So for the new study, which was published recently in the Journal of Endocrinology, scientists from Loughborough University in Britain and other institutions who have been studying exercise and appetite for years recruited 16 healthy, fit young men. (They did not include women because this was a small, pilot study, the authors say, and controlling for the effects of women’s menstrual cycles would have been difficult.)
They separated the men into two groups, each of which would concentrate on one element of exercise.
The first group focused on intensity. To accomplish this, the scientists had the men visit the university’s performance lab on three separate occasions. During one, they sat quietly for several hours. During another, they ran on a treadmill at an easy jog, with their heart rates hovering at about 50 percent of their maximum capacity, for 55 minutes, until they had burned about 600 calories. On the final visit, they ran at a much more vigorous pace, around 75 percent of heart rate capacity, for 36 minutes, until they had again burned about 600 calories.
Throughout their workouts and for an additional few hours, the scientists drew blood to check for levels of a particular hormone, acylated ghrelin, that is thought to influence appetite. Generally, when acylated ghrelin levels rise, so does hunger. They also asked the men how hungry they felt.
Meanwhile, the scientists performed the same tasks with the second group of volunteers. But these men’s workouts emphasized length. So, one day they ran for 45 minutes at a steady pace and on another, strode at the same pace, but for 90 minutes. During a final visit, they sat.
Then the scientists compared numbers. In general, exercise had lowered the men’s levels of acylated ghrelin, compared to when they had sat continuously. The effects were especially pronounced when the exercise had been intense or long. Vigorous running had blunted acylated ghrelin production more than gentler jogging and longer runs more than briefer ones. The effects also had lingered longest when the exercise had been most protracted. More than an hour after their 90-minute run, most of the men’s acylated ghrelin levels remained suppressed.
Interestingly, the men’s subjective feelings of hunger had also been affected, but not in precisely the same fashion. After the 90-minute run, the men reported feeling less hungry than when they had sat around the lab, even an hour and a half later. But after the short, intense workout, the volunteers soon felt peckish, despite still having low levels of acylated ghrelin in their blood.
Over all, these findings reveal that our appetites certainly are strange, influenced by many factors besides exercise and acylated ghrelin levels. But the results also intimate that if we hope to have workouts reduce our appetite, we may wish to increase the intensity or, even more, the duration of each session.
Of course, this study was small and looked only at young men in good shape, says David Stensel, a professor of exercise metabolism at Loughborough University who oversaw the experiments. It also did not follow them to see whether, in the hours after their workouts, they replaced the calories they had burned.
In the future, the researchers hope to mount longer-term studies that include women, as well as older, sedentary and overweight people, to better understand how different types of exercise influence each group’s hormones and hunger and to tease out the many physiological mechanisms involved.
But in the meantime, Dr. Stensel points out, we should exercise, whether or not the activity makes us thin. “There are so many other reasons, irrespective of the effects on appetite, why exercise benefits health,” he says.