td><td>4</td><td>4</td></tr><tr><td>Yb-EDTA</td><td>0.8</td><td>0.8</td><td>0.8</td><td>0.8</td></tr><tr><td>Phytase, FTU</td><td>500</td><td>500</td><td>500</td><td>500</td></tr><tr><td>Analysed chemical composition (g/kg as fed)</td><td></td><td></td><td></td><td></td></tr><tr><td>DM</td><td>905</td><td>905</td><td>903</td><td>912</td></tr><tr><td>CP (N × 6.25)</td><td>155</td><td>149</td><td>161</td><td>155</td></tr><tr><td>Starch</td><td>381</td><td>375</td><td>391</td><td>306</td></tr><tr><td>NSP<sup>IV</sup></td><td>82</td><td>70</td><td>92</td><td>167</td></tr><tr><td>Arabinosyl</td><td>2</td><td>4</td><td>4</td><td>6</td></tr><tr><td>Xylosyl</td><td>7</td><td>21</td><td>34</td><td>34</td></tr><tr><td>Mannosyl</td><td>16</td><td>1</td><td>1</td><td>1</td></tr><tr><td>Galactosyl</td><td>3</td><td>1</td><td>1</td><td>5</td></tr><tr><td>Glucosyl</td><td>50</td><td>32</td><td>48</td><td>48</td></tr><tr><td>Uronyl</td><td>4</td><td>11</td><td>3</td><td>73</td></tr><tr><td>Ash</td><td>38</td><td>41</td><td>52</td><td>56</td></tr><tr><td>Calculated chemical composition (g/kg as fed)</td><td></td><td></td><td></td><td></td></tr><tr><td>Dietary fibre<sup>V</sup></td><td>118</td><td>117</td><td>117</td><td>192</td></tr><tr><td>Crude fat<sup>VI</sup></td><td>61</td><td>60</td><td>60</td><td>60</td></tr><tr><td>ME, MJ/kg<sup>VII</sup></td><td>13.3</td><td>13.1</td><td>13.7</td><td>13.8</td></tr></tbody></table>

Abbreviations: FTU = phytase unit; ME = metabolisable energy; NSP = non-starch polysaccharides.

received six hourly meals, the second pig received eight hourly meals, and the third pig received ten hourly meals to remain in steady-state conditions. For each pig, feeding times on the dissection day were adjusted according to the time of euthanasia.

## Faeces and digesta collection

Faeces and digesta collection were performed as described in detail by Lannuzel et al. (2024b). Briefly, faeces were collected quantitatively using bags attached to rings glued around the anus. Following sedation and euthanasia, the abdominal cavity was

opened, and surgical clamps were positioned at the esophageal and duodenal ends of the stomach. A tie was placed between the proximal and distal regions of the stomach to prevent mixing of the stomach contents, and the stomach was removed (Bornhorst et al., 2013). The content of each stomach region was collected separately and weighed. Then, the digesta was mixed manually with a spoon, and pH was measured in each region using a XS Tester pH 1 (XS Instruments, Carpi, Italy). The rest of the GIT was removed. The SI was divided into three segments: the ileum was standardised, using the distal 1.5 m of the SI. The remaining SI was divided into two parts of equal length, resulting in a proximal SI and a mid-SI

<footnote>I Softwood flour (Fibrecell®M1, Agromed Austria GmbH, Kremsmünster, Austria).</footnote>

<footnote>II High-methylated citrus pectin (Classic CU 201, Herbstreith & Fox GmbH, Neuenbürg, Germany).</footnote>

<footnote>III Premix composition, /kg diet: 10 000 IU vitamin A, 2 000 IU vitamin D₃, 40 mg vitamin E, 1.5 mg vitamin K₃, 1.0 mg vitamin B₁, 4.0 mg vitamin B₂, 1.5 mg vitamin B₆, 20 µg vitamin B₁₂, 30 mg niacin, 15 mg D-pantothenic acid, 150 mg choline chloride, 0.4 mg folic acid, 0.05 mg Biotin; 100 mg Fe (as FeSO₄·H₂O), 20 mg Cu (as CuSO₄·5H₂O), 30 mg Mn (as MnO), 70 mg Zn (as ZnSO₄·H₂O), 0.70 mg I (as KI), 0.25 mg Se (as Na₂SeO₃). Carrier: milled maize.</footnote>

<footnote>IV Monosaccharides represent anhydrous sugar moieties.</footnote>

<footnote>V Calculated using dietary fibre contents reported by Slama et al. (2019).</footnote>

<footnote>VI Calculated using data from Centraal Veevoeder Bureau (2018).</footnote>

<footnote>VII ME (MJ) = (20.0 × digestible CP + 39.1 × digestible ether extract + 17.5 × starch + 16.6 × sugars + 17.2 × digestible non-starch polysaccharides)/ 1 000 (Noblet et al., 1994).</footnote>
segment. The LI was divided into four segments: caecum; colon divided into two parts of equal length, resulting in a proximal colon and a distal colon; rectum.

## Analysis and calculations

Diets, digesta, and faeces were analysed for DM content (ISO 6496, 1999), nitrogen (AOAC 990.03; AOAC International, 2019), total starch (ISO 15914, 2004). Crude fat content was analysed only in the digesta and faeces according to ISO 6492 (1999). Non-starch polysaccharides (NSP) were analysed based on Englyst et al. (1994) and Jonathan et al. (2012), with some modifications as described in detail by Lannuzel et al. (2024b). Tracer concentrations were measured after ashing and microwave digestion using inductively coupled plasma optical emission spectrometry, as described in detail by Lannuzel et al. (2024b). The MRT of digesta phases and their separation in the different segments of the GIT, the apparent nutrient digestibility in the three segments of the SI based on titanium, and the apparent nutrient digestibility total tract based on a quantitative collection of faeces were calculated as described by Lannuzel et al. (2024b). Nutrient digestibility was calculated using the calculated fat content in the diet for fat, while the analysed nutrient content was used for other nutrients. The buffering capacity of diets was quantified based on the method described by Mennah-Govela et al. (2019) with some modifications, as reported by Lannuzel et al. (2024a). Image analysis was used to quantify the particle size distribution, eccentricity, and maximum Feret diameter of the diets (Fig. 1), based on Swackhamer et al. (2019), as described by Lannuzel et al. (2024a). Due to the presence of large straw particles, the WS and WSP presented a bimodal particle distribution. Therefore, particles with a surface area ≤ 1 mm² and > 1 mm² were fitted separately to the Rosin-Rammler equation. Then, the final 10th, 50th, and 90th percentile particle areas (x₁₀, x₅₀, and x₉₀), the broadness of distribution (b), and the number of particles per sample were calculated as the weighted average:

