How much of a fat-burning hormone women secrete in response to exercise depends on the number of calories they burn, not on how hard they exercise, according to a new study. Results will be presented as a scientific poster Monday, June 4, at The Endocrine Society’s 89th Annual Meeting in Toronto.

The University of Michigan study showed the effects of intensity of cardiovascular exercise on stimulating the response of growth hormone, one of the hormones that help burn fat. The level of this hormone in the blood peaks immediately after exercise, according to the lead author, Katarina Borer, PhD, a professor in the university’s Department of Movement Science. Deficiencies of growth hormone have been linked to higher body fat and greater obesity around the middle.

Some people exercise with a more intense effort in an attempt to burn more calories, but the high intensity may cause them to exercise for a shorter time and thus burn fewer calories, Borer said. Her study assessed two different exercise intensities, in which exercisers worked off the same amount of calories by varying their time on a treadmill.

Borer’s group studied whether 75 minutes of high-intensity exercise would cause overweight, postmenopausal women to produce more growth hormone than when they did 150 minutes of moderate-intensity exercise on another occasion. All the women burned the same number of calories each day: 800. Moderate intensity was defined as 40 percent of a woman’s peak effort. High intensity was 80 percent of peak effort and was accomplished by increasing the speed and slope on the treadmill. Because the women were in their 50s and 60s and some were not physically fit, according to Borer, they divided each exercise bout into 10 short sessions over 2.5 hours for each type of exercise intensity.

Women who exercised at twice the intensity did not release more growth hormone into the bloodstream, the researchers found. The amount of growth hormone released was identical for both intensities of exercise, showing that calorie expenditure guided the growth hormone response, Borer said. The short-term study did not address weight loss.

“You don’t have to exercise at a high intensity,” Borer said. “You just have to exercise longer if you’re at a low or moderate intensity.”

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