Kataoka et al.
Journal of Physiological Anthropology(2024) 43:5
https://doi.org/10.1186/s40101-023-00353-0

ORIGINAL ARTICLE

Journal of
Physiological Anthropology

Open Access

Fructose‑induced FGF21 secretion
does not activate brown adipose tissue
in Japanese young men: randomized cross‑overand randomized controlled trials
Haruki Kataoka1, Shinsuke Nirengi2,3, Yuka Matsui1 and Hirokazu Taniguchi1*   

Abstract
Background Human brown adipose tissue (BAT) activity is associated with lower body fatness and favorable glucose
metabolism. Previous studies reported that oral fructose loading induces postprandial fibroblast growth factor 21(FGF21) secretion. FGF21 is a known inducer of adipose tissue thermogenesis; however, the effects of diet-induced
FGF21 secretion on BAT thermogenesis remain to be elucidated.
Methods The effects of both single load and daily consumption of fructose on BAT activity were examined usinga randomized cross-over trial and a 2-week randomized controlled trial (RCT), respectively. In the cross-over trial, 15
young men consumed a single dose of fructose solution or water and then consumed the other on a subsequent day.
The RCT enrolled 22 young men, and the participants were allocated to a group that consumed fructose and a groupthat consumed water daily for 2 weeks. BAT activity was analyzed using thermography with cold exposure. Plasma
FGF21 level was determined by enzyme-linked immunosorbent assay.
Results In the cross-over single-load trial, plasma FGF21 levels were significantly increased at 2 h after oral fructose
load (p < 0.01); however, there was no significant difference in BAT activity between the fructose load and drinking water. The 2-week RCT revealed that both plasma FGF21 levels and BAT activity were not significantly increased
by daily fructose consumption compared to water. Correlation analyses revealed that BAT activity at the baseline
and the final measurements were strongly and positively associated with the RCT (r = 0.869, p < 0.001). Changes
in BAT activity were significantly and negatively correlated with changes in plasma glucose levels during the 2-weekintervention (r = − 0.497, p = 0.022).
Conclusions Oral fructose load induces a temporary increase in circulating FGF21 levels; however, this does not activate BAT thermogenesis in healthy young men. Further studies are needed to elucidate the effect of endogenous
FGF21 on physiological function.
Trial registration This study is registered with the University Hospital Medical Information Network in Japan (number
000051761, registered 1 August 2023, retrospectively registered, https://​cente​r6.​umin.​ac.​jp/​cgi-​open-​bin/​ctr/​ctr_​view.​
cgi?​recpt​no=​R0000​52680).
Keywords Brown adipose tissue, Fibroblast growth factor 21, Thermogenesis, Fructose, Thermography, RCT​

*Correspondence:
Hirokazu Taniguchi
hirokazu-t@kpu.ac.jp
Full list of author information is available at the end of the article© The Author(s) 2024. Open Access This article is licensed under a Creative Commons Attribution 4.0 International License, which
permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to theoriginal author(s) and the source, provide a link to the Creative Commons licence, and indicate if changes were made. The images orother third party material in this article are included in the article’s Creative Commons licence, unless indicated otherwise in a credit line
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Kataoka et al. Journal of Physiological Anthropology(2024) 43:5