$$ Parameter_{final} = \frac{\alpha(\text{Parameter}_{\le 1mm^2}) + \beta(\text{Parameter}_{>1mm^2})}{\alpha + \beta} \quad (5) $$

where Parameter is either the x₁₀, x₅₀, x₉₀, b, and number of particles per sample, α is the number of particles ≤ 1 mm² and β is the number of particles > 1 mm². The water-binding capacity of diets was quantified as described by Dorado-Montenegro et al. (2023).

## Statistical analysis

To evaluate the effect of the experimental diets on MRT, phase separation, and digestibility within segments, a general linear mixed model was used, including diet as a fixed effect, and pen as a random effect (PROC MIXED, SAS 9.4, SAS Institute Inc., Cary, NC, USA). Least square means (LSmeans) were compared among diets using Tukey adjustment for multiple comparisons. To evaluate the effect of stomach region within diets on MRT and phase separation, a general linear mixed model was used, including diet, stomach region, and their interaction as fixed effects, and pen as a random effect. To account for repeated observations within the pig, the region was modelled as an R-side effect. Contrast statements were used to compare the LSmeans of stomach regions within diet. For both models, the pig was considered the experimental unit. The homogeneity and normality of model residuals were verified graphically and with the Shapiro-Wilk test. If model residuals were not normally distributed, statistical analyses were conducted on log-transformed data. If model residuals were not homogeneously distributed, variances were estimated separately by diet. Data are presented as raw means ± pooled standard deviation (SD).

## Results

### Diets, BW, and feed intake

Compared with SF and WS, WF contained more mannosyl- (+15 g/kg; Table 1), but less xylosyl residues (−14 to 27 g/kg). The SF contained more uronyl- (+8 g/kg) but less glucosyl residues (−17 g/kg) compared with WF and WS. Compared with WF, SF, and WS, WSP had a greater NSP content (167 g/kg vs ≤ 92 g/kg), coinciding with greater uronyl content originating from the pectin (73 g/kg vs > 11 g/kg). All diets had a comparable buffering capacity (~187 µmol H⁺/(g DM × Δ pH); Table 2), except for the WSP (243 µmol H⁺/(g DM × Δ pH)). Compared with WF and SF, WS and WSP had greater median particle area, i.e. x₅₀, (0.1–0.2 vs 0.6–0.8 mm²), and maximum Feret diameter (0.4–0.5 vs 0.7 mm). The WSP had a greater water binding capacity compared with the other diets (+0.5 to 1.0 g/g DM). The BW at the start (49. 1 ± 2.96 kg) and at the end (59.8 ± 4.37 kg) of the experiment did not differ among diets (P > 0.10). Compared with WF and SF, feed intake was lower for WSP (−309 to 205 g/day; P < 0.05).

<figure><img src="image_3.png" /><figcaption>Fig. 1. Example of binary images used to derive particle size parameters, eccentricity, and maximum Feret diameter of the diets fed to finishing pigs containing either softwood flour (WF), sunflower hulls (SF), or wheat straw without (WS) or with the addition pectin (WSP). The scale bar shown is 1 cm.</figcaption></figure>
<caption>**Table 2**</caption>

Physicochemical properties of the experimental diets fed to finishing pigs containing either softwood flour (WF), sunflower hulls (SF), or wheat straw without (WS) or with the addition of high-methylated pectin (WSP).

<table><thead><tr><th rowspan="2">Item</th><th colspan="4">Diets</th></tr><tr><th>WF</th><th>SF</th><th>WS</th><th>WSP</th></tr></thead><tbody><tr><td>Water binding capacity, g/g DM<sup>I</sup></td><td>1.4</td><td>1.3</td><td>1.8</td><td>2.3</td></tr><tr><td>pH<sup>II,III</sup></td><td>4.8</td><td>5.0</td><td>5.0</td><td>4.5</td></tr><tr><td>Buffering capacity, µmol H<sup>+</sup>/(g DM × Δ pH)<sup>II</sup></td><td>190</td><td>190</td><td>182</td><td>243</td></tr><tr><td>Particle area, x<sub>10</sub>, mm<sup>2</sup> <sup>IV</sup></td><td>0.0</td><td>0.0</td><td>0.1</td><td>0.1</td></tr><tr><td>Particle area, x<sub>50</sub>, mm<sup>2</sup> <sup>IV</sup></td><td>0.1</td><td>0.2</td><td>0.8</td><td>0.6</td></tr><tr><td>Particle area, x<sub>90</sub>, mm<sup>2</sup> <sup>IV</sup></td><td>0.2</td><td>0.4</td><td>2.0</td><td>1.7</td></tr><tr><td>Broadness of particle area, dimensionless<sup>IV</sup></td><td>1.5</td><td>1.2</td><td>1.4</td><td>1.4</td></tr><tr><td>Particle per mg DM<sup>IV</sup></td><td>169</td><td>68</td><td>18</td><td>23</td></tr><tr><td>Maximum Feret diameter, mm<sup>IV</sup></td><td>0.4</td><td>0.5</td><td>0.7</td><td>0.7</td></tr><tr><td>Eccentricity, dimensionless<sup>IV</sup></td><td>0.76</td><td>0.74</td><td>0.75</td><td>0.77</td></tr></tbody></table>

<caption>**Table 3**</caption>

pH, DM, mean retention time (MRT, h:min) of liquids (Yb-EDTA), fine solids (TiO₂), and fibrous particles (Chromium mordants), and separations between fine solids and liquids, and between fibrous particles and fine solids in the proximal and distal stomach of finishing pigs fed diets containing either softwood flour (WF), sunflower hulls (SF), or wheat straw without (WS) or with the addition of high-methylated pectin (WSP)<fnref n="1" />.

