Does intense workout blunt hunger? New study explores relationship between fitness and eating habits


Why are we so hungry after some workouts, but not interested in eating after others?

In a new study published June 15 in the journal Nature, an international team of scientists suggests that the answer lies in part in a single molecule released after exercise that blunts hunger. The molecule, found in the bloodstreams of mice, humans and racehorses, was found in much greater amounts after strenuous exercise than after light exercise, suggesting that heavy exercise may be the key to controlling how much we eat afterwards.

The relationship between fitness and food is notoriously complicated. Studies have shown that people who start exercising without controlling their calorie intake typically lose a few pounds over time, if at all, and may gain weight. This result is affected by many factors, including current fitness, body weight, diet, gender, genetics, metabolic rate, and even exercise time. Some experiments, though not all, show that morning exercise can burn more fat than the same exercise later in the day.


Appetite also matters. If you feel hungry a few hours after your workout, you can easily consume more calories than you burn. But what makes us feel hungry – or not – after we exercise has been a mystery. For decades, scientists have known that various substances, such as the hormones leptin and ghrelin, enter the brain and make us more or less interested in food.

Research shows that exercise changes the levels of these substances, but so does diet and sleep habits. Some researchers have begun to wonder if there might be some specific response to exercise that affects appetite.

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So scientists at Stanford University School of Medicine, Baylor College of Medicine, the University of Copenhagen and other institutions used newly developed methods to look for molecules that appeared in greater numbers in the bloodstream after exercise. They started with mice, putting them on tiny treadmills to run at increasing speeds until they were exhausted. They took blood before and after, and then compared the levels of thousands of molecules in the rodents’ blood.

One stood out, growing larger than any other molecule. It had been noted previously in several metabolic and exercise studies, but its chemistry and biological role remained unknown. The scientists found that this new molecule, a mixture of lactate and the amino acid phenylalanine, was apparently created in response to high levels of lactate released during exercise. Scientists named it lac-phe.

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Energy balance after training

Researchers have suggested that lac-phe may have something to do with post-workout energy balance, as cells in the blood and other places that create it are widely involved in energy consumption and body weight. Maybe, they thought, it affects the appetite. To find out, they gave a form of lakfe to obese mice, which usually eat with pleasure. But their consumption of pellets has dropped by more than 30%. Apparently they were less hungry with the extra lac-phe.


The researchers then returned to the exercises. They bred mice that produced virtually no lac-fe and forced them to run as hard as they could on treadmills five times a week for several weeks. After each race, the animals were given as much fatty food as they wanted. Typically, running helps mice prevent weight gain, even on a high-calorie diet. But the animals, unable to produce much lac-fee, bloated, ate more pellets, and gained about 25% more weight than the control group.

Lac-phe seemed to be the key to how strenuous exercise helped the mice avoid weight gain. Without it, the same exercise led to overeating.

Finally, the researchers tested for the presence of lac-phe in other exercising creatures. They first found it in the bloodstream of racehorses at much higher levels after a hard run than before. Then they asked eight healthy young people to work out three times: once on a bike at a leisurely pace for 90 minutes, another time with weights, and a third time with several 30-second sprints on a stationary bike. Blood lac-phe levels peaked after each type of exercise, but they were highest after sprints followed by strength training. Prolonged, light exercise produced the least.


In other words, the more intense the exercise, the more lac-phe is produced and, at least in mice, the more appetite drops.

“The results are impressive and add a new dimension to our understanding of exercise and weight management,” said Richard Palmiter, professor of biochemistry at the University of Washington in Seattle and an expert in behavioral neuroscience who was not involved in the study. new research.

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“We have always known that our current menu 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,” said Barry Brown. , Executive Director of the Clinical Human Performance Laboratory at Colorado State University Fort Collins, who 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 overeating after a workout, we may need to increase the intensity, says Jonathan Z. Long, professor of pathology at Stanford University School of Medicine and senior author of the new study.

He added that this idea makes intuitive and evolutionary sense. “If you’re running from a rhinoceros or any other threat, the autonomic nervous system screams to the brain to turn off digestion and any other unnecessary processes.”

However, his research doesn’t tell us how lac-phe might interact with our brain cells to affect appetite, or how strenuous exercise must be to produce goose lac-phe, or how long the effects of the molecule might last. Also, the people who exercised were healthy young men, which means we don’t know if lac-phe exists and if it works the same way in everyone else.

However, if you want to feel less hungry after your workout, you can increase your pace. Throw a few hills on your next walk, or run to the far corner of the street. “The data suggests that intensity matters” for exercise and appetite control, Long said.

This article originally appeared in The New York Times.


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