Background
Adipose tissues have different functional features. White
adipose tissue is a regulator of energy storage and systemic metabolic homeostasis [1], whereas brown adipose tissue (BAT) converts metabolic energy into heatand thereby increases energy expenditure [2, 3]. In adulthumans, the BAT depot is mainly localized in the supraclavicular region, and the heat production of BAT is
activated by cold exposure [2, 3]. The cold-induced activation of BAT (BAT activity) induces energy expenditureand glucose disposal [4–6]. High levels of BAT activity
are associated with lower body fatness [7, 8] and favorable glucose metabolism [9, 10]. Interventional studies
reported that BAT activity was increased by oral intake
of food ingredients such as capsinoids [4, 11] and tea catechins [12, 13]. Thus, BAT activity has health benefits,
and the activity could be induced by environmental and/or nutritional factors.
Fibroblast growth factor 21 (FGF21) is known as an
endocrine and metabolic regulator that induces adipose tissue thermogenesis [14, 15]. FGF21 administration prevents body weight gain and the developmentof an insulin-resistant state in rodents [16]. Consistent
results were shown in human studies, which reported
that administration of FGF21 analogs decreased bodyweight and increased insulin sensitivity in people with
type 2 diabetes [17, 18]. Other human studies suggested
that blood FGF21 levels were significantly and positively
associated with BAT activity [19, 20]. Therefore, endogenous secretion of FGF21 may have a physiological rolein human energy metabolism. Recent studies reported
that the secretion of FGF21 was stimulated by acute oral
fructose load [21–23], which secreted hepatic FGF21
in a carbohydrate response element binding protein(ChREBP)-dependent manner [24]. The results suggestthat the FGF21 response to fructose intake may activate
BAT thermogenesis. On the other hand, Richard et al.
reported that a short-term high-fructose diet suppressed
glucose uptake by cold-stimulated BAT in young men[25], suggesting the adverse effects of fructose consumption on BAT activity.
As the effects of diet-induced secretion of FGF21 on
BAT thermogenesis activation remain to be elucidated,
the present study aimed to evaluate the acute and shortterm effects of fructose loading on BAT activity using arandomized cross-over trial and a randomized controlled
trial (RCT).
Methods
Ethics approval and trial registration

The present study was approved by the Ethical Committee of Kyoto Prefectural University (approval number

Page 2 of 10216). All participants provided written informed consent
before enrolment. The study is registered with the University Hospital Medical Information Network in Japan(number 000051761) and was conducted in accordancewith the Declaration of Helsinki.
Study design and participants

The effects of both single load and daily consumption offructose on BAT activity were examined in the winter of2022 (Fig. 1) by randomized cross-over trial and RCT,
respectively. Participants were healthy young men aged
over 18 years.
In the cross-over trial (Fig. 1A), 15 young men (aged
20–24 years) participated and were randomly divided
into a group that received a single load of fructose(n = 9) and a group that received water (n = 6). The
subjects switched to the other group after 3–7 days ofthe first dose. The data of one participant who did notcomplete the study was excluded; thus, the data of 14
participants were finally analyzed.
The RCT included 22 young men aged 18–24 years(Fig. 1B). The participants were randomized and allocated to a group that consumed fructose (n = 11) and agroup that consumed water (n = 11). After baseline measurements, one participant in the group that consumed
water declined to participate due to syncopal reaction
after blood collection, and their data was excluded. The
remaining 21 participants completed the 2-week intervention, and final measurements were performed.
Fructose intake

A fructose solution was prepared by dissolving 30 g offructose in 225 mL of a commercial brand of water, as
described in a previous study that examined a quantitative relationship between fructose load and FGF21 secretion [23]. The fructose solution was used in both thecross-over and controlled trials. A commercial brand ofwater was used as a control.
Measurement of BAT activity

BAT activity was determined using mild cold exposure,as described previously [20, 26]. In brief, subjects wereseated at rest in a temperature-controlled room at 27°C for 30 min. Baseline body surface temperature wasmeasured using a thermal imaging camera (DETC1000T;
D-eyes, Osaka, Japan). The subjects entered the climaticchamber (TBRR-9A4GX; ESPEC, Osaka, Japan), which
was set at 19 °C. After the start of cold exposure, bodysurface temperature was collected at 30 min and 1 h of
mild-cold exposure. Before and during cold exposure,
subjects were asked to rate their shivering, cold sensation, and discomfort using a visual analog scale [20, 26].

Kataoka et al. Journal of Physiological Anthropology(2024) 43:5

Page 3 of 10

Fig. 1 Flow diagram of the participants in the study. A Randomized cross-over trial for single fructose load. B Randomized controlled trial
consisting of daily fructose consumption for 2 weeks

The supraclavicular temperature adjacent to the BAT
location on both the right and left sides was measured from each image. Chest temperature was simultaneously measured as a control for underlying BAT
depots. The images of body surface temperature wereanalyzed using a modified (D-eyes) version of the
Thermal-Cam v.1.1.0.9 software (Laon People, Seoul,
Korea), and the average of supraclavicular temperatureminus chest temperature was estimated as BAT activity. For the 2-week RCT, maximal BAT activity wasdefined as the higher BAT activity between 30 min and
1 h after cold exposure.
Anthropometric characteristics, dietary food intake,and physical activity