<table><thead><tr><th rowspan="2">Item</th><th rowspan="2">Region</th><th colspan="4">Diets</th><th rowspan="2">Pooled SD</th><th>p<sup>III</sup></th></tr><tr><th>WF</th><th>SF</th><th>WS</th><th>WSP</th><th>Diet</th></tr></thead><tbody><tr><td rowspan="3">pH</td><td>Proximal</td><td>2.4</td><td>2.5</td><td>3.7</td><td>3.4</td><td>0.640</td><td>0.030</td></tr><tr><td>Distal</td><td>2.1</td><td>2.3</td><td>2.8</td><td>3.1</td><td>0.876</td><td>0.338</td></tr><tr><td>P<sup>IV</sup></td><td>0.505</td><td>0.738</td><td>0.016</td><td>0.409</td><td></td><td></td></tr><tr><td rowspan="3">DM<sup>I</sup>, g/kg</td><td>Proximal</td><td>152<sup>b</sup></td><td>217<sup>ab</sup></td><td>282<sup>a</sup></td><td>180<sup>b</sup></td><td>51.06</td><td>0.010</td></tr><tr><td>Distal</td><td>186<sup>ab</sup></td><td>220<sup>a</sup></td><td>193<sup>a</sup></td><td>139<sup>b</sup></td><td>27.39</td><td>0.008</td></tr><tr><td>P<sup>IV</sup></td><td>0.175</td><td>0.902</td><td>&lt;0.001</td><td>0.080</td><td></td><td></td></tr><tr><td rowspan="3">MRT, h:min<br>Liquids</td><td>Proximal</td><td>00:10<sup>b</sup></td><td>00:20<sup>ab</sup></td><td>00:42<sup>a</sup></td><td>01:04<sup>a</sup></td><td>00:23</td><td>0.005</td></tr><tr><td>Distal</td><td>00:11<sup>b</sup></td><td>00:16<sup>ab</sup></td><td>00:21<sup>ab</sup></td><td>00:30<sup>a</sup></td><td>00:09</td><td>0.037</td></tr><tr><td>P<sup>IV</sup></td><td>0.941</td><td>0.447</td><td>0.017</td><td>0.009</td><td></td><td></td></tr><tr><td rowspan="3">Fine solids</td><td>Proximal</td><td>00:29<sup>b</sup></td><td>00:50<sup>ab</sup></td><td>02:20<sup>a</sup></td><td>01:44<sup>ab</sup></td><td>00:48</td><td>0.040</td></tr><tr><td>Distal</td><td>00:39<sup>b</sup></td><td>00:40<sup>b</sup></td><td>01:43<sup>a</sup></td><td>01:09<sup>ab</sup></td><td>00:27</td><td>0.018</td></tr><tr><td>P<sup>IV</sup></td><td>0.348</td><td>0.567</td><td>0.323</td><td>0.237</td><td></td><td></td></tr><tr><td rowspan="3">Fibrous particles</td><td>Proximal</td><td>00:27<sup>b</sup></td><td>01:04<sup>b</sup></td><td>03:41<sup>a</sup></td><td>05:00<sup>a</sup></td><td>02:04</td><td>&lt;0.001</td></tr><tr><td>Distal</td><td>00:41<sup>b</sup></td><td>00:54<sup>b</sup></td><td>03:17<sup>a</sup></td><td>05:29<sup>a</sup></td><td>01:30</td><td>&lt;0.001</td></tr><tr><td>P<sup>IV</sup></td><td>0.196</td><td>0.604</td><td>0.786</td><td>0.505</td><td></td><td></td></tr><tr><td rowspan="3">Separation, h:min<br>Fine solids</td><td>Proximal</td><td>00:18<sup>b</sup></td><td>00:30<sup>ab</sup></td><td>01:38<sup>a*</sup></td><td>00:40<sup>ab*</sup></td><td>00:28</td><td>0.016</td></tr><tr><td>Distal</td><td>00:29<sup>b*</sup></td><td>00:25<sup>b*</sup></td><td>01:22<sup>a*</sup></td><td>00:39<sup>b*</sup></td><td>00:19</td><td>0.005</td></tr><tr><td>P<sup>IV</sup></td><td>0.401</td><td>0.640</td><td>0.150</td><td>0.982</td><td></td><td></td></tr><tr><td rowspan="3">Fibrous particles<br>-Fine solids</td><td>Proximal</td><td>00:02<sup>b</sup></td><td>00:14<sup>ab</sup></td><td>01:21<sup>a*</sup></td><td>03:16<sup>a*</sup></td><td>01:22</td><td>&lt;0.001</td></tr><tr><td>Distal</td><td>00:04<sup>b</sup></td><td>00:14<sup>b*</sup></td><td>01:34<sup>a*</sup></td><td>04:20<sup>a*</sup></td><td>01:07</td><td>&lt;0.001</td></tr><tr><td>P<sup>IV</sup></td><td>0.960</td><td>0.996</td><td>0.694</td><td>0.062</td><td></td><td></td></tr></tbody></table>

## Stomach

Digesta pH (+0.2 to 0.9 units; Table 3) and DM (−34 to +89 g/kg) were similar or greater in the proximal stomach compared with the distal stomach. For WS, digesta pH (+0.9 units; *P* = 0.016) and DM (+89 g/kg; *P* < 0.001) were greater in the proximal stomach compared with the distal stomach. In the proximal stomach, digesta pH differed among diets (0.1 to 1.3 units; *P* = 0.030), but not in the distal stomach (*P* = 0.338). In the proximal stomach, digesta DM was greater for WS compared with WF and WSP (+102 to 130 g/kg; *P* ≤ 0.05). In the distal stomach, digesta DM was greater for SF and WS compared with WSP (+54 to 81 g/kg; *P* ≤ 0.05). Overall, liquids were emptied quickest from the stomach

(MRT of 00:21 to 01:34 h:min), followed by fine solids (MRT of 01:08 to 04:03 h:min), and fibrous particles (MRT of 01:08 to 10:29 h:min). For WS and WSP, the MRT of liquids was longer in the proximal stomach compared with the distal stomach (+21 to 34 min; *P* ≤ 0.05). In the proximal stomach, the MRT of liquids (+32 to 54 min; *P* ≤ 0.05) was greater for WS and WSP compared with WF. The MRT of fine solids was greater for WS compared with WF (+01:51 h:min, *P* = 0.043). In contrast to WS and WSP, there was no phase separation, neither between liquids and fine solids nor between fibrous particles and fine solids, for the WF and SF in the proximal stomach.

