How exercise affects your appetite


Intense training on Thanksgiving morning may make it easier to skip a second helping of stuffing or pie, according to fascinating new science about physical activity and appetite.

The results suggest that strenuous physical activity curbs hunger, at least for a few hours.

This research has practical implications if we don’t want to overdo Thanksgiving, suggesting we might want to “run a turkey trot” or otherwise get into fast, sweaty exercise first, said Jonathan Z. Long, professor of Pathology at Stanford University School of Medicine studying the cellular effects of exercise and starvation.

But the research also raises the question of whether eating less is really what we most want from our Thanksgiving celebration or from our movement.

Training intensity affects appetite

The effects of exercise on appetite are powerful but strange. Exercise requires energy. Appetite, by fueling the food, helps deliver it. So it makes intuitive sense that exercise would make us hungry. And often it does. In many studiesPeople who exercise moderately, for example by going for a walk, become hungry and ready to snack afterwards.

But not if they are self-propelled. Most people “don’t feel hungry after a hard workout,” Long said.

But why and how? Long, himself an avid runner and scientist, wondered if molecules circulating in our bloodstream after exercise might play a role. These molecules would presumably travel to the brain or other organs, where they initiate processes that increase or suppress hunger.

To find out, he and more than two dozen colleagues looked deep inside mice before and after they sprinted to exhaustion on tiny treadmills. For a to learn The scientists, published in Nature this summer, used a technique called mass spectrometry to enumerate any changes in the levels of all metabolic molecules in the animals’ bloodstream after exercise.

They found plenty. But one in particular shot up in abundance after the animals ran. It was an obscure molecule that scientists had never named or typed before. When the researchers worked out the chemical composition of the molecule, they found that it was a mixture of lactate, a substance that cells produce in abundance during strenuous exercise, and phenylalanine, an amino acid. The scientists dubbed it Lac-Phe, and from their data they realized that the more lactate mice shed during exercise — that is, the harder they ran — the more Lac-Phe appeared in their blood.

A molecule that suppresses appetite after exercise

Next, they set out to see if Lac-Phe affects hunger by injecting it into dormant mice that normally enjoy their food. The animals “immediately reduced their food intake by half over a 12-hour period,” Long said. When they bred mice that couldn’t produce Lac-Phe and ran them hard on treadmills, the animals subsequently became constipated compared to mice runners with high levels of Lac-Phe. Without the molecule, intense exercise stimulated appetite.

Finally, they checked the Lac-Phe elevations in people’s bloodstreams after either gentle cycling, lifting weights, or sprinting through high-intensity intervals. “We found that sprinting produced the highest levels of Lac-Phe,” Long said, “followed by strength training and then cardio.”

In other words, intense exercise produced more of the appetite-suppressing molecule than lighter exercise.

The study caused a scientific stir and led some commentators to speculate Miscellaneous papers This Lac-Phe could eventually be purified for pharmaceutical use to curb people’s appetites without the need for rigorous training first.

Exercise will not help you “earn” food.

But most exercise scientists believe the effects of exercise on hunger go well beyond the effects of a single molecule. Exercise also acutely affects various hormones that help regulate how much we eat, studies show. In general, moderate or light activity increases levels of hormones that make you eat more, particularly one called acetylated ghrelin (or just ghrelin).

“Exercise-induced ghrelin suppression is consistent in our studies with vigorous exercise,” said Tom Hazell, a professor of kinesiology at Wilfried Laurier University in Waterloo, Ontario, who has done extensive research on exercise and eating behaviors.

In a new, yet unpublished study from his lab, nine middle-aged participants ended up with significantly reduced ghrelin levels almost immediately after a workout involving repeated, intense 15-second sprint intervals, he said. The results match those of his group earlier Work, who also found that ghrelin crashed soon after a hard workout and stayed low for up to two hours.

Interestingly, in some of his group’s studies, people’s ghrelin levels were inversely related to their blood lactate levels, similar to the Lac-Phe study. As their lactate levels rose, indicating hard exertion, their ghrelin tended to drop, which may suppress hunger.

A bewildering array of other bodily processes and parts also play a role in movement and appetite, including our brains. In some recent animals studiesFor example, intense exercise temporarily altered the firing of specialized neurons dedicated to hunger, increasing activity in those that appear to reduce appetite and increasing it in others that keep hunger at bay. This process has not yet been observed in humans.

It also remains a mystery how all of these systems and processes work together and whether they vary between males and females, old and young, heavy and slender, or mice and us.

Perhaps most fundamentally, “It’s a bad idea to think of exercise as a way to ‘earn’ food,” said Glenn Gaesser, a professor of exercise physiology at Arizona State University in Phoenix, who studies physical activity and weight management.

First, exercise burns fewer calories. “In one of our studies,” he said, “we had subjects eat two donuts,” which totaled 520 calories. “It took less than five minutes to devour the donuts but nearly an hour or more to burn them” with exercise.

More importantly, exercise has its own priceless rewards, as does the Thanksgiving buffet, and arming one to keep you from assaulting the other could dull the joys of both.

If you still want to slip into a Turkey Day workout and also consume a little less, “a vigorous-intensive training how high-intensity interval training would be the way to go,” Hazell said.

Have a fitness question? E-mail and we may answer your question in a future column.

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