Body weight and body fat percentage were measured
using an electronic scale (V-body HBF-359; Omron,
Kyoto, Japan). Body mass index was calculated as bodyweight (kg) divided by the square of height (m). Energy,
protein, carbohydrate, and fat intake were also assessedKataoka et al. Journal of Physiological Anthropology(2024) 43:5

using a Food Frequency Questionnaire based on the foodgroups (Ver. 6.0; Kenpakusha, Tokyo, Japan) [27]. Physical activity was estimated by the International Physical Activity Questionnaire and expressed as metabolic
equivalents per hour per week [28].
Measurements in the randomized cross‑over trial and RCT​For all measurement days, subjects were instructed toarrive by 9:00 AM after an overnight fast, and the measurements were performed within 3 h.
In the cross-over trial (single load), baseline body surface temperature and blood sample were collected, andthen the subjects consumed fructose solution or control
water. The initiation of cold exposure was started 1.5 hafter the oral fructose intake to match BAT thermogenesis [26] and a peak increase in blood FGF21 level 2 hafter fructose load [21–23]. Body surface temperatureand blood sample were collected at 30 min and 1 h ofcold exposure (2 h and 2.5 h after the oral fructose load,
respectively).
In the RCT (daily consumption), the same baseline andfinal measurements were performed before and after the
2-week daily consumption of fructose solution. Body
surface temperature was measured at baseline (before
cold exposure) and after 30 min and 1 h of cold exposure. Blood sampling was performed before cold exposure. The subjects did not consume fructose solution
and could only consume water in the baseline and final
measurements.
Blood analysis

The collected blood samples were centrifuged at 3000×g
for 5 min. Plasma and serum were stored at – 80 °C until
the time of analysis. The concentrations of plasma glucose and free fatty acid (FFA) were measured using Glucose C2-test Wako and NEFA C-test Wako (FUJIFILM
Wako Pure Chemical, Osaka, Japan), respectively. Serumtriglyceride, total cholesterol, high-density lipoproteincholesterol, and low-density lipoprotein cholesterol were
determined by Kyoto Biken Laboratories, Inc. (Kyoto,
Japan). Plasma FGF21 concentration was determinedusing a commercially available enzyme-linked immunosorbent assay kit (DF2100; R&D Systems, Minneapolis,
USA) according to the manufacturer’s instructions.
Statistics

All statistical analyses were performed using SPSS, version 29.0 (SPSS, Chicago, IL, USA). The KolmogorovSmirnov test was performed to assess the normality ofdata distribution. Two-way ANOVA analysis (time ×
condition) and post-hoc Tukey’s HSD test were used todetermine the absolute value difference in the cross-overtrial. Baseline characteristics and changes in variables

Page 4 of 10

during the RCT were compared using Student’s t test fornormally distributed data or the Mann-Whitney U test
for nonnormally distributed data. Relationships betweenchanges in all variables were determined by Pearson’s
correlation coefficients. All measurements and calculated
values are presented as the mean ± SD, and the level ofstatistical significance was set at p < 0.05.

Results
Effects of single fructose load on FGF21 and BATin the cross‑over trial

Subject characteristics for the cross-over single-load trial
are shown in Table 1. Shivering response, cold sensation,and discomfort during cold exposure were not statistically different between the fructose load and drinking
water conditions (data not shown). Compared to thebaseline level, plasma FGF21 levels were significantly
increased at 2 h after oral fructose load (p < 0.01), but
decreased to non-significant levels at 2.5 h after loading(Fig. 2A). Plasma FGF21 levels at both 2 h and 2.5 h after
drinking water were significantly lower than at the baseline level (p < 0.05). Plasma FFA levels were significantly
decreased at both 2 h and 2.5 h after fructose loading(p < 0.05) in the group compared to the baseline level(Fig. 2B). There were no significant changes in plasma
glucose levels during the single-load trial (Fig. 2C). As
shown in Fig. 3, BAT activity, which was evaluated as
standardized supraclavicular temperature, was significantly increased by 30-min and 1-h mild cold exposure
at both 2 h and 2.5 h after fructose loading, respectively
Table 1 Subject characteristics for the cross-over singleconsumption trial (n = 14)
Age (year)

21.8 ± 1.1

Height (cm)173.1 ± 5.4

Weight (kg)61.9 ± 7.6

BMI (kg/m2)

