NEW YORK — Why are we so hungry after some workouts but not interested in eating after others?
In a new study published June 15 in Nature, an international team of scientists suggests that the answer lies in part in the actions of a single molecule produced after exercise that alleviates hunger. The molecule, found in the bloodstream of mice, humans and racehorses, appeared in much greater profusion after strenuous workouts than easy ones, suggesting that vigorous exercise could be the key to controlling how much we eat after.
The relationship between being fit and eating is famously thorny. Studies have shown that people who start exercising without also monitoring their calorie intake typically lose few or no pounds over time and may gain weight.
Many factors influence that outcome, including someone’s current fitness level, body mass, diet, gender, genetics, metabolic rate, and even the timing of the exercise. Some, but not all, experiments suggest that morning sessions can burn more fat than the same efforts later in the day.
Appetite matters too. If you feel hungry in the hours after a workout, you can easily end up consuming more calories than you burned. But what makes us feel hungry, or not, after exercising has been a mystery. For decades, scientists have known that various substances, such as the hormones leptin and ghrelin, travel to the brain and prompt us to be more or less interested in eating.
Studies show that exercise alters the levels of these substances, but so do diet and sleep habits. Some researchers began to wonder if there might be some kind of specific reaction 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 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 drew blood before and after and then compared the levels of thousands of molecules in the rodents’ blood.
One stood out, increasing more than any other molecule. It had been observed before in some metabolism and exercise studies, but its chemical and biological function was still unknown. The scientists discovered that this new molecule, a mixture of lactate and the amino acid phenylalanine, was apparently created in response to the high levels of lactate released during exercise. Scientists called it lac-phe.
The researchers speculated that lac-phe might have something to do with energy balance after exercise, since the cells in the blood and elsewhere that create it are largely involved in energy intake and body mass. Perhaps, they thought, it affects the appetite. To find out, they gave a form of lac-phe to obese mice, which normally eat with relish. But their kibble consumption dropped by more than 30 percent. Apparently they were less hungry with the extra lac-phe.
The researchers then went back to exercising. They bred mice that produced little or no lac-phe and had them run on treadmills five times a week for several weeks. After each run, the animals were allowed to eat as many high-fat kibble as they wanted. Normally, running helps mice avoid weight gain, even on a high-calorie diet. But the animals unable to produce much lac-phe became bloated, ate more kibble and gained about 25 percent more weight than the control group.
Lac-phe, it seemed, had been the key to how strenuous exercise helped the mice avoid weight gain. Without it, that same exercise resulted in overeating.
Finally, the researchers looked for lac-phe in other exercising creatures. They first found it in the bloodstream of racehorses at much higher levels after a hard race than before. They then asked eight healthy young men to exercise three times: once by pedaling at a leisurely pace for 90 minutes, again by lifting weights, and a third time by doing multiple 30-second runs on a stationary bike.
Blood levels of lac-phe peaked after each type of exercise, but were highest after sprints, followed by weight training. Long, gentle exercise produced the least.
In other words, the more intense the exercise, the more lac-phe was produced and, at least in the mice, the more appetite seemed to decrease.
“The results are exciting and add a new dimension to our thinking about exercise and body weight regulation,” said Dr. Richard Palmiter, a professor of biochemistry at the University of Washington in Seattle and an expert in behavioral neurobiology who was not involved. . of 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 and this new metabolite/signaling molecule is a potentially important addition to that list.” Dr. Barry said. Braun, executive director of the Human Performance Clinical Research Laboratory at Colorado State University in Fort Collins, which studies exercise and weight management. He was not involved in the new study.
Assuming that this process works the same in humans as it does in mice, the discovery of lac-phe provides a useful lesson. If we want to avoid binge eating after a workout, we may need to increase the intensity, said Dr. Jonathan Z. Long, a professor of pathology at Stanford University School of Medicine and lead author of the new study.
This idea makes intuitive and evolutionary sense, he added. “If you’re running from a rhino or some other threat, the autonomic nervous system screams at the brain to stop digestion and any other unnecessary processes.”
However, their study doesn’t tell us how lac-phe might be interacting with our brain cells to affect appetite, or how strenuous the exercise needs to be to speed up lac-phe formation, or how long the molecule’s effects might last. . Also, the human exercisers were healthy youngsters, which means we don’t know if lac-phe exists or operates in the same way in everyone else.
Still, if you want to be less hungry after working out, you may want to pick up the pace. Add some hills during your next walk or run to the farthest street corner. “What the data is saying is that intensity matters” for exercise and appetite control, Dr. Long said.
This article originally appeared on The New York Times.
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