Obesity causes a lot of medical issues, some of which are life threatening, but now they may even be affecting our five senses. Researchers from Florida State University have opened up the doors to new possibilities in obesity research by discovering that obese mice developed a loss of smell. Their study was published in The Journal of Neuroscience.

"It was surprising to us that mice that were genetically obese were not perturbed in terms of anatomical loss but that those that consumed fat in the diet had anatomical and sensory behavioral losses," Nicolas Thiebaud and co-author Debra Fadool, of the Department of Biological Science and Program in Neuroscience and the Institute of Molecular Biophysics at Florida State, told Medical News Today.

Researchers said there isn’t a lot of evidence regarding how obesity affects the body’s sensory system, which includes taste, touch, sight, hearing, and of course, smell. Considering that 34.9 percent of Americans weigh in as clinically obese, according to the Centers for Disease Control and Prevention, almost a third of the country could be experiencing sensory loss, without ever recognizing its true cause: excessive weight gain. 

"Because olfaction (sense of smell) is linked with ingestive behavior to guide food choice, its potential dysfunction during obesity could evoke a positive feedback loop to perpetuate poor ingestive behaviors," Thiebaud and Fadool said.

For the study, researchers fed mice a high-fat diet every day over a period of six months, which made them obese; while another group of mice were fed standard balanced meals. Mice that were fed the high-fat diet only had 50 percent of the brain cells they needed to encode odor signals, the researchers found. They were also slower at making associations between odor and reward, when compared to healthy normal-weight mice.

Even when researchers switched the diets of the obese mice from high-fat to standard, balanced meals, they lost weight and showed normal blood chemistry, but still had a reduced sense of smell.

“We do not exclude that a high-fat diet also disturbs the function of other brain areas involved in processing the olfactory signals. Obesity also causes high levels of glucose and insulin, and we know that ion channels in the olfactory system act as sensors to these internal chemical cues. Obesity may disrupt the normal homeostatic sensors," the researchers said.

This isn’t the first time this connection's been hypothesized among the medical community. Last year, the Harvard Nurses’ Health Study surveyed 68,000 women and found that those who were obese also developed hearing loss at the age they began the excessive weight gain. However, it wasn’t until now that researchers have performed a study on animals to support the link.

Obesity wreaks unpredictable havoc on the human body, and now that it has been found that mice lose half of the brain function that controls their sense of smell, many researchers want to move forward with human models. They're interested in seeing if the same problems occur in our own biological systems, as well as whether or not obesity affects other senses. Obesity-related conditions that we’re currently aware of include heart disease, stroke, type 2 diabetes, and certain types of cancers.

"There are numerous epidemiological factors that affect human health that complicate scientifically controlled experiments, and that is why we utilize rodent models," researchers said. "However, we would not be surprised if similar principles exist in humans. There are many similarities between the structure and molecular functioning of the olfactory system between the two species."

Source: Thiebaud N, Fadool D, Bell G, et al. Hyperlipidemic Diet Causes Loss of Olfactory Sensory Neurons, Reduces Olfactory Discrimination, and Disrupts Odor-Reversal Learning. The Journal of Neuroscience. 2014.