In the distal stomach, the MRT of liquids was longer for WSP compared with WF (+19 min; *P* = 0.032). The MRT of fine solids

<footnote n="1">¹ Presented values are the average of duplicate measurements.</footnote>

<footnote>II Presented values are the average of triplicate measurements.</footnote>

<footnote>III Measured on diets mixed with water in a 1:1 ratio of feed to water (w/w), as fed to the animals.</footnote>

<footnote>IV Presented values are the average of quintuplicate measurements.</footnote>

<footnote n="1">¹ Data are presented as raw means ± pooled SD. In each region, n = 6 for WS and WSP, and n = 5 for WF and SF.</footnote>

<footnote>II DM content of fresh digesta was determined gravimetrically on single aliquots by recording the weight before and after freeze-drying.</footnote>

<footnote>III Model established *P*-values for the fixed effect of diet. Within a row, means without a common superscript (a,b) differ (*P* ≤ 0.05).</footnote>

<footnote>IV Model established *P*-values for fixed effects of the region within diet.</footnote>

<footnote>* Indicates that separation significantly differs from 0 (*P* ≤ 0.05).</footnote>
was longer for WS compared with WF and SF (+01:04 h:min, $P \le 0.05$), while the separation between fine solids and liquids was greater for WS compared with the other diets (+43 to 57 min; $P \le 0.05$). As in the proximal stomach, there was no phase separation between fibrous particles and fine solids for the WF. The separation between fibrous particles and fine solids was greater in the distal stomach for the WS and WSP compared with the SF (+01:20 to 04:16 h:min; $P \le 0.05$). In both stomach regions, the MRT of fibrous particles was longer for WS and WSP, compared with WF and SF (+02:23 to 04:48 h:min; $P \le 0.05$).

### Small intestine

In the SI (i.e. proximal and mid-SI, and ileum), there was neither difference in MRT nor separation of digesta phases among diets ($P > 0.05$; Table 4). Apparent starch digestibility was numerically greater in the proximal SI (+10.9%-units; Table 5), and statistically greater in the mid-SI for WSP compared with WS (+9.4%-units; $P < 0.001$). Nitrogen digestibility at the ileum was numerically lower for the WSP compared with the WS ($-7.5%-units$).

### Large intestine

In the cecum, the MRT of liquids was longer for SF compared with WS and WSP (+02:57 to 03:18 h:min; $P \le 0.05$; Table 4). The MRT of liquids was also longer for WF compared with the WS (+01:47 h:min; $P = 0.029$). The MRT of fine solids was longer for SF compared with WS and WSP (+02:29 to 02:47 h:min; $P \le 0.05$). The MRT of fibrous particles was also longer for SF compared with the other diets (+02:07 to 02:59 h:min; $P \le 0.05$). In the proximal colon, the MRT of liquids and fine solids were longer for WF and SF and compared with WS and WSP (+06:23 to 13:17 h:min; $P \le 0.05$).

The MRT of fibrous particles was also longer for SF compared with WS and WSP (+06:48 to 11:07 h:min; $P \le 0.05$). The MRT of fibrous particles was longer for WF compared with WSP (+08:49 h:min; $P = 0.004$). In the distal colon, the MRT of liquids was shorter for WSP compared with the other diets ($-05:24$ to $02:38$ h:min; $P \le 0.05$). The MRT of fine solids (+03:33 to 04:38 h:min; $P \le 0.05$) and fibrous particles (+03:05 to 04:46 h:min; $P \le 0.05$) were longer for WF and SF compared with WSP. Total tract digestibility of DM was greater for SF compared with WSP (+4.7%-units; $P = 0.015$; Table 5). Digestibility of nitrogen was lower for WSP compared with the other diets ($-7.9$ to $7.6%-units; $P \le 0.05$). However, the digestibility of NSP was greater for WSP compared with the other diets (+34.1 to 44.8%-units; $P \le 0.05$). Digestibility of arabinosyl residue was greater for SF and WSP compared with WF and WS (+22.2 to 32.4%-units; $P \le 0.05$). Digestibility of uronyl residues was greater for SF compared with the WF and WS (+32.1 to 35.1%-units; $P \le 0.05$), but lower compared with WSP ($-50.1%-units; P \le 0.05$).

### Discussion

This study aimed at quantifying the effects of (1) insoluble fibres differing in botanical origin, cell wall composition, and physicochemical properties such as particle size, and (2) the addition of high-methylated pectin to coarse insoluble fibres, on the MRT of digesta phases, and nutrient digestion kinetics throughout the GIT of finishing pigs. To that end, three lignified and poorly fermentable insoluble fibres sources differing in origin, cell wall composition, and physicochemical properties, were selected; i.e. softwood flour, sunflower hulls, and wheat straw. To study the effects of adding pectin to coarse insoluble fibres, a high-methylated pectin was used. This type of pectin can have a gelling

<caption>**Table 4**</caption>

Mean retention time (MRT, h:min) of liquids (Yb-EDTA), fine solids (TiO₂), and fibrous particles (Chromium mordants) in different segments of the gastrointestinal tract of finishing pigs fed diets containing either softwood flour (WF), sunflower hulls (SF), or wheat straw without (WS) or with the addition of high-methylated pectin (WSP)<fnref n="1" />.