20.6 ± 2.2

Body fat (%)

15.7 ± 3.5

Blood metabolic parameters
Glucose (mg/dL)

93.7 ± 13.0

FFA (mEq/L)

0.54 ± 0.15

Triglyceride (mg/dL)

71.8 ± 33.3

Total cholesterol (mg/dL)

180.4 ± 23.6

HDL cholesterol (mg/dL)

65.2 ± 11.9

LDL cholesterol (mg/dL)

98.2 ± 19.9

Dietary consumption (/day)
Energy (kcal)

1767 ± 435

Protein (g)60.2 ± 15.5

Fat (g)64.4 ± 18.3

Carbohydrate (g)221.6 ± 54.9

PA (METs-h/week)

52.9 ± 38.4

BMI body mass index, FFA free fatty acid, HDL high-density lipoprotein, LDL lowdensity lipoprotein, MET metabolic equivalent, PA physical activity

Kataoka et al. Journal of Physiological Anthropology(2024) 43:5

Page 5 of 10

Fig. 2 Changes in plasma FGF21, FFA, and glucose levels in the randomized cross-over trial. A Plasma FGF21 levels. B Plasma FFA levels. C Plasma
glucose levels. Two-way ANOVA analysis and the post-hoc Tukey’s HSD test were performed. * and † indicate statistical significance comparedto the baseline levels (p < 0.05). FGF21, fibroblast growth factor 21; FFA, free fatty acid(p < 0.01). However, a significant increase in BAT activity was similarly observed after drinking water (p < 0.01).
Two-way ANOVA analysis found that there was no significant difference in BAT activity between the fructose
load and drinking water conditions (p = 0.923).
Effects of daily fructose consumption on FGF21 and BATin the RCT​

Table 2 shows anthropometric values, blood metabolic
parameters, dietary consumption, physical activity, andBAT activity before and after the 2-week RCT. There wereno significant changes in body weight and body fat percentage between the groups. The mean value of plasma
glucose levels was reduced in the fructose group, but
the difference was not significant (p = 0.173). Changes
in blood lipid parameters were not different between
the groups. Plasma FGF21 levels during the trial wereincreased both in the fructose and water groups, whereas
the changes in the values between the groups were notsignificantly different in the RCT (p = 0.505). There wasno difference in dietary consumption and physical activity between the groups. BAT activity at baseline and after
cold exposure was not increased by daily fructose consumption and was not different between the fructose andwater groups. For cold exposure, there was no significant

difference in shivering response, cold sensation, and discomfort between the two groups (data not shown).
Associated factors of BAT activity in the RCT​

Correlation analyses were performed to determine theassociated factors of BAT activity changes during the
2-week intervention (Table 3). As shown in Fig. 4A, BAT
activity at the baseline and the final measurements werestrongly and positively correlated in the RCT (r = 0.869,
p < 0.001). The changes in BAT activity during the trial
were not associated with changes in plasma FGF21 levels(r = − 0.065, p = 0.780), whereas the changes in maximal
BAT activity were significantly and negatively correlatedwith changes in plasma glucose levels during the 2-weekintervention (Fig. 4B; r = −0.497, p = 0.022). Changes inanthropometric values, lipid metabolic parameters, dietary consumption, and physical activity were not associated with the changes in BAT activity.

Discussion
We performed a randomized cross-over trial andan RCT to evaluate whether dietary-induced FGF21
secretion activates BAT thermogenesis. In the present
study, oral fructose load increased plasma FGF21 levels more than 2-fold but did not influence BAT activity in both the acute and short-term interventions. The

Kataoka et al. Journal of Physiological Anthropology(2024) 43:5

Fig. 3 Changes in BAT activity in the randomized cross-over trial.
Two-way ANOVA analysis and the post-hoc Tukey’s HSD test were
performed. * indicates statistical significance compared to baseline
levels (p < 0.05). BAT, brown adipose tissue

