Olfactory Dysfunction in Obesity and Type 2 Diabetes

The article surveys literature data on a close relationshipbetween energy balance and the sense of smell. Olfaction is oneof the main modalities for hedonic food evaluation. Odor is oneof the most important sensory signals, which predicts food qualityand plays a key role in food selection and consumption. Appetitecan be stimulated by a variety of factors, but the leading rolebelongs to olfactory cues (tasty smells) and hormone levels thatsignal hunger or satiety. Olfactory perception is subject to hormonalmodulation. In this regard, a special focus in the article is onthe modulating function of insulin. Insulin, one of the main metabolichormones, controls food intake and has an anorexigenic effect atthe level of not only the hypothalamus but also the olfactory pathway,being particularly strong in the olfactory bulb. It is the olfactorybulb where the rate of insulin transport is two to eight times higher thanin other parts of the brain, and that contains highest levels ofinsulin and insulin receptor kinase. Thus, insulin is not only ableto penetrate to the site of olfactory information processing butdoes it quickly. Insulin and its receptors localize in the olfactoryepithelium, in mature olfactory sensory neurons. Therefore, insulininfluences the primary stage of odorous molecule perception, odor detection,which occurs at the level of the olfactory epithelium. In obesityand type 2 diabetes, the sense of smell is impaired up to its completeloss, worsening the quality of life in such patients. The articlealso addresses the effectiveness of intranasal insulin administrationfor restoring olfactory function in metabolic disorders and otherdiseases.