In a new study published June 15 in the journal Nature, an international team of scientists says the answer lies in part in the hunger-reducing effects of a single molecule released after exercise. The molecule, found in the blood of mice, humans and racehorses, appeared in much greater numbers after strenuous exercise than after light exercise, suggesting that heavy exercise may be the “key” to controlling how much we eat afterwards.

Studies have shown that people who start exercising without controlling their calorie intake usually lose little or even gain weight over time. Many factors play a role in this effect, including current fitness, body weight, diet, and even time of day.

A matter of appetite

Appetite also matters. If you feel hungry after a workout, you can easily consume more calories than you burn. But what makes us feel hungry — or not — after a workout has been a mystery. For decades, scientists have known that various substances, such as the hormones leptin and ghrelin, are transmitted to the brain and cause us to have more or less appetite for food. Research shows that exercise changes the levels of these substances, but so does diet and sleep habits. Scientists at Stanford University School of Medicine, Baylor School of Medicine, the University of Copenhagen and other institutions have used newly developed methods to look for molecules that appear in high amounts in the blood after exercise. They started with mice, having them run on tiny treadmills at increasing speeds until they were exhausted. They took before and after blood and compared the levels of thousands of molecules in the rodents’ blood.

One molecule stood out and became larger. The scientists found that this molecule — a mixture of lactic acid and the amino acid phenylalanine — appeared to be created in response to high levels of lactic acid released during exercise, and they named it lac-phe.

The researchers hypothesized that lac-phe may be related to post-exercise energy balance, as cells in the blood and elsewhere that produce it are generally associated with energy intake and body weight. Perhaps, they thought, it affects the appetite. To find out, they gave the lac-phe form to obese mice, which usually eat with pleasure. Food intake has been reduced by more than 30%. Apparently they were less hungry with the extra lac-phe.

Research has shown that sprinting helps prevent slouching.

The researchers then returned to the exercises. They bred mice that produced little or no lac-phe and made them run on treadmills five times a week for several weeks. After each race, the animals were allowed to eat as many greasy croquettes as they wanted. Regular running helps mice avoid weight gain even on a high-calorie diet. But animals that couldn’t produce much lac-phe got fat because they ate more pellets and gained about 25% more weight. lac-phe seems to have been the key to how vigorous exercise helped the mice avoid weight gain. Without it, the same exercises led to overeating.

Experiments

The researchers also tested for lac-phe in other creatures. First, they found it in the blood of racehorses, and then asked eight healthy young men to work out three times: once by cycling at a leisurely pace for 90 minutes, another by lifting weights, and a third by various 30-second sprints on a stationary bike. . Blood lac-phe levels peaked after all types of exercise, but were highest after sprints followed by strength training. Prolonged light exercise gave the least result. In other words, the more intense the exercise, the more lactate was produced and, at least in mice, the more their appetite was reduced. “The results are impressive and add a new dimension to our theory about exercise and body weight regulation,” said Richard Palmiter, professor of biochemistry at the University of Washington in Seattle and an expert in behavioral neuroscience.

Update list

“We have always known that the current list of molecules that regulate appetite and food intake, such as leptin, ghrelin, etc., was incomplete, and this new metabolite/signaling molecule is a potentially important addition to that list,” Barry said. . Brown, executive director of the Clinical Human Performance Laboratory at Colorado State University Fort Collins, which studies exercise and weight management. He did not participate in the new study.

Assuming this process works the same way in humans as it does in mice, the discovery of lac-phe provides a useful lesson. If we want to avoid post-workout cramps, we may need to increase the intensity, says Jonathan Z. Long, professor of pathology at Stanford University School of Medicine and lead author of the new study.

However, his research does not tell us how lac-phe interacts with our brain cells to influence appetite, or how intense exercise must be to increase lac-phe production, or how long the effect of the molecule lasts. Also, the people involved were healthy young men, so we don’t know if lac-phe exists or if it works the same for everyone.