<table><thead><tr><th rowspan="2">Item</th><th colspan="4">Diets</th><th rowspan="2">Pooled SD</th><th rowspan="2">p<sup>ii</sup><br>Diet</th></tr><tr><th>WF</th><th>SF</th><th>WS</th><th>WSP</th></tr></thead><tbody><tr><td>MRT, h:min</td><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>Liquids</td><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>Proximal SI</td><td>00:35</td><td>00:31</td><td>00:19</td><td>00:32</td><td>00:13</td><td>0.234</td></tr><tr><td>Mid-SI</td><td>01:33</td><td>02:02</td><td>01:26</td><td>01:25</td><td>00:38</td><td>0.413</td></tr><tr><td>Ileum</td><td>01:19</td><td>01:15</td><td>01:07</td><td>00:44</td><td>00:47</td><td>0.775</td></tr><tr><td>Cecum</td><td>03:45<sup>ab</sup></td><td>05:16<sup>a</sup></td><td>01:58<sup>c</sup></td><td>02:19<sup>bc</sup></td><td>00:46</td><td>&lt;0.001</td></tr><tr><td>Proximal colon</td><td>17:36<sup>a</sup></td><td>20:06<sup>a</sup></td><td>09:17<sup>b</sup></td><td>06:49<sup>b</sup></td><td>02:29</td><td>&lt;0.001</td></tr><tr><td>Distal colon</td><td>08:34<sup>a</sup></td><td>10:06<sup>a</sup></td><td>07:20<sup>a</sup></td><td>04:42<sup>b</sup></td><td>01:58</td><td>0.002</td></tr><tr><td>Rectum</td><td>03:01</td><td>02:18</td><td>01:33</td><td>00:50</td><td>01:29</td><td>0.248</td></tr><tr><td>Fine solids</td><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>Proximal SI</td><td>00:27</td><td>00:24</td><td>00:21</td><td>00:26</td><td>00:13</td><td>0.894</td></tr><tr><td>Mid-SI</td><td>01:29</td><td>01:38</td><td>01:29</td><td>01:14</td><td>00:34</td><td>0.700</td></tr><tr><td>Ileum</td><td>01:11</td><td>01:01</td><td>00:56</td><td>00:39</td><td>00:42</td><td>0.765</td></tr><tr><td>Cecum</td><td>03:30<sup>ab</sup></td><td>04:49<sup>a</sup></td><td>02:20<sup>b</sup></td><td>02:02<sup>b</sup></td><td>00:49</td><td>0.001</td></tr><tr><td>Proximal colon</td><td>15:54<sup>a</sup></td><td>18:48<sup>a</sup></td><td>09:31<sup>b</sup></td><td>06:31<sup>b</sup></td><td>02:10</td><td>&lt;0.001</td></tr><tr><td>Distal colon</td><td>07:50<sup>a</sup></td><td>08:55<sup>a</sup></td><td>06:20<sup>ab</sup></td><td>04:17<sup>b</sup></td><td>01:42</td><td>0.002</td></tr><tr><td>Rectum</td><td>02:43</td><td>02:01</td><td>01:17</td><td>00:47</td><td>01:20</td><td>0.219</td></tr><tr><td>Fibrous particles</td><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>Proximal SI</td><td>00:27</td><td>00:28</td><td>00:26</td><td>00:23</td><td>00:16</td><td>0.953</td></tr><tr><td>Mid-SI</td><td>01:43</td><td>01:48</td><td>02:00</td><td>01:20</td><td>00:49</td><td>0.568</td></tr><tr><td>Ileum</td><td>01:25</td><td>01:18</td><td>00:45</td><td>00:44</td><td>00:47</td><td>0.600</td></tr><tr><td>Cecum</td><td>03:47<sup>b</sup></td><td>05:54<sup>a</sup></td><td>03:10<sup>b</sup></td><td>02:55<sup>b</sup></td><td>01:00</td><td>0.003</td></tr><tr><td>Proximal colon</td><td>16:42<sup>ab</sup></td><td>19:00<sup>a</sup></td><td>12:12<sup>bc</sup></td><td>07:53<sup>c</sup></td><td>02:40</td><td>&lt;0.001</td></tr><tr><td>Distal colon</td><td>08:31<sup>a</sup></td><td>10:12<sup>a</sup></td><td>06:56<sup>ab</sup></td><td>05:26<sup>b</sup></td><td>01:55</td><td>0.006</td></tr><tr><td>Rectum</td><td>02:57</td><td>02:19</td><td>01:31</td><td>00:52</td><td>01:29</td><td>0.275</td></tr></tbody></table>

Abbreviations: SI = small intestine.

<footnote n="1">¹ Data are presented as raw means ± pooled SD. In the proximal SI, mid-SI, cecum, proximal and distal colon, n = 6 for WS and WSP, n = 5 for WF and SF, in the ileum, n = 4 for all diets, in the rectum, n = 5 for WF, SF and WS, n = 6 for WSP.</footnote>

<footnote>ii Model established P-values for the fixed effect of diet. Within a row, means without a common superscript (a,b,c) differ ($P \le 0.05$).</footnote>
<caption>**Table 5**</caption>

Apparent digestibility of DM and nutrients in different segments of the gastrointestinal tract of finishing pigs fed diets containing either softwood flour (WF), sunflower hulls (SF), or wheat straw without (WS) or with the addition of high-methylated pectin (WSP)¹.

