Olfactory influences on appetite and satiety in humans

Marina Bernabeu Cantó. Master´s student of Biotechnology and Bioengineering of the University Miguel Hernandez of Elche, Course 2015-2016.

 

Reference: Yeomans MR. Olfactory influences on appetite and satiety in humans. Physiolgy and Behavior 2006, 89: 10-14.

 

Flavour arises from the combination of sensorial signals such as smell, taste or appearance. It is difficult to discern the level of influence each sense has on the flavour, although olfactory stimulation has been shown to have an important influence. For example, individuals with anosmia (loss of olfaction) have a distorted perception of the flavours. Also, it must be mentioned that there is no evidence concerning the innate preferences or aversions for any food-related smell.

 

The olfactory system generates very complex signals, since each olfactory molecule binds to several different receptors. How the brain interprets these signals is largely unknown, although several reports suggest that each smell is identified by a specific pattern of stimulated receptors. This seems to indicate that the lack of innate preferences for smells could be a consequence of such a complex olfactory system.

 

Considering this lack of innate preferences or aversions, and knowing that smell is very important for flavour, it could be considered that the majority of food preferences may be acquired. Also, the evidence shows that food-related smells have a direct impact on appetite stimulation, as well as other responses including insulin or gastric acid secretion. One study observed that smell could better predict the ingestion of a food product than its taste, indicating that olfactory perception could have a great impact on short-term ingestion.

 

The global increase of obesity is generally attributed to the availability of high-caloric foods that possess high palatability. However, since olfactory perception represents an important component of flavour perception, palatability is very influenced by the smell during ingestion. Therefore, understanding the role of smell in appetite is very important.

 

On the other hand, understanding how the smell preferences are acquired is a key point. In a previous study, smells were paired with sweet or sour flavours. Specifically, new smells were combined with sucrose or citric acid, and later with only the smell. The results showed that new smells acquired the sensitive qualities of the flavour it was presented, perceiving them as sweet or sour depending on its combination.

Also, other studies have shown that the flavour can be modulated by the physiological or nutritional state of the consumer. For example, sweetness is a reflection of energy necessity. The hedonic evaluation of a food has evolved as an efficient method to direct the attention of the stimulus, in order to obtain the necessary nutritional requirements. Several studies have confirmed that food preferences depend on the nutritional necessities, although in these works the flavours studied were very complex and the authors were not capable of discerning if the flavour as a whole or certain components of it were responsible for the effect.

 

Other studies have focused on analysing food preferences associated with flavour based on the consequences of ingestion. However, not all flavour preferences are acquired in this manner. The association of a flavour with another has also been studied. In this case, when a new flavour is associated with a previously known one, the consumer´s preference for the new flavour will be conditioned by his/hers preference for the old one.

 

The recent discovery that pairing new smells with sweet flavours allows the acquirement of olfactory preferences allows an opportunity to investigate how these acquired preferences are related to hunger. A previous study concluded that the acquired taste for a food by associating it with sweet smells while hungry was non-existent when this was performed while the consumer was satiated. This indicates that the short-term expression of a flavour is a reflection of the physiological necessities of that moment.

 

Specific sensorial satiety is referred to the reduction of the sense of pleasure during the consumption of a food with respect to another, and this hedonic change can contribute to the decision to finish a meal. It has been shown that this not only occurs in response to a flavour, but also to smells that contribute to the final flavour of the meal. Therefore, smell plays an important role in satiety as well as in appetite stimulation.

 

Considering all the previously mentioned points, it can be concluded that the smells play an important role in flavour perception, although how smell is perceived can vary by its association with certain tastes (sweet, sour…). Also, our understanding of the neuronal basis of this smell-taste integration is increasing. Although smells are only part of the sense of flavour of a food, it allows to modulate in short-term the appetite and prepare the individual for food consumption.