Study suggests intense workouts decrease appetite

Why are we so hungry after some workouts but not in the mood to eat after others?

In a study published in Nature in June, an international team of scientists suggested that the answer lies in part in the actions of a single molecule produced after exercise that reduces hunger.

Found in the blood of mice, humans and racehorses, the molecule appeared in much greater profusion after intense physical training than after light exercise sessions. This suggests that exercising intensively may be the key to controlling how much you eat afterwards.

The relationship between fitness and food is notoriously thorny. Studies have already shown that people who start exercising without also controlling their caloric intake usually lose a few pounds or nothing over time, and may even gain weight.

​There are several factors that contribute to this result, including the person’s current fitness level, their body mass, diet, gender, genetics, metabolic rate, and possibly even the time of day they exercise. Some, though not all, experiments suggest that morning workouts can burn more fat than the same workouts done later in the day.

Appetite also makes a difference. If you feel hungry in the hours following your workout, you could easily end up consuming more calories than you burned. But what makes us feel or not feel hungry after physical exercise was anyone’s guess.

Scientists have known for decades that various substances, such as the hormones leptin and ghrelin, travel to the brain and cause us to feel more or less interested in eating.

Studies show that exercise alters the levels of these substances, but diet and sleep habits do too. Some researchers have begun to wonder if there might be some kind of reaction specifically linked to exercise that influences appetite.

So scientists at Stanford University School of Medicine, Baylor College of Medicine, the University of Copenhagen and other institutions used newly developed techniques to look for molecules that appear in the blood stream in greater numbers after exercise.

They began by studying mice, putting them to run at increasing speeds on baby treadmills until they were exhausted. They took blood from the mice before and after exercise and then compared the levels of thousands of molecules present in the rodents’ blood.

One molecule stood out for having leveled up more than any other. It had been noted before in some metabolism and exercise studies, but its chemistry and biological role were unknown.

Scientists found that this new molecule — a mix of lactate and the amino acid phenylalanine — was apparently created in response to the high levels of lactate released during exercise. Scientists named the molecule lac-phe.

The researchers speculated that lac-phe might have something to do with energy balance after exercise, as the blood cells and other parts of the body that create it are largely involved in energy intake and body mass.

Perhaps it affects the appetite, they speculated. To find out the answer, they gave a form of lac-phe to obese mice, which normally eat with gusto. But the mice’s feed intake dropped by more than 30%. Apparently, they felt less hungry with the additional lac-phe.

The researchers then turned their attention to exercise again. They bred mice that produced little or no lac-phe and ran them on treadmills until they were exhausted five times a week for several weeks on end.

After each race the animals were given as much high-fat feed as they wanted. Normally, running helps mice avoid weight gain, even when they are on a high-calorie diet. But the animals unable to produce much lac-phe bloated, eating more feed and gaining 25% more weight than the control group.

It appears that lac-phe was the explanation of how strenuous exercise helped the mice gain weight. Without it, the same amount of strenuous exercise caused them to overeat.

Finally, scientists looked for lac-phe in other beings that exercise. They found it first in the blood of racehorses, at much higher levels after intense racing than before the race.

They then asked eight healthy boys to exercise three times: once by pedaling for 90 minutes at a leisurely pace, once by lifting weights, and a third time with several 30-second sprints on a stationary bike.

Participants’ blood lac-phe levels rose after each type of exercise, but peaked after sprints, with weight training coming in second. Prolonged but light exercise produced the lowest level of lac-phe.

In other words, the more intense the exercise, the more lac-phe was produced, and, at least among mice, the more their appetite declined.

“The results are fascinating and add a new dimension to how we view exercise and body weight regulation,” commented Richard Palmiter, professor of biochemistry at the University of Washington in Seattle and an expert in behavioral neurobiology. He did not participate in the new study.

“We always knew that our current menu of molecules that appear to regulate appetite and food intake, such as leptin, ghrelin, etc., was incomplete. This new metabolite/signaling molecule is a potentially important addition to the list,” said Barry Braun, executive director of the Human Performance Clinical Research Laboratory at Colorado State University in Fort Collins. He is a student of physical exercise and weight control and was not involved in the study.

Assuming that this process works in humans as well as in mice, the discovery of lac-phe offers a useful lesson. If we want to avoid overeating after a bout of exercise, we may need to increase the intensity of the workout, said Jonathan Z. Long, a professor of pathology at Stanford University School of Medicine and senior author of the new study.

For him, this idea makes intuitive and evolutionary sense. “If you’re running at high speed, running away from a rhino or other danger, the autonomic nervous system tells the brain to suspend digestion and any other unnecessary processes.”

But their study doesn’t reveal how lac-phe might be interacting with our brain cells to affect appetite, or how strenuous exercise needs to be to stimulate lac-phe formation, or how long the molecule’s effects last.

In addition, the human participants were young, healthy men; therefore, we do not know if lac-phe exists or acts in the same way in other people.

Still, if you want to feel less hungry after working out, you might want to step up your exercise. Include some climbing on your next walk or run to the far corner. According to Long, “what the data says is that intensity makes a difference” for exercise and appetite control.

Translation by Clara Allain