The weight of smells

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Does our sense of smell, involved in detecting the taste of food and in regulating hunger, play a role in metabolic pathologies such as obesity or psychiatric disorders such as anorexia? Researchers are studying this approach and exploring the therapeutic possibilities it presents. On the occasion of the World Obesity Day this Saturday, March 4, we propose you to rediscover an article by Eléonore de Bonneval and Hirac Gurden originally published in Nez, the olfactory magazine #10 – From the nose to the mouth.

Can our nose influence our choice of dessert? This is what Stéphanie Chambaron, who has a doctorate in cognitive psychology and is a research fellow at the Centre for Taste and Feeding Behaviour (CSGA) in Dijon, studied in a controlled experiment. After having called in participants under a false pretext, she exposed some of them to the smells of either fruit or pastries – aromas so faint they weren’t aware of their presence – and then observed which foods they chose from a buffet. The results were revealing. People were more naturally drawn to “a low-calorie dessert (a compote) when they were [exposed to] the smell of pear” than when no smell was diffused. On the contrary, the aroma of a chocolate croissant attracted more adults to a high-calorie dessert (a waffle). Stéphanie Chambaron insists, however, that though a smell can influence our urge to “go towards” a certain type of food, it can’t do the opposite and curb it. Furthermore, she adds, “the effect of olfactory priming is even more pronounced in people who are obese”.
In France, this metabolic disease affects over 9 million people. Related to the body’s defective management of energy, it is characterised by an excess accumulation of adipose tissue and a body mass index (BMI) that is greater than 30 kg/m2 for adults. The indicator is calculated by dividing weight (in kilogrammes) by height squared (in metres), as though we were laid out flat and the density of our surface area were measured. It isn’t easy to demonstrate the correlation between obesity and olfactory disorders. In scientific literature, ten or so articles address the subject, but their results are contradictory. One of the studies shows no correlation, while another concludes there is a hypersensitivity to the aromas of food. According to the latter, authored by Lorenzo Stafford of the Centre for Comparative and Evolutionary Psychology at the University of Portsmouth (UK), people whose BMI exceeds 30 kg/m2 are more sensitive to the smell of chocolate than those whose BMI is lower, and they find it more pleasing. Yet, out of all the articles, eight show a significantly lower capacity to detect odours in cases of obesity, which corresponds with data collected from studying rodents made obese by a high-calorie diet. Brynn Richardson of the University of Nebraska Medical Center (USA) has demonstrated a connection between decreased olfactory acuity and a high BMI. And in 2018, Mei Peng of the Department of Food Science at the University of Otago (New Zealand) found a lower olfactory acuity in subjects whose BMI was higher than 40 kg/m2. She noted, however, that it is reversible in the case of patients who have undergone a gastric bypass to lose weight.

The satiety and reward circuits

What could then be the correlation between olfactory sensitivity and obesity? According to some authors, obesity might be the consequence of defective perception rather than the cause of it, as Stéphanie Cambaron points out. A normally functioning sense of smell provides information to the satiety and reward circuits in our brain that enable us to assess the quality and quantity of food we need to ingest. The cognitive psychologist cites another study by Mei Peng, according to which subjects with weaker olfactory capacities are driven to consume more food to achieve an equivalent level of sensation from eating than people whose weight and sense of smell fall within the norm. This could be explained by a widespread hypothesis that is still being discussed, supported notably by Serge Ahmed of the Institute of Neurogenerative Diseases (IMN) at the University of Bordeaux, who serves as the director of research at the French National Centre for Scientific Research (CNRS). In his view, this behaviour could be related to the fact that sweet and fatty foods trigger the same reward and pleasure pathways in the brain as do drugs: the dopaminergic system.
Obesity is characterised by inflammation of the brain, which is harmful to the olfactory neurons of the mucous membrane and to the neurons of the brain’s olfactory system. As the mass of adipose tissue increases, it releases more toxic pro-inflammatory molecules, causing inflammation of the joints, liver, intestines and brain. One hypothesis would be that the neurogenesis which enables the renewal of neurons in the mucous membrane and olfactory bulb becomes weaker in response to this inflammation.

Leptin, insulin, ghrelin

At the other end of the BMI spectrum – between 14 and 17.5 kg/m2 – anorexia is classified as an eating disorder, like bulimia and hyperphagia. Eating disorders affect 1 to 2% of the world population, mostly women (eight out of ten patients), a third of whom suffer from a chronic form of the disease.
The works of Nora Rapps and her team at the Department of Psychosomatic Medicine and Psychotherapy at Tübingen University Hospital (Germany) have shown a distinct decrease in the olfactory sensitivity of anorexia sufferers for reasons that remain unknown. Just like obese subjects, this decrease seems to reverse as soon as patients regain some weight. In addition, olfactory capabilities are affected by the duration of patients’ condition: At the early stage of the disease and during pathological weight loss, hypersensitivity may be noted, before turning into hyposensitivity, which sets in very quickly. Bulimia sufferers whose BMI doesn’t change significantly don’t seem to have the problem.
Why do variations in BMI affect the brain and sense of smell? Behind weight gain or loss hides a complex hormonal machinery in charge of regulating energy metabolism, for instance, glycaemia, according to needs and food intake. This requires coordination among the brain, liver, pancreas, adipose tissue, intestines and muscles. These communicate via hormones: leptin, insulin, ghrelin. When we’re hungry, the scale tips in favour of ghrelin; our sense of smell becomes keener, which facilitates the quest for food until we feel full. Smell is one of the senses that inform the centre of satiety, which controls the synthesis of these hormones. And the balance among the three changes. But if the BMI varies pathologically, the system malfunctions. It just so happens that the cells of the olfactory bulb contain a great number of leptin, ghrelin and insulin receptors. Therefore, sudden variations in the concentration of these substances disrupt not only food intake but also the sense of smell (the sensitivity of olfactory neurons, the functioning of the olfactory bulb). This means that the changes in olfactory sensitivity observed in anorexic patients may be a consequence of hormonal imbalance.

