A new study found exercise changes how our brain reacts to images of food and internal stimuli such as hunger.
Exercise helps us lose weight by altering our energy balance; weight is lost when the energy balance is negative. Thus, the term "burning calories" refers to a negative shift in the body's energy balance. Research by a team in Colorado suggests that exercise may also affect weight profile by changing how the brain responds to food and appetite. The team first published work on the subject two years ago (The Effects of Exercise on the Neuronal Response to Food Cues, 2012. Cornier M, et al.). The Colorado-based research team then published a more in-depth study in the Summer of 2013 (Effects of exercise on resting-state default mode and salience network activity in overweight/obese adults, 2013. McFadden KL, et al.).
The study participants performed a six month exercise program consisting of treadmill-walking. Subjects underwent a behavioral assessment and cerebral MRI analysis at baseline and then following the six-month exercise intervention. Behavioral measures were taken using a variety of questionnaires and assessments. For example, participants assessed appetite using a visual analog scale to rate prospective food consumption. All measurements were taken after a overnight fast and during rest.
The MRI scans focused on two areas of the brain: the default mode network and the salience network. Studies have found that obese people have altered activity in these cerebral regions. The deault mode network is dedicated to self-relevant mentalizing and interoception. Activity in the default mode network is increased in obese individuals. The salience network is responsible for assessing relevance of external and internal stimuli and reward-driven behavior. The salience network also has increased activity in obese individuals.
The researchers found no changes in behavior, but significant changes in brain activity in the default mode network. As the figure above demonstrates, activity in the default mode network was reduced from baseline following the six-month exercise intervention. However, no changes in the salience network were observed. No differences in appetite or anticipated food consumption were measured following the six-month exercise intervention. The authors suggest that default mode network activity may serve as a more sensitive measure of changes in appetite. Furthermore, leptin concentration was found to be reduced following the exercise intervention. Leptin is an important energy-balance regulating hormone as it plays a key role in regulating appetite and metabolism.
Further analysis of the results reveals that a greater reduction in default mode network activity was associated with a greater drop in perceived hunger and bodyweight. Interestingly, although the salience network activity was unchanged at rest, the 2012 study by this group found salience network activity to be reduced in response to food stimuli in the exercised participants.
Exercise reduces resting-state default mode network activity and salience network activity in response to food stimuli. Thus, this presents a model by which exercise exerts control over excess weight gain. By reducing salience network activity in response to food stimuli, exercised persons have a tampered response to food causing them to plausibly eat less at a time. Reducing resting-state default mode network activity causes people who exercise to have less focus on their hunger causing them to eat less often.
The authors suggest that exercise may have a normalizing effect on obese individuals' default mode network and salience network activity.