temporary increase in circulating FGF21 level may
have little or no effect on BAT thermogenesis in young
healthy adults.
The thermogenic action and mechanism of FGF21
have been explored using transgenic and high-dosemodels in rodents [16, 29], which showed abnormal levels of circulating FGF21. Because recombinant human
FGF21 has a short half-life of less than 2 h in mice andprimates [30], the pharmacological properties in clinical
trials were examined using FGF21 analogs with a prolonged half-life [17, 18]. Unlike the above studies, thepresent study found that the increased FGF21 levels 2
h after oral fructose load were decreased within 30 min.
Therefore, the weak and short actions of FGF21 wereconsidered as the reason why the fructose intake did not
activate BAT thermogenesis. It was reported that BAT
activity is increased by methods of sympathetic nervous
system activation such as cold exposure [5, 6] and oral
intake of food ingredients [4, 11–13]. At this point, thesemethods have more advantages than FGF21 induction
regarding BAT activation. Moreover, it was reported

Page 6 of 10

that a hypercaloric fructose diet was associated withrisks of obesity [31] and fatty liver [32]. Both obesityand fatty liver accumulation are independent factors ofchronic high FGF21 levels [33, 34], which is considered
an FGF21-resistant state [35] and related to the onsetof type 2 diabetes and cardiovascular diseases [36–39].
Thus, the present study supports these previous studies
that found that fructose intake has health disadvantages.
The present study found a dose-response relationship between 30 g oral fructose and blood FGF21 levels,
which supports the findings of a previous study in the
USA [23]. This suggests that genetic and lifestyle background do not affect oral fructose-induced FGF21 secretion and that the oral amount of fructose was adequate
to increase circulating FGF21 levels. On the other hand,
we found a significant decrease in plasma FGF21 levels
after drinking water in the single-load trial. This finding may be explained by a diurnal rhythm of circulating FGF21, which decreases from morning to afternoon[40]. Daily fructose intake at breakfast may influence
the diurnal rhythm of FGF21, although the physiological role requires further investigation. Circulating FFA
levels were decreased around 2 h after oral fructose load
as seen in this and a previous study [41]. Fructose consumption activates hepatic ChREBP [24], which induces
triglyceride synthesis in the liver [42]. It is likely that
hepatic FFA uptake increased due to lipogenesis, and atemporary decrease in FFA levels was observed. Previous
studies reported that FGF21 administration induced glucose uptake in both adipocyte and adipose tissue [16, 43];
however, in the present study, a single load did not showchanges in plasma glucose levels during increased plasma
FGF21. Our findings suggest that an acute increase inendogenous FGF21 may not have an impact on whole
glucose homeostasis in healthy young men.
The present study found that both anthropometric andblood metabolic parameters did not significantly change
after 2 weeks of daily consumption of 30 g of fructose.
Our findings suggest that short-term fructose consumption does not have both beneficial and adverse effects
on the parameters of healthy young adults. Daily fructose consumption did not change baseline FGF21 levels; thus, daily FGF21 secretion may not modulate thediurnal rhythm of FGF21. Previous studies reported that
resting levels of circulating FGF21 were increased by alow protein diet [44] and were decreased by endurance
exercise [45, 46]. We assessed macronutrient intakes andphysical activity using standardized questionnaires; however, these associated factors were unchanged between
the baseline and final measurements. For the correlation analyses, a highly reproducible correlation wasobserved between the baseline and final measurementsof BAT activity in the RCT. Our findings suggest that

175.9±5.8
63.8±7.2
20.6±2.1
14.6±3.8

Height (cm)Weight (kg)BMI (kg/m2)

Body fat (%)

0.24±0.13
63.0±23.0
165.0±28.0
60.0±11.0
92.0±24.0
125±74

FFA (mEq/L)

Triglyceride (mg/dL)

Total cholesterol (mg/dL)

HDL cholesterol (mg/dL)

LDL cholesterol (mg/dL)

FGF21(pg/mL)

