Exercising in the heat may help you eat less

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Exercisers who are trying to cut calories might want to take a run in the sun instead of a climate-controlled gym, a small study suggests.

The study, of 11 physically active men, found that participants ate less immediately after working out in hot conditions—about 97 degrees Fahrenheit—than in a more moderate, 77-degree environment.

On average, the men ate roughly 300 calories more when they worked out in moderate temperatures than when they rested in those same conditions. In contrast, when they exercised in the heat, they subsequently downed about the same amount of calories as they did after relaxing.

"Our findings suggest that if you exercise in a warmer environment you will eat less in the subsequent meal,” senior researcher Dr. Kym J. Guelfi, of the University of Western Australia’s School of Sports Science, told Reuters Health in an email.

In theory, she said, that would be a good strategy for exercisers trying to lose weight.

However, Guelfi added, future studies should look at whether warm-weather exercisers just make up for the smaller means with larger ones later in the day.

For the study, which is published in the American Journal of Clinical Nutrition, the researchers had 11 young, regularly active men make several visits to the exercise lab.

During one visit, the men ran on a treadmill for 40 minutes in 97- degree heat; on another visit, they performed the same workout under 77-degree conditions. On a third visit, they rested in a moderately warm room.

In all three situations, the men were presented with an all-you-can- eat breakfast buffet after exercising or relaxing.

Overall, the study found, the men downed more calories after the moderate-temperature workout versus the resting condition, while there was no significant difference when they worked out in the heat.

The researchers then calculated the men’s relative calorie intake—which accounted for calories burned during exercise—and found that the hot workout looked even better. On average, the men’s relative calorie intake was almost 200 calories less compared with the resting condition.

The study also found clues as to why exercising in the heat may dull appetite. After the hot workout, the men typically showed higher blood levels of peptide YY, a hormone produced in the digestive tract that serves a “fullness” signal.

The men’s smaller appetites also appeared to be related to the increases in core body temperature that came with the hot workouts.

The reasons for this are not clear, but Guelfi noted that some researchers believe that, because eating produces heat in the body, food intake is one of our natural mechanisms for controlling body temperature.

“This is why, anecdotally, cold environments have been said to lead to an increase in appetite,” she said.

But while a warmer environment may help curb post-exercise appetites, Guelfi cautioned against taking that idea to extremes.

“Exercise should not be performed in overly hot environments due to the risk of dehydration and heat illness,” she said.

SOURCE: American Journal of Clinical Nutrition, November 2009.

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