If You Want to Eat Less, Work Out in the Heat

Trying to curb your appetite? According to a new study, you might want to try exercising outdoors, under a hot sun. The research, published in the American Journal of Clinical Nutrition, reports that people who worked out in a warmer climate ate less afterwards than people who exercised in an air−conditioned room.

Kym Guelfi and her team at the University of Western Australia's School of Sports Science had 11 fit young men make three trips to the school’s exercise lab. During one visit, they ran on a treadmill in 97° temperatures; in the second, they ran under a more temperate 77°; and on the third visit, they simply rested in a moderate climate. After all sessions, the men were offered an all−you−can−eat buffet, and the researchers monitored how many calories the men consumed.

After working out in the hot climate, the men had higher circulating levels of peptide YY, suggesting that the body was registering "fullness" sooner than it did after exercising in the moderate climate.

The team found that the men ate the most when they exercised under moderate conditions – they consumed 300 calories more, in fact, than they did under hotter conditions. Why might this be? The researchers took blood samples from the participants and measured a hormone called peptide YY, which acts a fullness signal in the body: rising levels of the compound essentially tell one when to stop eating. After working out in the hot climate, the men had higher circulating levels of peptide YY, suggesting that the body was registering “fullness” sooner than it did after exercising in the moderate climate.

Guelfi also says that changes in the core body temperature may also be at play: since the acts of eating and digestion put off heat, reducing food intake may be a natural mechanism to help the body cool down. “This is why, anecdotally, cold environments have been said to lead to an increase in appetite," she said.

However, she cautions that working out under very hot conditions also has obvious risks and should not be overdone: "Exercise should not be performed in overly hot environments due to the risk of dehydration and heat illness,” she warns.