“Progressive cognitive restriction”

In addition, anorexia sufferers engage in a real battle against the smell stimuli, observes Vincent Dodin, an associate professor of psychiatry at Lille Catholic University. Eliminating anything from their diet that seems high-calorie, they generally avoid being exposed to the smells of such dishes. “A classic with anorexics is that they restrict their panel of food smells, eating an apple and two or three vegetables, no more; the rest is gradually eliminated from their olfactory semantics, or even forgotten”, the psychiatrist summarises, speaking of “progressive cognitive restriction”. For those who are anorexic, appetising aromas represent the threat of weight gain: They are wary of them. This perception will therefore reinforce their restrictive behaviour: “There is a habituation in the struggle, cognitive distortions, treatment mechanisms of ‘odour’ information which induce a behaviour of rejection”, explains Dodin.
In his view, many people suffering from anorexia consciously alter the taste or flavour of certain foods so they can’t enjoy eating them. Adding large amounts of salt, spices, pepper, vinegar, mustard or even burning a dish before eating it are part of “a quasi-masochistic or painful process”. This type of pathological strategy overstimulates the three sensorial structures involved in savouring: smell, taste and especially the trigeminal system, which perceives ‘hot’ spices [see page 82]. Aversion is therefore triggered even before the first stage of digestion. In turn, digestion reinforces disgust, up to the point of inducing vomiting. Signals are sent to the brain to alert it to those foods so that they will no longer be consumed in the near future. The rejection of positive food smells thus becomes quasi-categorical for anorexic persons.

“Sensory awakening” workshops

Anorexics can also perceive smells as being too invasive because of their volatility, their long-lastingness and their capacity to infiltrate everything. In Judith du Pasquier’s documentary Chère anorexie, Emmanuelle Dor-Nedonsel, a child psychiatrist at the University Hospital of Nice, cites the example of a patient whose anxiety was triggered by the mere presence of flour suspended in the air: She was afraid of getting fat if she inhaled it. The young girls she treats often suffer from cognitive distortions. For instance, they feel “full after smelling fat: When they smell shortbread, it’s as though they’d eaten it.”
Conversely, in cases of bulimia or hyperphagia – other eating disorders – “smells will act as starters”, notes Vincent Dodin. Walking past a bakery wafting the aromas of croissants can trigger an attack. The psychiatrist and his team in Lille conduct “sensory awakening” workshops to help patients reappropriate food in order to regain a normal BMI. The aim is to “relearn to broaden the palette of smells even before ingesting the foods”. A dietician presents a certain number of food aromas, adding to them from one session to another, so that patients can develop the olfactory competence they have lost or insufficiently used from the start. Dodin also works on smells during therapeutic meals, an exercise he compares to a form of mindful meditation. During these meetings led by local culinary instructors or chefs, anorexic patients are encouraged to concentrate on the smell of foods, as well as on their texture, their taste, and on the negative or positive emotions triggered by these sensations. It is a time-consuming process: Cycles last six weeks, with one to three weekly sessions.

“An emotional block”

Emmanuelle Dor-Nedonsel uses a similar approach in her hospital in Nice. Initially centred around taste, the workshop soon evolved to focus on smell, after observing how hard it was for anorexic patients to put food in their mouth: “They would sniff at the offerings rather than eat them.” Sessions take place in small groups of six or seven young girls with two healthcare workers. Two smells are presented: one of a food, the other of a plant (woody or floral). Patients are then asked about their hedonic perceptions of these odours and are invited to express themselves freely, in writing, so they will not be influenced by other participants and so that the team will have a record of their individual progress. White sheets of paper, pencils and markers are put at their disposal. The medical staff try to identify, first and foremost, the emotions elicited by these scents. During the initial sessions, patients claim they can’t smell anything and hand in blank pages. Yet Dor-Nedonsel postulates that this springs from an ‘emotional block’. Cold and rigid, the young girls often seem indifferent, she notes, “though it’s more a matter of control, of containing themselves: Giving way to emotions would be too potent.”
Both psychiatrists agree that the senses, especially smell, are a pathway to emotions and that their use can reactivate mnesic traces in those suffering from eating disorders. Vincent Dodin is convinced that odour is an excellent mediator. “We’ve realised that many patients with eating disorders had suffered traumas that led them to develop defense mechanisms to keep from remembering them. Many have problems accurately remembering early events in their lives because there’s a sort of haunting, of anxiety.” Smells have a direct connection to what is known as autobiographical memory; they can unearth painful memories, “repressed to make sure [patients] won’t be overwhelmed by emotions they can’t control”, and help to translate these emotions into words.
Could they also be used in the fight against obesity? The disease, which affects more than 17% of French adults and 13% of the world population, was declared a global epidemic by the World Health Organisation in 2017. The possibility of using olfactory therapy for people affected by the disease is presently being studied. The aim would be for them to learn – or relearn – to smell and taste the different flavours in their food individually, to remedy the sensory and cognitive losses associated with their pathology.
Research to better understand the connection between olfaction and obesity or BMI is just beginning. Smell is unquestionably one of the avenues that might lead to the more effective treatment of eating disorders, helping sufferers find food pleasurable again. It would also be a way for them to regain a balanced weight but, above all, to embrace their feelings and their body again.

Main visual: Edouard Manet, La Brioche, 1870. Source: Wikipedia


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