Does the nose know? Physiological and behavioural responses upon food odours

Imagine yourself walking into your preferred supermarket or in the main street of your home town. Suddenly, a smell of freshly baked chocolate cake interrupt your thoughts, your mouth starts watering and you start craving some sweet, high-caloric food. Through repeated exposure, which leads to learned conditioning, food odours may act as conditioned stimulus that transmit specific information about the food and produce physiological and behavioural responses in anticipation of consumption. Exposure to food cues increases the appetite for food products with similar taste and energy-density characteristics: sensory-specific appetite. This infers that food odour cues may transmit vital information associated to the macronutrient content of the food and consequently induce specific responses such as salivation, appetite and even food intake. However, there is still uncertainty how, and under what circumstances, food odours may specifically impact physiological and behavioural responses. This thesis was geared towards a better understanding on the role of food odours and its impact on those responses is crucial to improve eating patterns towards the healthier option.

Two main questions were addressed:

1. To what extent can food odours (and other sensory food cues) trigger specific cephalic-phase salivary responses? (Chapter 2 and 3).
2. How does the level of awareness of food odours influence specific eating behaviour responses (appetite, preference, choice, and intake)? (Chapter 4 - 6).

In Chapter 2 we investigated the role of odours on saliva secretion and composition in two studies: study 1 involved odours that signal taste qualities (sweet, savoury and sour) and study 2 involved odours that signal macronutrient content (high in carbohydrate, protein, fat and low-calorie). Our results showed that food odour exposure significantly increased saliva secretion rate compared to non-food and non-odour conditions. Saliva secretion rate was similar across odours that signal different taste qualities (study 1) and specific macronutrient content (study 2). However, salivary composition remained stable across odour and control conditions. This demonstrated that food odours play a role in saliva secretion as part of physiological response in anticipation of food. Nevertheless, the use of more sensory modalities could be necessary to impact salivary composition. Therefore, a follow-up study, was conducted to investigate the role of (multi)sensory cues and the type of stimuli on saliva secretion and its composition (α-amylase concentration and secretion rate, pH level, buffering capacity, MUC5B concentration, and total protein content) (Chapter 3). We systematically varied levels of sensory stimulation (odour exposure, + vision, + taste, + mastication) and macronutrient content of the stimuli (bread, high-in-starch; cucumber, low-in-starch; and parafilm as control). Our results showed that saliva secretion rate increased with increasing levels of sensory stimulation. In general, bread secreted the highest amount of salivation compared to parafilm.α-amylase secretion rate increased upon the exposure of the highest level of stimulation (odour + vision + taste + mastication) compared to the lowest levels (odour and odour + vision stimulation). Other salivary characteristics differed with the level of sensory stimulation, which might be related to the total volume of salivation. Importantly, the nutritional content of the stimuli did not influence any salivary characteristics. Cumulative sensory information, mainly taste and mastication, may play a crucial role in anticipatory salivary responses.

Chapter 4 and 5 were conducted to understand the role of odours that signal specific macronutrient content on eating behaviour responses and to disentangle the influence of the level of awareness. In Chapter 4, participants were consciously exposed, while in Chapter 5 were non-consciously exposed to odours in a cross-over design. In each test session, participants were (non-)consciously exposed to one odour and assessed their congruent appetite, food preferences and intake (by an ad libitum lunch consisting of various macronutrients). Conscious exposure increased appetite for congruent food products. However, this effect was mainly driven by the protein-related odours. Nevertheless, food intake was similar across the different odour exposures.  On the other hand, non-conscious exposure did not impact specific appetite, and, although contrary to our hypothesis, also did not influence food preference nor food intake. Our results suggest that the exposure to conscious odours mainly influences sensory-specific appetite. We hypothesized that intake of a main course - salad during lunch in this case - might not be prone to modification by non-conscious odour exposure as it is part of our habitual dietary patterns. However, non-conscious odour exposure could influence food choice of rewarding foods that are more susceptible to be steered by external cues. Therefore, we conducted a final study where we investigated the role of non-conscious exposure of sweet and savoury odours on snack choice, as well as visual attention by means of eye-tracking (Chapter 6). Our results showed that sweet snacks were mainly chosen regardless of the type of odour exposure. However, congruent snacks were fixated upon first, suggesting that non-conscious exposure might influence the initial orientation. Furthermore, similar to the snack choice, sweet snacks were more frequently and longer fixated upon compared to savoury snacks, regardless of the type of odour exposure.

In general, the type of odours, time and intensity of the exposed odour, and different measures to investigate the same outcome are crucial to understand the role of (non-) conscious odour exposure and its influence on eating responses.

In summary, food odours play a key role in anticipatory responses such as saliva secretion rate, initial orientation, and appetite. However, their effect under laboratory circumstances might be too small and therefore difficult to grasp. Physiological and behavioural responses are not specifically influenced by the macronutrient content of the food cues. The nutrient information that is signalled by the food cues might be disrupted by the complex mix of macronutrient available in our current food environment. Further research should focus on a larger sample of odours consisting of different mixtures of macronutrients and on the role of multisensory cues to fully understand the role of odours on physiological and behavioural responses.  More research is needed to better understand the potential impact of odours to steer people’s healthier food choices.