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By a simple food-in/energy-out model, a run on the treadmill or swim in the pool should make you want to eat more. But recent findings have suggested that exercise can actually help to slow overeating. And a new study presents evidence that the body's physiologic response to exercise can help retune the nervous system's cues and make the body feel less hungry, rather than more so.
Hunger is a complex sensation, but it is determined in part by neurons located in the hypothalamus, which send signals to the brain telling it that you're either hungry or sated. Those neurons get their message from hormones, including insulin and leptin. When the body develops a resistance to these messengers, people become more prone to overeating and weight gain. And scientists have begun to suspect that cellular inflammation might be at least partly responsible for allowing these signals to get out of whack.
Researchers behind the new work found that "physical activity reorganizes the set point of nutritional balance through anti-inflammatory signaling," they reported in their paper, which published online August 24 in PLoS Biology.
The key to the signaling seemed to be interleukin-6 (IL-6) and IL-10, which are proteins secreted by immune cells. The compound IL-6 gets released from muscles when they contract and has been found to "play a central role in the regulation of appetite, energy expenditure and body composition," the researchers noted. But just how these compounds might be acting on the nervous system's components, such as the hypothalamus, remained murky.
To further explore this association, the Brazil-based research team examined energy use in both lean and obese rats that swam or ran on a treadmill. After the exercise, both the lean and the obese rats had lower insulin levels, but the rats that had been fed to become obese went back to eating more like their lean peers. By sampling the biological profiles of some of these animals, the scientists found that the exercise had changed the obese rats' hypothalamic chemistry, which included boosting IL-6. Rats that were given an antibody to inhibit IL-6 before exercise did not show the same biochemical or feeding patterns afterward.
"These molecules were crucial for increasing the sensitivity of the most important hormones, insulin and leptin, which control appetite," José Carvalheira, of the Department of Internal Medicine at the State University of Campinas in São Paulo and coauthor of the new study, said in a prepared statement.
Although the intense bursts of exercise seemed to spur these noticeable shifts in chemical profiles, in this study the activity only reduced the food intake in rats that were already obese, and the activity did not seem to directly relate to immediately apparent weight loss. But this chemical change alone suggests that physical activity "could help to reorganize the set point of nutritional balance and, therefore, aid in counteracting the energy imbalance induced by overnutrition-related obesity," Carvalheira and his colleagues noted in the study.