<table><thead><tr><th rowspan="2">Item</th><th colspan="4">Diets</th><th rowspan="2">Pooled SD</th><th rowspan="2">p<sup>ii</sup><br>Diet</th></tr><tr><th>WF</th><th>SF</th><th>WS</th><th>WSP</th></tr></thead><tbody><tr><td>Digestibility, %</td><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>Proximal SI</td><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>Starch</td><td>40.8</td><td>47.4</td><td>37.9</td><td>48.8</td><td>13.8</td><td>0.657</td></tr><tr><td>Nitrogen</td><td>22.2</td><td>-3.8</td><td>9.8</td><td>3.5</td><td>24.6</td><td>0.419</td></tr><tr><td>Mid-SI</td><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>Starch</td><td>57.9<sup>ab</sup></td><td>65.6<sup>ab</sup></td><td>56.2<sup>b</sup></td><td>65.6<sup>a</sup></td><td>4.48</td><td>0.021</td></tr><tr><td>Nitrogen</td><td>53.5</td><td>54.9</td><td>56.3</td><td>42.3</td><td>12.8</td><td>0.284</td></tr><tr><td>Ileum</td><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>DM</td><td>57.0</td><td>55.9</td><td>60.2</td><td>49.0</td><td>6.86</td><td>0.499</td></tr><tr><td>Starch</td><td>67.3</td><td>62.7</td><td>67.2</td><td>69.0</td><td>8.40</td><td>0.859</td></tr><tr><td>Nitrogen</td><td>77.2</td><td>76.3</td><td>73.2</td><td>65.7</td><td>4.90</td><td>0.088</td></tr><tr><td>Fat</td><td>92.7</td><td>91.4</td><td>90.3</td><td>86.5</td><td>4.39</td><td>0.588</td></tr><tr><td>Total tract</td><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>DM</td><td>79.6<sup>ab</sup></td><td>82.4<sup>a</sup></td><td>80.9<sup>ab</sup></td><td>77.7<sup>b</sup></td><td>2.20</td><td>0.019</td></tr><tr><td>Starch</td><td>99.0</td><td>99.2</td><td>99.3</td><td>99.1</td><td>0.59</td><td>0.037</td></tr><tr><td>Nitrogen</td><td>79.8<sup>a</sup></td><td>79.8<sup>a</sup></td><td>79.5<sup>a</sup></td><td>71.9<sup>b</sup></td><td>4.02</td><td>0.011</td></tr><tr><td>Fat</td><td>88.9</td><td>88.3</td><td>86.3</td><td>80.5</td><td>5.02</td><td>0.091</td></tr><tr><td>NSP<sup>iii</sup></td><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>Total</td><td>8.3<sup>b</sup></td><td>16.6<sup>b</sup></td><td>19.0<sup>b</sup></td><td>53.1<sup>a</sup></td><td>7.70</td><td>&lt;0.001</td></tr><tr><td>Arabinosyl</td><td>10.5<sup>b</sup></td><td>42.8<sup>a</sup></td><td>20.6<sup>b</sup></td><td>42.9<sup>a</sup></td><td>6.40</td><td>&lt;0.001</td></tr><tr><td>Xylosyl</td><td>8.6</td><td>15.7</td><td>21.1</td><td>19.9</td><td>8.42</td><td>0.122</td></tr><tr><td>Glucosyl</td><td>8.2</td><td>6.9</td><td>20.6</td><td>15.1</td><td>9.75</td><td>0.132</td></tr><tr><td>Mannosyl</td><td>18.1</td><td>14.0</td><td>2.6</td><td>-12.0</td><td>15.75</td><td>0.383</td></tr><tr><td>Uronyl</td><td>9.9<sup>c</sup></td><td>45.0<sup>b</sup></td><td>12.9<sup>c</sup></td><td>95.1<sup>a</sup></td><td>6.87</td><td>&lt;0.001</td></tr></tbody></table>

Abbreviations: SI = small intestine; NSP = non-starch polysaccharides.

¹ Data are presented as raw means ± pooled SD. In the proximal SI, n = 6 for WS and WSP, n = 5 for SF, and n = 4 for WF, in the mid-SI, n = 6 for WS and WSP, n = 5 for WF and SF, in the ileum, n = 4 for all diets, total tract, n = 6 for all diets and all nutrients except for NSP where n = 6 SF, WS and WSP, n = 5 for WF.

ii Model established P-values for the fixed effect of diet. Within a row, means without a common superscript (a,b,c) differ (P ≤ 0.05).

iii Monosaccharides represent anhydrous sugar moieties.

action under different physicochemical conditions, and reach the large intestine (partly) unfermented, thus also acting in the distal GIT. We also aimed to study the interactions between these fibres and the digestion kinetics of protein and starch not originating from these fibre sources. To this end, a combination of poorly and highly digestible protein and starch sources was selected.

The present study further demonstrated that gastric emptying of digesta phases is a gradual process in pigs (Lannuzel et al., 2024b; Martens et al., 2019b; Schop et al., 2019, 2020; Wilfart et al., 2007a). Liquids are emptied more rapidly from the stomach (MRT of 20 min to 1 h 30 min) than fine solids (MRT of 1 to 4 h). Depending on their size, fibrous particles may separate from fine solids (MRT of 1 to more than 10 h), illustrating the ability of the stomach to sieve particles based on size (Lannuzel et al., 2024b). In line with previous studies in growing pigs, the MRT of digesta phases ranged between 2 and 4 h in the SI (Lannuzel et al., 2024b, Schop et al., 2019, 2020; Wilfart et al., 2007a). Digestibility at the end of the SI ranged from 63 to 69% for starch and from 66 to 77% for nitrogen, as expected from the combination of poorly and highly digestible protein and starch sources used (CVB, 2018; Hodgkinson et al., 2022; Martens et al., 2019a; Sun et al., 2006). In agreement with previous studies (Hendriks et al., 2012; Lannuzel et al., 2024b; Low et al., 2020), the LI was the main contributor to the total tract retention time, having MRT of digesta phases ranging from 13 to 38 h in this study, depending on the type of insoluble fibres included in the diet.

Unlike straw, softwood flour and sunflower hulls reduce gastric accumulation of solids, and delay transit in the large intestine

Diets containing WF and SF were characterised by a uniform intragastric digesta, reflected in a comparable digesta pH and DM between the proximal and distal stomach. Due to their fine particles (x50 ≤ 0.2 mm²), these diets were presumably homogenously

