Regular exercisers have long sworn that working out is a natural appetite suppressant. But the relationship between exercise and appetite has been difficult to pin down, particularly with regard to the hormones that produce the sensation of hunger. Now, a new study suggests that exercise does change levels of appetite-related hormones—but not all exercise is equal. In fact, cardiovascular exercise and weight-training will have different implications for the grumble in your stomach.
The study, conducted by researchers from the British universities of Sheffield Hallam and Loughborough, sought to sort out the effects of aerobic and resistance exercise on appetite. In particular, the study's authors write, "Resistance (muscle strengthening) exercise is a key component of exercise recommendations for weight control yet very little is known about the effects of resistance exercise on appetite." Partly, this is because appetite is very difficult to measure. But what can be measured are the hormones that produce and suppress appetite—and only recently have those been conclusively identified. The hormone ghrelin, discovered only 10 years ago, is (in addition to being a growth hormone) a known appetite stimulant; on the other hand, peptide YY, discovered within the last couple of decades, is an appetite suppressant. By combining measurements of the levels of these hormones before, during, and after exercise with subjects' reports of their experience of hunger, researchers hoped to identify the form of exercise that most impacts each hormone.
Eleven male university students participated in the study for three eight-hour sessions each. One session had the participant run for 60 minutes on a treadmill, followed by seven hours of rest. Another session involved 90 minutes of resistance training—weight-lifting—followed by six hours and 30 minutes of rest. And in a third session the participants did not exercise at all. In each session, the participants filled out surveys about their experiences of appetite, ate a couple of meals, and had their hormones checked repeatedly. The result was a marked change in hormone levels, associated with different types of exercise.
Weight-lifting gave some appetite benefits. The resistance training session caused the participants' ghrelin levels to go down, meaning that they generated less hunger; but their peptide YY levels remained unchanged, indicating that appetite was not suppressed, even if it was reduced. Aerobic exercise, however, was really a winner. The treadmill sessions produced both a drop in ghrelin and a rise in peptide YY, indicating both less appetite and suppression of that appetite. The surveys supported this conclusion: while both weight-lifting and aerobic exercise produced reports of decreased appetite, this effect was stronger after the treadmill sessions. "The finding that hunger is suppressed during and immediately after vigorous treadmill running is consistent with previous studies indicating that strenuous aerobic exercise transiently suppresses appetite," lead author David Stensel of Loughborough University said. "The findings suggest a similar, although slightly attenuated response, for weight lifting exercise."
It's important to know that the effect of exercise on these hormones is short-lived: the researchers noted that the changes only lasted about two hours, including the time spent exercising, for both types of workouts. And, the next step is to work out whether the appetite suppression induced by exercise actually leads to less eating.
RealJock members have the chance to participate in a real-world test of just that question. Many times on our site, San Francisco nutritionist Manuel Villacorta, MS, RD, CSSD, founder of the interactive weight-management web site Nutrition for You, and creator of our Weight Loss Challenge, has warned of the dangers of letting ghrelin levels get too high. Not only does this lead to increased appetite and over-eating, he tells us, but the food eaten at high ghrelin levels is more likely to be processed as fat. Getting ghrelin under control is key—and can partly be accomplished by eating often and regularly, as the hormone builds up between meals.
Exercise, however, can also play a role. This January, Villacorta and the Nutrition for You team will run a second RealJock/Nu4You Weight Loss Challenge, this time in tandem with Billy Polson and Mike Clausen of Diakadi Body. For the first time, this team of experts will put together a combined nutrition and workout program designed to put our participants' bodies on a whole new track. Our last group of Weight Loss Challenge participants lost over 300 pounds—and are still going! If you want to be one of the 30 lucky guys to go on this journey in 2009, keep your eyes on RealJock for how to sign up. And either way, get in that gym—it does more than just burn calories.