68.6±20.6
239.4±72.3

Fat (g)Carbohydrate (g)0.87±0.43
0.93±0.42
1.00±0.40

30-min cold exposure

60-min cold exposure

Maximal activity

1.00±0.34

0.92±0.40

0.86±0.35

0.55±0.24

67.8±54.7

206.6±78.2

63.6±27.0

59.6±25.51726±647

182±17490.0±22.0

59.0±9.0

168.0±31.0

83.0±58.0

0.20±0.1393.4±6.0

15.3±3.1

20.5±2.0

63.6±6.6

Post

0.00±0.09− 0.01±0.11− 0.02±0.21

0.06±0.14

15.8±45.4− 32.8±101.0− 4.9±18.0− 4.1±17.7− 188.0±589.7

57±144− 1.0±8.0

0.0±6.0

3.0±10.0

20.0±59.0− 0.04±0.19− 4.6±9.3

0.7±1.2− 0.1±0.3− 0.3±0.8⊿

0.94±0.25

0.93±0.25

0.78±0.32

0.47±0.2159.7±69.5

180.2±56.0

51.9±15.949.7±15.2

1433±415

158±59100.0±24.0

56.0±9.0

167.0±23.0

58.0±26.0

0.27±0.08

90.9±4.415.1±3.3

20.7±1.860.6±6.1

171.0±5.3

21.7±1.6

Pre

Control (n = 10)0.87±0.32

0.76±0.36

0.70±0.35

0.47±0.1376.8±122

185.4±70.0

54.4±18.0

53.4±20.3

1492±518

227±18398.0±22.0

54.0±10.0

167.0±27.0

59.0±21.0

0.29±0.27

91.7±7.8

15.8±4.0

20.7±1.960.5±6.1

Post−0.07±0.23−0.17±0.31−0.08±0.26

0.00±0.19

17.1±58.5

5.2±56.6

2.4±9.4

3.7±12.958.4±310.8

68±1692.0±14.0−2.0±4.0−1.0±13.0

1.0±27.0

0.02±0.28

0.9±8.4

0.7±1.1−0.1±0.2−0.1±0.5⊿*Results of the Mann-Whitney U test

BAT brown adipose tissue, BMI body mass index, FFA free fatty acid, FGF21 fibroblast growth factor 21, HDL high-density lipoprotein, LDL low-density lipoprotein, MET metabolic equivalent, PA physical activity

Student’s t test (for normally distributed data) or Mann-Whitney U test (for non-normally distributed data) were performed

0.49±0.22

Baseline

BAT activity (°C)

51.9±36.9

63.8±20.7

Protein (g)PA (METs-h/week)

1914±524

Energy (kcal)

Dietary consumption (/day)

98.0±8.5

Glucose (mg/dL)

Blood metabolic parameters22.1±1.4

Age (year)

Pre

Fructose (n = 11)

Table 2 Changes in the variables during 2-week fructose consumption in the randomized controlled trial

0.366

0.128

0.536

0.433

0.123*

0.308

0.262

0.267

0.253

0.505*

0.884

0.256*

0.477

0.505*

0.835*

0.173

0.926

0.736

0.591

0.056

0.568

p

Kataoka et al. Journal of Physiological Anthropology(2024) 43:5
Page 7 of 10

Kataoka et al. Journal of Physiological Anthropology(2024) 43:5

Table 3 Correlations between changes in BAT activity and othervariables in the randomized control trial (n = 21)
Dependent variable⊿Maximal BAT activity (°C)
rp

⊿BMI (kg/m2)− 0.053

0.819⊿Body fat (%)− 0.0860.710

Blood metabolic parameters⊿Glucose (mg/dL)− 0.497

0.022⊿FFA (mEq/L)− 0.224

0.329⊿Triglyceride (mg/dL)

0.009

0.970

⊿Total cholesterol (mg/dL)

0.128

0.579

⊿HDL cholesterol (mg/dL)

0.062

0.791

⊿LDL cholesterol (mg/dL)

0.109

0.639

⊿FGF21(pg/mL)− 0.065

0.780

Dietary consumption (/day)⊿Energy (kcal)

0.128

0.580

⊿Protein (g)

0.260

0.256

⊿Fat (g)

0.166

0.473

⊿Carbohydrate (g)

0.082

0.724

− 0.368

0.101⊿PA (METs-h/week)

The results of Pearson’s correlation coefficients are shown
BAT brown adipose tissue, BMI body mass index, FFA free fatty acid, FGF21
fibroblast growth factor 21, HDL high-density lipoprotein, LDL low-density
lipoprotein, MET metabolic equivalent, PA physical activity
Boldface indicates significance (p < 0.05)

BAT activity was stable within individuals throughout
the study period and was not influenced by the intervention. In addition, changes in glucose levels were only
negatively associated with BAT activity in young men.
The role in glucose disposal of activated BAT has beenreported [4–6]; thus, unlike the consumption of fructose,