mixed with gastric secretions (Nadia et al., 2022). The WF and the SF were also rapidly emptied from the stomach, having overall stomach MRT shorter than ~30 min for liquids, ~90 min for fine solids, and ~120 min for fibrous particles. These findings are in accordance with a recent study from our group, which showed that pigs fed a diet containing finely ground straw had shorter gastric MRT of digesta solids compared with pigs fed a diet containing coarsely chopped straw (Lannuzel et al., 2024b). As finer particles do not require extensive mechanical breakdown to pass the pylorus, they quickly leave the stomach (Lannuzel et al., 2024b; Nadia et al., 2021). Despite a comparable initial buffering capacity with the WF and SF, the WS had a greater difference in pH between the proximal and distal stomach (~0.3 vs 0.9 units), as typically observed when feeding coarser and more rigid diets (Bornhorst et al., 2014; Lannuzel et al., 2024a; Nadia et al., 2022). These variations in pH differences among diets containing different insoluble fibres may have impacted amylase- (i.e. salivary and bacterial amylases) and pepsin activity, thereby influencing protein and starch hydrolysis in the stomach (Martens et al., 2020; Mennah-Govela et al., 2021; Nadia et al., 2022). The 2-fold longer MRT of liquids in the proximal stomach compared with the distal stomach for WS illustrates the prominent reservoir function of the proximal stomach when feeding diets that require greater physical disintegration (Regina et al., 1999). In addition, the greater dietary water binding capacity (+30%) of WS compared with WF may have contributed to the longer MRT of liquids in the proximal stomach. The WS was also characterised by a pronounced sieving of coarse solids, reflected in a long retention time of fibrous particles in the stomach (~7 h) for WS, and a greater separation between fibrous particles and fine solids compared with WF and SF (Bornhorst et al., 2013; Lannuzel et al., 2024b). This likely modulated the timing and the sequence of nutrient delivery to more distal segments of the GIT (Lannuzel et al., 2024a; Nadia et al., 2022), thereby influencing the kinetics of nutrient absorption, and the
release of nutrient-stimulated peptides, such as peptide tyrosine tyrosine and glucagon-like peptide-1, involved in transit regulation (Black et al., 2009; Boulier et al., 2023; Nadia et al., 2023; Owusu-Asiedu et al., 2006). Marked differences in gastric sieving among insoluble fibres may also have influenced the exposure of dietary nutrients to enzymes present in the gastric environment, and thus their degree of hydrolysis (Nadia et al., 2021, 2022). Despite the marked differences among insoluble fibre sources observed in the stomach, the extent of starch and nitrogen digestion along the SI and digesta transit of digesta phases remained comparable. The changes in the physicochemical environment (e.g. meal dilution and pH increase), the tubular structure, the propulsive peri-staltic contractions typically found in the SI, and the steady state conditions induced by frequent feeding may have limited the differences among insoluble fibres in this segment, as previously reported (Lannuzel et al., 2024b). Although the selective entry of digesta phases in the cecum cannot be excluded, differences in tracer ratios were limited among WF, SF and WS in the cecum, suggesting that diets containing fine insoluble fibres were overall retained longer in this compartment, especially for SF, compared with WS (+1h 30 min to 3 h). Finer and denser particles from SF, and to a lower extent from WF, may have sedimented and accumulated longer in this sac-shape segment (Yen, 2000) compared with WS. In the colon, WS but not WF or SF resulted in short MRT, indicating that not only the amount (Wilfart et al., 2007a) but also the type of insoluble fibres and their physicochemical properties modulate digesta transit in the LI. Previous research have shown that the provision of coarse wheat bran to humans was more effective in accelerating digesta transit than finely ground wheat bran. This acceleration coincided with a greater capacity of coarse wheat bran to hold water in the faeces (Heller et al., 1980). In the present study, differences in MRT were also correlated with differences in faecal DM (r = 0.91, P < 0.001, between faecal DM and MRT of fine solids in the LI). Together with differences in bulk among fibres, variations in water reabsorption among diets may have influenced colonic motility (Low et al., 2020). Although NSP from softwood flour, sunflower hulls, and wheat straw greatly differ in composition and structure, their total tract degradation was poor, regardless of fibre source (8–19%).

While softwood flour typically contains galactoglucomannans, wheat straw contains xylosyl-rich hemicelluloses such as xylans or (arabino) glucuronoxylans (Carpita and McCann, 2020). In sunflower, glucuronoxylans are present, together with xyloglucans and (gluco) mannans, while pectic polysaccharides, e.g. homogalacturonans, arabinogalactans, and arabinans, are also abundant (Lannuzel et al., 2022). Despite these differences, all three insoluble fibre sources are rich in poorly degradable lignin and cellulose (Bach Knudsen, 2014; Carpita and McCann, 2020). These polymers can interact with hemicellulosic and pectic polysaccharides within the plant cell wall, and limit the degradation of NSP from WF, SF, and WS (Lannuzel et al., 2022; Yoo et al., 2020). Degradation of arabinosyl- and uronyl-residues from SF ranged from 43 to 45%. In sunflower, these residues mainly originate from pectic polysaccharides which are typically better degraded by pigs compared with cellulose and hemicelluloses (Lannuzel et al., 2022). In contrast, degradation of arabinosyl- and uronyl-residues from WF and WS was below 21%, as in WF and WS, these residues are typically found in recalcitrant hemicellulose (Carpita and McCann, 2020).

### Addition of pectin to coarse straw reduces gastric separation of liquids-fine solids, and increases starch digestibility in the proximal small intestine

The addition of pectin to the coarse straw diet suppressed the differences in pH and DM between the proximal and distal stomach. The greater initial buffering capacity of the WSP (+60 µmol H⁺/(g DM × ΔpH)) likely contributed to the reduced pH gradient in the stomach (Mennah-Govela et al., 2021). The lower DM in both stomach regions in response to pectin addition was concomitant with a greater dietary water binding capacity, which certainly limited the percolation of the liquids (Low et al., 2021). The numerical increase in the MRT of liquids and decrease in the MRT of fine solids following the addition of pectin reduced the separation between fine solids and liquids, especially in the distal stomach. By increasing water binding capacity, the addition of pectin may also have limited the sedimentation of fine solids and accelerated

