Obesity is a significant risk factor for cardiovascular disease, diabetes, and cancer. Although obesity can stem from behavioural factors such as diet and physical inactivity, central nervous system changes can also mediate appetitive behaviour and restraint. As the understanding of hypothalamic and hindbrain regulation of food intake has rapidly expanded, less is understood about how gut and adipose tissue hormonal signaling integrates with environmental cues, memory, motivation, and appetitive behaviour.
Food aromas can be powerful appetitive cues that are known sensory elements in the ‘tasting’ of food, as well as the texture and look. Researchers have shown that odour-based cues are key determinants of actual intake that can facilitate overeating. Whilst it is known that palatable food aromas, arguably more naturalistic Pavlovian-conditioned stimuli than food images, help drive consumption, small inroads have been made to neuroimage and map this crucial circuitry in the brain. Now, a study from researchers at Hospital Infantil de México Federico Gómez has shown that the area of the brain associated with impulsivity and the development of obsessive-compulsive disorder is activated in obese children when introduced to food smells. The team state that in order to fight obesity, it is crucial to understand the brain mechanisms of odour stimulus. The study was presented at the annual meeting of the Radiological Society of North America (RSNA).
Previous studies show that studies of differential brain responses to food cues between normal-weight and obese subjects are nevertheless largely limited to visual images of food. While visual images have proven useful, a food’s aroma could be considered as a more visceral conditioned stimulus that is more suggestive of a food’s immediate presence. So far, feasibility of using naturalistic food odours in brain imaging studies of obesity, with robust activation effects compared to nonfood odours in a well-documented medial frontal/mesolimbic reward network. Trait impulsivity is implicated in developing and maintaining obesity and may also work against internal satiety and satiation signals. The specific form of impulsivity known as negative urgency (NU) reflects a tendency to act rashly as a consequence of strong negative affect. A similar eating-specific behavioural trait, eating provoked by negative or depressive-based emotions, has also been associated with obesity. With so many unmapped traits, emotion-led problems, as well as, addiction-based ventral frontal responses being implicated in obesity; it is now urgent that these traits, depression-based problems and addictive risk-factors are mapped via neuroimaging to uncover large unknown parts of the obesity-disorder.
The current study neuroimaged 30 children between the ages of 6 and 10 years old. Half of the children had a normal body mass index (BMI) between 19 and 24, and the other half exhibited a BMI over 30, which is classified as obese. Each child was presented with three odour samples, chocolate, onion and a neutral odour of diluted acetone. As the participants smelled the samples, two MRI techniques, functional MRI (fMRI) and functional connectivity MRI (fcMRI), were used to measure brain activity.
An evaluation of the fMRI results showed that in the obese children, the food odours triggered activation in the areas of the brain associated with impulse and the development of obsessive-compulsive disorder, while the areas of the brain associated with impulse control exhibited no activity. In contrast, data findings show in the children with a normal BMI, the areas of the brain associated with pleasure regulation, organization and planning, as well as regions governing emotional processing or memory function, became more active.
In addition, the fcMRI results showed that when the normal-weight children smelled the onion, there was a connection between the gustatory cortex, which processes taste, and the area of the brain linked to reward anticipation, this connection did not occur in the obese children. Data findings show that the chocolate smell elicited significant brain connections in obese children, compared to the normal-weight children.
The team surmise that their findings provide the global medical community with a better understanding of the neurological disorder component of obesity, with potential to affect treatment for obese patients. For the future, the researchers state that if they can identify the mechanisms that cause obesity, they will be able to change treatment paths, and in turn, reduce obesity prevalence and save lives.