David Broom, Ph.D., the lead study and senior lecturer of exercise science at Sheffield Hallam University in England, says the seemingly backwards reaction probably has to do with evolution: “Walking long distances or foraging at a low intensity and feeling hungry would give you the drive to find food. However, if you encountered danger and had to flee or fight, the last thing you want to be worrying about is your hunger.”
British researchers found that going for a run reduced both. "Blood is shunted away from the stomach when you exercise, so there is less supply of acylated ghrelin," explains Broom, senior lecturer of exercise science at Sheffield Hallam University in England.
But workout length didn’t make much of a difference: As long as it was at the same easy to moderate pace, guys who finished a 90-minute run had the same suppressed ghrelin levels as after a 45-minute run. Essentially, your ghrelin dips to a certain point and remains steady along with your pace until you stop moving, at which point your hunger hormone levels will start to rise again, Broom explains.
The interesting part, though, comes when you kick the intensity up a notch: athletes felt less hungry post-workout when they had a heart rate above 75 percent of their max compared to below it, no matter the duration. In fact, Broom adds that even a 30-second sprint proved intense enough to affect hunger hormone levels.
The bottom line:
While the study was small and exclusively on men, previous research supports exercise’s repression of ghrelin and appetite, and Broom notes that his team has shown these results would probably be the same for women. If you’re trying to curb your calorie intake, sweating at a higher intensity may help.