<figure><img src="image_4.png" /><figcaption>**Fig. 2.** Relation between starch digestibility in the proximal small intestine (SI) and digesta pH in the proximal and distal stomach of finishing pigs fed diets containing either softwood flour (WF), sunflower hulls (SF), or wheat straw without (WS) or with the addition of pectin (WSP).</figcaption></figure>
their drag from the stomach with the liquids (Lannuzel et al., 2024a). In the SI, the addition of pectin did not affect digesta transit behaviour. As previously mentioned, the physicochemical conditions encountered in the SI certainly altered the influence of pectin on digesta physicochemical properties (Lannuzel et al., 2024b). Still, pectin addition increased apparent starch digestibility numerically in the proximal SI (11%-units), and significantly in the mid SI by 10%-units. This greater starch digestibility in the proximal part of the SI may be related to the gastric pH, especially in the proximal stomach where the high intragastric pH (≥ 3) typically found allows bacterial and salivary amylases to be active (Martens et al., 2020; Nadia et al., 2022). Therefore, we explored the relationship between gastric pH and starch digestibility. In contradiction with our expectations, starch digestibility in the proximal SI was positively correlated with digesta pH in the distal stomach (r = 0.46, P = 0.034 Fig. 2), but not in the proximal stomach. Possibly, the pH > 3 in the distal stomach observed for the WSP, but not for the other diets, prolonged amylase activity (Martens et al., 2020; Nadia et al., 2022). This may have increased gastric hydrolysis of starch, thereby increasing starch digestibility in the proximal part of the SI. Despite similar net nitrogen disappearance in the LI (~6.3% units), total tract digestibility of nitrogen was lower after pectin addition (~7.6%-units). Although the presence of fermentable pectin may have increased the formation of microbial biomass, possibly decreasing ammonia absorption and increasing urea influx in the colon (Heijnen and Beynen, 1997; van der Meulen et al., 1997), the lower total tract digestibility of nitrogen seemed to be mainly related to the lower ileal digestibility of nitrogen (~7.5%-units). The presence of pectin in the proximal GIT may have increased endogenous secretions, and hindered accessibility to digestive enzymes by creating a gel network, notably in the soluble phase where proteins are abundantly found in the SI (Chen et al., 2019; Low et al., 2021; Tian et al., 2017). The apparent digestibility of uronyl residues, which represent 93% of the NSP in this pectin, was nearly null at the end of the SI (≤ 4%), indicating that a substantial fraction of the pectin reached the LI (Tian et al., 2017). Total tract, the degradation of high-methylated pectin was complete, as shown by the large degradation of uronyl residues (95%). This complete pectin fermentation did not affect the degradation of xylosyl- and glucosyl residues which constitute 89% (w/w) of the NSP in wheat straw, indicating that pectin did not alter the activity of the microbiota degrading wheat straw fibres (Lannuzel et al., 2024b).

## Conclusion

The present study confirmed the role of fibre sources and their physicochemical properties in regulating digestion. In the stomach, fine softwood flour and sunflower hulls, suppressed regional differences in pH and DM between the proximal and distal stomach, observed for coarse wheat straw. Softwood flour and sunflower hulls also accelerated gastric emptying and reduced gastric sieving compared with coarse wheat straw. The addition of pectin to a wheat straw diet reduced the regional differences in pH and DM between the proximal and distal stomach, and limited separation between fine solids and liquids which was concomitant with an increased dietary water binding and buffering capacity. In the SI, there were no differences in digesta transit nor nutrient digestion among insoluble fibre sources. The physicochemical environment and the tubular structure found in the SI likely explain the lower importance of the physical structure of insoluble fibres compared with the stomach. Although the extent of starch digestibility at the end of the SI was unaffected by the addition of pectin, it increased starch digestibility in the proximal part of the SI. This was concomitant with a greater digesta pH in the distal stomach,

which may have influenced amylase activity and starch hydrolysis in the stomach. The addition of pectin also decreased nitrogen digestibility total tract, reflecting a reduction in digestibility already present at the ileal level. The presence of pectin may have hindered the accessibility of enzymes to nutrients or increased endogenous secretions. In the LI, coarse insoluble fibres accelerated digesta transit compared with fine insoluble fibres, indicating that the type of insoluble fibres and their associated physicochemical properties influence transit in this segment. The different fibres present in softwood flour, sunflower hulls, and wheat straw were all poorly degraded. Furthermore, the complete fermentation of pectin did not affect the degradation of fibres from straw. By altering intragastric pH, digesta transit behaviour, especially in the stomach, and the site of nutrient digestion, fibre sources and their physicochemical properties are likely to influence the absorption kinetics of nutrients and their metabolic use. The present results can be useful to better predict the effects of fibre-rich ingredients on the digestive utilisation of diets.

## Ethics approval

All experimental procedures were approved by the Dutch Central Committee of Animal Experiments (the Netherlands) under the authorisation number AVD40100202010104.

## Data and model availability statement

None of the data were deposited in an official repository. The data that support the study findings are available from the authors upon request.

## Declaration of Generative AI and AI-assisted technologies in the writing process

During the preparation of this work, the authors used Grammarly to check the grammar and clarity of the text. After using this tool, the authors reviewed and edited the content as needed and took full responsibility for the content of the publication.

## Author ORCIDs

C. Lannuzel: https://orcid.org/0009-0007-2284-3155.

R.J. Veersma: https://orcid.org/0000-0002-4761-6297.

G. van Erven: https://orcid.org/0000-0002-1544-1744.

M.A. Kabel: https://orcid.org/0000-0002-2787-7852.

W.J.J. Gerrits: https://orcid.org/0000-0003-0494-9259.

S. de Vries: https://orcid.org/0000-0002-3842-8411.

## CRediT authorship contribution statement

C. Lannuzel: Writing – original draft, Visualisation, Software, Investigation, Formal analysis, Data curation. R.J. Veersma: Writing – review & editing, Investigation, Formal analysis, Data curation. G. van Erven: Writing – review & editing, Conceptualisation. M.A. Kabel: Writing – review & editing, Project administration, Methodology, Funding acquisition, Conceptualisation. W.J.J. Gerrits: Writing – review & editing, Supervision, Methodology, Funding acquisition, Conceptualisation. S. de Vries: Writing – review & editing, Supervision, Methodology, Funding acquisition, Conceptualisation.

## Declaration of interest

None.
## Acknowledgements

We thank Kim de Wit, Nienke Kerssen, Harold de Louw, Sus- anne van Uden, Wilko Verkuijlen, Bo Hanegraaf, and Pieter Roskam for their technical support during the experiment, Saskia van Laar, Niels Wever, Mary Maxfield, Natalia Hutnik and Giovanni van Kralingen for their guidance and help with the laboratory analyses. We are also grateful to Prof. Hans-Ulrich Endress who kindly sup- plied the pectin, Christine Potthast for her information about the softwood flour, Paul Bikker for his help regarding the ethical appli- cation, and Prof. Gail Bornhorst for her advices concerning the par- ticle and buffering capacity analyses. This work was preliminarily published as a chapter in the PhD dissertation of Corentin Lan- nuzel: Featuring fibres on the menu of pigs - The role of fibres in regulating digestion, 2024. Wageningen University, Wageningen, The Netherlands (Lannuzel, 2024).

## Financial support statement

This project was performed within the framework of the research program Open Technology, with project number 17874, jointly financed by the Dutch Research Council (NWO), Wagenin- gen University & Research, IsoLife, Royal Agrifirm Group, Schothorst Feed Research, and Topigs Norsvin.

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