When we exercise, our bodies become hot and we start to feel flushed. But something else happens: our appetites decrease after the workout. Researchers set out to explore exactly why and how this happens.
For a long time, I lived with the conviction that the more physically active I was, the more my appetite would increase. Makes sense, right?
Surely, I would think, the body will call for a replacement of all the calories burnt while jogging or dancing.
But my appetite never increased, and all of my expectations had been entirely wrong.
Studies have now shown that aerobic exercise — such as running, cycling, and swimming — actually decreases appetite by changing the levels of hormones that drive our state of hunger.
However, the underlying biological mechanisms that are therefore set in motion, and which tell our bodies to secrete fewer of the hormones that drive hunger, have remained uncertain.
But recently, one researcher decided to take steps toward understanding what goes on in the body after a decent workout.
Young-Hwan Jo, of the Albert Einstein College of Medicine in the Bronx, NY, was intrigued by how his regular 45-minute runs always left him craving less food than usual.
He believed that the fact that body heat goes up during exercise may play a role in signaling to the brain that appetite needs to go down. He thought the process might be similar to what happens in the body when we eat very spicy foods.
Heat sensations lower appetite
When we eat foods that contain hot chili peppers, our body temperature seems to go up, and our appetite decreases. That is because chili peppers contain a compound called "capsaicin," which interacts with sensory receptors (TRPV1 receptors) in the body, bringing about the sensation of being hot and flushed.
Capsaicin has also been shown to create a decrease in appetite, which has made this compound a target of research for weight loss treatments.
Following this train of thought, Jo wondered whether the increased body heat felt after exercising might not stimulate neurons in brain areas responsible with homeostasis, the regulation of basic bodily processes, including eating.
"I'm a neuroscientist," says Jo, "who studies the hypothalamus — the portion of the brain that plays the central role in regulating metabolism and weight," adding:
"I wondered if certain hypothalamic neurons sense temperature increases and respond to exercise-induced warming by releasing a 'stop eating!' message."
And, sure enough, the results of the research that followed — which have now been published in the journal PLOS Biology – indicate that he was on the right track.
Neural receptors 'regulate feeding'
Jo and team decided to zoom in on a set of neurons that coordinate appetite suppression, called "proopiomelanocortin" (POMC) neurons. These cells are found in a region of the hypothalamus known as the "arcuate nucleus," and some are not screened by the brain-blood barrier.
This is a membrane that prevents most of the cells in the brain from being exposed to serious fluctuations in blood plasma composition, thus protecting neural function.
But since some POMC brain cells have more direct communication with the rest of the system and interact with hormones released into the blood, Jo thought they may also be able to respond to fluctuations in body temperature.
In order to test this hypothesis, the researchers first experimented with mouse hypothalamus tissue that contained POMC brain cells. They exposed this tissue first to capsaicin, and then to heat, to see whether these cells would be able to respond to both stimuli.
The researchers were not disappointed — both the presence of heat and that of the chili pepper compound activated POMC neurons, meaning that they had TRPV1 receptors. Two thirds of these cells responded to these two different heat stimuli, the researchers noted.
At the next stage of their research, the scientists conducted various tests using mice, in order to understand how POMC neurons reduced appetite after their TRPV1 receptors were activated.
So, Jo and colleagues saw that when they exposed the arcuate nuclei of mice to capsaicin, the animals tended to eat less food over the following 12 hours.
The scientists, however, were able to block the loss of appetite associated with capsaicin exposure either by blocking the TRPV1 receptors of POMC neurons before administering the compound, or by turning off the gene that encodes such receptors in mice.
Jo and team also put some mice on treadmills, leaving them to run for a period for 40 minutes. In this way, they created the conditions that are typical of a regular workout session.
As a result of this exercise, the animals' body temperatures initially shot up, and then they reached a plateau after 20 minutes. Body heat remained high for over an hour, and the mice's appetite visibly decreased.
The exercising rodents had an approximately 50 percent lower food intake after the treadmill session than their counterparts that had not taken part in the exercise.
And, finally, exposure to treadmill exercise had no effect on the appetite of the mice whose TRPV1 receptors had been suppressed. This suggests that heightened body heat due to physical activity stimulates relevant receptors in the brain to decrease the desire for food.
"Our study provides evidence," Jo concludes, "that body temperature can act as a biological signal that regulates feeding behavior, just like hormones and nutrients do."
He adds that this knowledge may eventually lead to improved strategies for weight loss.