We’ve all had the experience of coming down with a cold and losing our sense of smell due to congestion. Food temporarily seems like an afterthought, and something we likely attribute to just not feeling like ourselves. The sense of smell plays a large role in our lives. Olfaction, or the act of smelling, contributes up to 80% of the flavor of our food, so losing our sense of smell not only impacts taste, but also our enjoyment of food, contributes to an altered appetite, as well as feeding habits.
Researchers are working to map brain circuits to understand which pathways impact obesity. One recent study carried out by Celine Riera, PhD, an assistant professor in the Center for Neural Science and Medicine at Cedars-Sinai (and was a U.C. Berkeley postdoctoral fellow at the time of the study) uncovered a connection between olfactory neurons, which control smell, and metabolism, the body’s process of converting food into energy. Riera explains that her research “is one of the first studies that really shows if we manipulate olfactory inputs we can actually alter how the brain perceives energy balance, and how the brain regulates energy balance”. This is in addition to how smell can impact the hypothalamus, an area of the brain which controls hunger- which explains why our appetite diminishes when we experience temporary loss of smell.
The study was completed using mice- some with their olfactory neurons temporarily inhibited, and others with their normal sense of smell. The mice without the ability to smell, despite eating the same fatty diet as those with normal olfactory neurons, stayed leaner. They also burnt more calories, stayed more active, took in more oxygen, and had a higher metabolism. It didn’t stop there, the mice with the inhibited olfactory neurons only gained 10 percent more weight, at most. Meanwhile, the mice that retained their senses of smell gained “about 100 percent of their normal weight, ballooning up to 60 grams” from 25 to 30 grams. The mice with normal smell became obese and developed a sensitivity to insulin and glucose intolerance, but the group of mice without the ability to smell did not develop the same issues.
The data shows that smell doesn’t just impact the experience surrounding food, but that it also impacts how bodies metabolize food. “Weight gain isn’t purely a measure of the calories taken in; it’s also related to how those calories are perceived,” said senior author of another study, Andrew Dillin, professor of molecular and cell biology and Howard Hughes Medical Institute Investigator. “If we can validate this in humans, perhaps we can actually make a drug that doesn’t interfere with smell but still blocks that metabolic circuitry.”
Additional research supports what we already know about fragrance and how it influences different circuits in our brain and body. One study, led by author Giada Brianza from the University of Sussex, UK, claims that the smell or fragrance can influence the images we have in our mind related to our body. Much as scent can influence pain levels, Brianza’s uncovered further evidence “that our brain works like a computer. There are different ways to put inputs in the brain. Taste and smell are part of sensory input.” The olfactory bulb, which begins inside the nose and to the bottom of the brain is the initial step in that input. The study also indicates that whatever we feel after having any type of meal is not psychological but is scientific. It is a result of what brain reacts to our food habits,” explains Dr. Anjali Chhabria of Mind Temple, Mumbai.
The tie to COVID-19
One of the most recognizable symptoms of COVID infection is the loss of smell. While still not yet understood completely, it is believed that the SARs-COV-2 virus infects cells that support neurons in the nose, leading to smell loss. Most often, the symptom only lasts a couple of weeks. But new studies have revealed that up to 10% of people continue to experience loss of smell even six months after infection.
In a survey conducted by Virginia Commonwealth University Medical Center of 322 respondents, it was found that COVID-induced loss of smell could disrupt eating habits and lead to weight loss. With 55% of the respondents reporting loss of appetite and 37% showing signs of weight loss, experts indicated a possibility of malnourishment. This can all be attributed to the significant role that olfaction has on feeding habits, from appetite to sensory pleasure.
Data shows that the loss of smell (and taste) isn’t linked to the severity of infection, with people who had the most mild symptoms being just as likely to experience the loss of taste and smell as those with severe outcomes. For those who experienced a continuous loss of smell, many report that their ability to smell slowly improved. But for most, the progression is not consistent- some people report moments of total loss of smell, and others when they can smell fragrances as they did prior to infection.
Some also experience parosmia, or a distorted sense of smell. Parosmia can last for several months or years, researchers say. Some who have the condition have turned to smell training, which involves smelling four specific scents repeatedly over a period of time. Proponents of the method liken it to physiotherapy after a stroke. The repeated exposure to the four scents is meant to help stimulate and restore the olfactory bulb. More research is needed as results aren’t consistent, but many experts say as it has no risk, smell training may be worth the time and effort. Data also shows that for most people, parosmia is a symptom of recovery from the total loss of smell.
It’s well understood that smell isn’t a singularly functioning sense. If we can better understand how to target the aspects of the brain that control metabolism as it relates to the olfactory bulb, there is potential to use that to address obesity and its consequent health issues.