Exercise and the Brain

The results of two recent studies have provided further insight into how exercise may influence our desire to eat—and not through changes in hormones as previously thought, but through changes in the brain.  The research suggests that exercise influences appetite through altering how specific parts of the brain react to the sight of food.

The first study involved 30 physically-fit adults in their 20’s.  Researchers from California Polytechnic State University studied responses in the regions of the brain known as the ‘food-reward system’ by attaching magnetic resonance imaging (MRI) coils to the participants’ heads. The food-reward system controls whether we like and want food. In general, the more cells firing in the region, the higher the desire to eat.  The participants first either spent an hour riding a stationary bike or sat quietly for an hour before being tested. They then swapped activities for a second MRI session. After performing the activities, the participants viewed a series of images of food, varying from healthy fruit and vegetables to cheeseburgers and ice cream. There were also a few non-food images in the series.  The food-reward system consistently lit up in the participants after they had been sitting for an hour, especially when they saw the high fat or sugary images.  However, after these same participants had exercised vigorously on the bike for the hour before, their brain scans displayed much less interest in food. Portions of the food-reward system remained fairly quiet, even when viewing ice cream and burgers. The reduction in response was spread over many areas of the brain, including those that are related to liking and wanting food, as well as the motivation to seek out food. The participants indicated on questionnaires that they felt much less interested in seeking out food after the exercise than after resting for an hour.

Dr. Bob Michaelson - a bariatric surgeon and avid cyclist

The article pointed out that these results may not be typical, however, since the participants were all physically fit and in their 20’s. In fact, previous research, has found that some overweight, sedentary people respond to exercise by revving their food-reward systems, not dampening them. In a study published last year, 34 overweight people were put on a supervised exercise routine, designed to burn around 500 calories each workout. The participants were allowed to eat as they chose throughout the study.  After 12 weeks, 20 people had shed considerable weight– 11 pounds, on average. The other 14 had lost at most a pound or two. These 14 were called  ‘nonresponders’, and had the highest brain responses to food cues following exercise when the study began. After 12 weeks, they still had the highest brain responses with their food-reward systems lighting up after exercise at the sight of food.  The nonresponders actually showed more enthusiasm when viewing food at 12 weeks than at the beginning.

The study author suggests that these findings indicate that exercise may influence appetite differently based on the individual’s body as well as the length and vigor of the exercise.  For physical activity to noticeably to decrease appetite, it may be important to exercise for an hour or more and it may help to be already lean and in shape.  They encourage everyone to exercise, as even if exercise makes you ravenous, being active can have psychological effects, perhaps increasing your desire to eat a better diet and ultimately lose weight in the long term.

Dr. Bob Michaelson, expert weight loss surgeon in Washington State, spoke with us about the findings. He said: “Our brains allow us to adapt to our environment in ways that ensure not just our individual survival, but the survival of the human race. Our skin and sensory organs are obvious interfaces between our bodies and the environment: is this hot or cold; wet or dry; friend or foe?” Dr. Michaelson explained that similarly, our digestive tracts are in constant communication with the environment through what we eat and drink. “Subtle signals between these organs and the brain communicate to subconsciously direct our food choices”, he said. “It is not surprising that these recent findings demonstrate that the brain plays a central role in our food choices based on our fitness level.”

Dr. Michaelson pointed out that these studies highlight some findings we have known for years, but have yet to work out how all of the pieces of the puzzle fit together. “For instance, there are more nerve cells in the gastrointestinal tract than there are in the brain, and almost all of the neurochemicals present in the brain are also found in the GI tract,” Michaelson explained. “We also know that weight loss surgery is a tool that allows people to lose weight, in part, by appetite suppression, through communication between the brain and gastrointestinal tract. At some point, we will eventually solve the puzzle and understand exactly how the brain and GI tract interact to control hunger and weight.” Dr. Michaelson concluded that “until then, continue to exercise!”

Related Reading:

Addicted to Food – a 2011 study suggests that compulsive food eating may be driven in part by an enhanced anticipation of the rewarding properties of food, similar to how drug addicted individuals react to substance-related cues.