Patients with type 2 diabetes and obesity know what these diseases can do to some senses: Damaged blood vessels can affect feeling, sight, and hearing. 

Perhaps to be expected, research is growing on the other two senses, namely smell and taste. "Obesity and type 2 diabetes are accompanied by alterations in endocrine regulation and whole-body metabolism that are tightly linked to olfaction," wrote researchers from Max Planck Institute for Metabolism Research in May. 

Medscape asked an important obesity-olfactory dysfunction researcher, M. Hirac Gurden, PhD, director, Research and Neuroscience, Centre National de la Recherche Scientifique, Université Paris Cité, to discuss his own work in this area. 

Medscape: Please discuss your 2023 paper, in which you showed how GLP-1 is integral in insulin production, and also your 2026 review, in which you discuss how those with obesity can lose their sense of smell.

Gurden: In the 2023 paper, we showed that in the (mouse) brain, GLP-1 in the olfactory bulb plays an integral role in regulating pancreatic insulin secretion. Brain-derived GLP-1 appears to be as important as intestinal GLP-1 in this regulation. 

But these mechanisms still need to be demonstrated in humans. A major challenge in human studies is that GLP-1 receptor agonists are administered exclusively through peripheral routes, making it difficult to specifically investigate the contribution of central GLP-1 signaling.

A somewhat counterintuitive study argued that GLP-1 receptor agonists may decrease olfactory and gustatory sensitivity. However, this study did not provide a direct assessment of olfactory and gustatory function, as it is a meta-analysis combining highly heterogeneous datasets, including self-reported evaluations of sensory abilities, which are known to be subject to bias, particularly in individuals with reduced olfactory or gustatory function. 

But well-controlled clinical studies have reported that GLP-1 receptor agonists increase gustatory sensitivity in female patients with obesity. Further well-designed clinical studies are required before drawing definitive conclusions.

Medscape: Do all people with type 2 diabetes experience olfactory dysfunction? 

Gurden: Like any biological mechanism, there is no absolute effect; a 100% occurrence does not exist. 

The proportion of individuals with type 2 diabetes and/or obesity affected by olfactory dysfunction varies between 20% and 50%. Cumulative deleterious effects have been observed when obesity is combined with type 2 diabetes or hypertension, suggesting that the coexistence of multiple metabolic disorders may further increase olfactory dysfunction. 

Reports of olfactory dysfunction in type 2 diabetes date back to the 1960s.

Medscape: Is the extent of smell loss inversely proportional to weight gain?

Gurden: Yes. Several reports have addressed this issue, although the scientific community does not fully agree on this point. What has been clearly demonstrated is that severe obesity is associated with anosmia, which bariatric surgery can rapidly reverse, even before significant weight loss occurs. This suggests that metabolic improvements, such as better glycemic control and restored insulin sensitivity, may play a key role in olfactory function recovery.

Medscape: Though the consequences of obesity and type 2 diabetes on olfactory system function is poorly understood, and little data exist on the olfactory system’s involvement in active energy metabolism regulation here, is there enough evidence-based information to be included in disease management guidelines? 

Gurden: Yes. Several major studies have shown that olfactory function is either reduced or biased toward highly palatable foods in individuals with obesity. Restoring an appropriate sensory connection between patients and food could represent an important strategy to help them regain control over eating behavior, including excessive eating frequency and increased food intake during each meal.

Individuals with obesity need to be able to sense, perceive, and consciously engage with what they are eating to move away from the automatic and potentially addictive eating patterns associated with eating disorders. Losing olfaction can profoundly impact their quality of life, including their nutritional status.

Medscape: Do you think PCPs, and pediatricians, can use the available information to test their patients' sense of smell? 

Gurden: We are addressing this question in collaboration with the French patient organization Anosmie.org— I am a member — which supports individuals with olfactory disorders, including children and adolescents. Also, olfactory training by itself is good for brain function.

Medscape: Can you help PCPs direct their patients back to healthy eating habits, in relation to the work you are doing?

Gurden: Yes. Stimulate your senses and you stimulate your brain, including areas involved in food intake. The simplest advice is: Smell your food whenever possible, and especially before eating. Take your time. Smell it again and again. This sensory stimulation can trigger specific metabolic responses in the body that may help prepare the organism for the processing of energy-rich foods. This is known as the cephalic phase. It is driven primarily by the brain, before the mouth and the gut are engaged through food intake.

Also, diversify your diet. Eat fresh fruits and vegetables, and reduce consumption of highly processed foods like pizzas and sugary drinks. Stimulate your body through varied sensory and nutritional experiences.