Page 8 of 10changes, such as environmental and/or daily variationin BAT activity, may be associated with glucose disposal
and negative correlation in the RCT.
Body temperature is controlled by shivering thermogenesis in skeletal muscle and non-shivering thermogenesis in BAT [47]. Cold-induced shivering is limited
in neonates due to underdeveloped skeletal muscle.
As a result, human neonates have various BAT sites
including the interscapular, neck, and mediastinum
regions [47]. Although BAT has a role in acclimatization to cold exposure after adolescence, the presence
and volume of BAT decline with age [47]. The underlying factors of BAT adaptability are unclear. Previous
studies reported that FGF21-deficient mice displayed an
impaired response to cold stress [48], and the diurnal
reduction of FGF21 was suppressed by cold exposure in
healthy adults [49, 50]. Thus, FGF21 may be a mediatorof cold-induced thermogenesis in adults; however, thepresent study did not find either the suppressed FGF21
reduction by cold exposure or the association of plasma
FGF21 level with BAT activity in young men. Because
female subjects participated in these previous studies[49, 50], our findings may be partly explained by sex differences in FGF21 responsiveness and/or BAT thermogenesis in cold adaptation.
This study has several limitations. The sample size wasrelatively small. Blinding was not performed because
fructose has a sweet taste. Young healthy subjects participated; therefore, our findings may not be generalizable to other age categories and patients with metabolic
diseases. BAT activity was not assessed by positron emission tomography-computerized tomography, which is
the gold standard method for evaluating BAT [2]. In the
cross-over trial, cold-induced changes in diurnal FGF21

Fig. 4 Correlation analyses in the RCT. A Correlation of maximal BAT activity between the baseline and final measurements. B Correlation
between changes in maximal BAT activity and plasma glucose levels. Open square shows the control group, and closed circle shows the fructose
group. Results of Pearson’s correlation coefficients are shown. BAT, brown adipose tissue; RCT, randomized controlled trial

Kataoka et al. Journal of Physiological Anthropology(2024) 43:5

rhythm should be considered [49, 50]. Although dietaryinduced FGF21 secretion could not activate BAT, theeffects of longer FGF21 action, such as using an FGF21
analog, on thermogenesis were unclear. Further clinicalresearch is necessary to explore the physiological role of
FGF21 in human BAT.

Conclusions
Oral fructose load induces a temporary increase in circulating FGF21 levels; however, the increased FGF21
does not activate BAT thermogenesis in healthy youngmen. Although it tends to evaluate the increase and/or decrease levels in the hormone, the implications ofendogenous FGF21 on physiological function require
further exploration.
Abbreviations
BAT	Brown adipose tissue
ChREBP	Carbohydrate response element binding protein
FGF21	Fibroblast growth factor 21
RCT​	Randomized controlled trial
Acknowledgements
The authors thank Fumika Sano, Shiori Yoshida, and Kazuo Nagano for theirassistance in the BAT activity measurements.
Authors’ contributions
HK and HT designed this study. HK, SN, and HT interpreted the data. HK and
YM collected and analyzed the data. HT was a major contributor to writing themanuscript, and SN substantively revised it. HT supervised the entire study.
The authors read and approved the final manuscript.
Funding
This work was supported by JSPS KAKENHI Grant Numbers 20K19680 and
23K10849.
Availability of data and materials
The datasets used and/or analyzed during the present study are available fromthe corresponding author on reasonable request.

Declarations
Ethics approval and consent to participate
All participants provided written informed consent before enrolment in thepresent study, which was approved by the Ethical Committee of Kyoto Prefectural University (Approval number 216).
Consent for publication
Not applicableCompeting interests
The authors declare that they have no competing interests.
Author details
1
Division of Applied Life Sciences, Graduate School of Life and Environmental
Sciences, Kyoto Prefectural University, Kyoto, Japan. 2 Clinical Research Institute,
Division of Preventive Medicine, National Hospital Organization Kyoto Medical
Center, Kyoto, Japan. 3 Dorothy M. Davis Heart and Lung Research Institute,
Department of Physiology and Cell Biology, The Ohio State University Wexner
Medical Center, Columbus, OH, USA.
Received: 29 August 2023 Accepted: 22 December 2023

Page 9 of 10

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