The appetizing and satiating effects of odours

Background and aim Unhealthy eating habits such as unhealthy food choices or overeating increase the prevalence of obesity, diabetes, cancer, cardiovascular and other diseases. Therefore, it is important to understand how separate factors, such as sensory processes, influence our eating behaviour. As one of the sensory modalities, olfaction has a relationship with food intake regulation. Previous research reveals that food odours can induce both appetite and satiation. In this thesis, we split appetite and satiation into a ‘general’ part and a ‘food specific’ part. General appetite and general satiation refer to the desire to eat in general. General satiation measured by subjective ratings (e.g. by using line scales) is also named ‘subjective satiation’. The specific part refers to the desire to eat a specific food: e.g. the appetite for a banana or the appetite for tomato soup. The main objective of this thesis was to investigate under which circumstances odours are appetizing or satiating in order to identify factors that influence our eating behaviour.Odours arrive at the odour receptors via two routes: the orthonasal route via the nose to perceive the outside world or retronasally via the mouth to ‘taste’ the food. The appetizing and satiating effects of ortho- and retronasally smelled odours were investigated by varying the odour exposure time , the odour concentration (retronasal only), the odour type , passive versus active sniffing (orthonasal only) and by switching between odour types . Methods We conducted six within-subject experiments. All participants were healthy normal-weight women (age 18-45 y and BMI 18.5-26 kg/m 2 ). In four experiments (studies 2A, 2B, 3A and 3B), we investigated the appetizing and satiating effects of orthonasal odours, with two experiments addressing odours that were smelled passively in rooms with ambient odours (chapter 2) and two addressing actively smelled odours by sniffing the contents of a cup (chapter 3). In studies 2A (passive, n=21), 2B (passive, n=13) and 3A (active, n=61), we investigated the effects of exposure time and odour type on appetite, the appetite for specific foods, food preference and food intake. Differences between passive and active exposure were investigated by comparing the data from 2A and 3A . In the fourth experiment (n=30) using a similar set-up, sweet and savoury odours were presented directly after each other, to explore the effects of daily encounters with a variety of food odours (i.e. switching ). In all orthonasal studies, general appetite and the appetite for specific foods were monitored over time, using visual analogue scales. General appetite comprised hunger and desire-to-eat ratings. The appetite for specific products addressed the appetite for smelled products and the appetites for a set of other products that were congruent and incongruent with the odour (studies 2A, 2B, 3A and 3B). Food preference was assessed using a computerised program offering pairs of food pictures (studies 2A, 2B and 3B). Furthermore, two experiments addressed the satiating effects of retronasal odours while consuming tomato soup ad libitum (studies 4A and 4B). The retronasal odour exposure was disconnected from the soup base consumptionby use of a retronasal tube that was connected to an olfactometer. The odours were delivered directly into the nasal cavity at the moment a sip of soup base was swallowed. In study 4A (n=38), the satiating effects of odour exposure time (3 and 18 s) and odour concentration (5x difference) were investigated. In study 4B(n=42),we investigated whether addition of cream odour to tomato soup, in combination with a low or high viscosity, affected satiation. Hunger and appetite ratings were monitored over time during odour exposure, by using 100 mm visual analogue scales (VAS). Results The results showed that orthonasal exposure to food odours influenced the appetite for specific foods via a typical pattern : the appetite ratings for the smelled foods increased by +6-20 mm (SSA; all P +5 mm and the appetite for incongruent sweet and savoury foods decreased by -5 mm (all P 15 mm; P 4 mm ; P=0.01; study 2A). Actively smelled food odours had no significant effect on general appetite or food intake (studies 3A and 3B). Non-food odours appeared to suppress general appetite slightly ( -2 mm, P=0.01). The appetizing effects did not change over time during a twenty-minute odour exposure (studies 2A, 2B, 3A and 3B) and the typical pattern of odour effects on the appetite for specific foods was not affected by switching between sweet and savoury odours (study 3B). The pleasantness of the odour decreased by - 4 mm during active smelling (P=0.005), whereas the appetite for the smelled food remained high (P retronasal studies showed that an increase in both retronasal odour exposure time and concentration reduced ad libitum intake by 9 % (i.e. 3 sips and 22 kJ; P=0.04) and had no effect on subjective satiation (study 4A). Adding cream odour decreased subjective satiation with circa 5 % between 7 and 13 minutes after the start of consumption (P=0.009), but did not affect ad libitumintake (study 4B). Retronasally smelled odour significantly contributed to the development of sensory-specific satiety (study 4A). Conclusions Orthonasally smelled odours affect to a larger extend what you eat, than how much you eat. They influence the appetite for specific foods via a typical pattern: the appetite for the smelled foods and for congruent sweet or savoury foods increases, whereas the appetite for incongruent sweet or savoury foods decreases. This typical pattern is independent of exposure time, passive or active smelling and switching between odours. The reason for this pattern is unknown, however, it may be caused by the preparation of the body for the intake of the smelled food, as food odours may provide information about the nutrient composition of their associated foods. Furthermore, passive odour exposure may enhance general appetite (how much), whereas active smelling appears to have no effect. Interestingly,the appetite for the smelled foods remained elevated during the 20-minute smelling, althoughthe pleasantness of the smelled odour decreased a little over time. This shows an earlier assumption from literature incorrect: a decrease in pleasantness of the odour does not lead to less appetite for the smelled food. This seeming contradiction may result from different mechanisms, such as a decrease in hedonic value during prolonged sensory stimulation on the one hand and anticipation of food intake on the other hand. Furthermore, food odours were found to change preference in circa 20% of the cases. Probably, food odours shift food preference, but do not overrule strong initial preferences in circa 80% of the cases. Moreover, retronasally smelled odours probably have a small influence on satiation, though the evidence is not very strong. An increase in both retronasal odour concentration and odour exposure time may enhance satiation. Adding cream odour may temporarily affect subjective satiation but does not affect food intake. However, the satiating effects that were found in these studies with retronasal odour exposure were borderline significant and data on food intake and subjective appetite ratings were not consistent, which probably reflects thesmall effect size. Orthonasal odours influence food preference and could potentially be used to encourage healthy eating behaviour. The studies in this thesis were conducted under controlled circumstances and the results possibly deviate from behaviour in daily life. Therefore, it is unclear how strong the influence of odours is on our eating behaviour in daily situations. Finally, we advise product developers not to focus on changing retronasal odour characteristics in order to enhance satiation of products, seen the small effects that were found in this thesis.