But the research also raises questions about whether eating less is what we really want most from our Thanksgiving, or from our exercise.
The intensity of exercise affects appetite
The effects of exercise on appetite are powerful but strange. Exercising takes energy. Appetite, by propelling the food, helps it deliver. So it makes intuitive sense that exercise would make us hungry. And often it is. In many studiespeople who exercise moderately, for example by walking, then get hungry and ready to eat.
But not if they push themselves. Most people “are not hungry after a hard workout,” Long said.
But why, and how? An avid runner and scientist himself, Long wondered if molecules circulating in our bloodstream after exercise might be involved. These molecules would presumably migrate to the brain or other organs and initiate processes there that cause or reduce hunger.
To find out, he and more than two dozen colleagues looked deep into mice before and after they sprinted to exhaustion on small treadmills. For a study published this summer in Nature, the scientists used a process called mass spectrometry to list any change in the levels of each molecule involved in metabolism in the animals’ bloodstreams after exercise.
They found enough. But one in particular shot up in abundance after the animals ran away. It was an obscure molecule that scientists had not previously named or typed. Now, when they worked out the chemical composition of the molecule, the researchers found that it was a mix of lactate, a substance abundantly produced by cells during strenuous exercise, and phenylalanine, an amino acid. Called it lac-phe, the scientists realized from their data that the more lactate mice pumped out during exercise — meaning the faster they ran — the more lac-phe showed up in their blood.
A molecule that blunts appetite after exercise
They then set out to see if lac-phe affected hunger by injecting it into inactive mice, which normally enjoy their food. The animals “immediately reduced their food intake by half in a 12-hour period,” Long said. Similarly, when they bred mice that couldn’t produce lac-phe and raced them hard on treadmills, the animals gorged themselves afterwards, compared to mouse runners with high levels of lac-phe. Without the molecule, intense exercise stimulated appetite.
Finally, they checked for lac-phe increases in people’s bloodstreams after they either cycled gently, lifted weights or sprinted 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 created more of the appetite-suppressing molecule than easier exercise.
The study caused a scientific stir and prompted some commentators to speculate other paper that lac-phe could eventually be purified for pharmaceutical use, to curb people’s appetites, without the need for strenuous training first.
Exercise doesn’t help you ‘earn’ food
But most sports scientists believe that the effects of exercise on hunger extend far beyond the action of a single molecule. Exercise also has an acute impact on several hormones that help regulate how much we eat, studies show. In general, moderate or easy activities increase the hormone levels that make you want to eat more, especially one called acetylated ghrelin (or just ghrelin).
“The exercise-induced suppression of ghrelin is consistent across all of our studies using vigorous exercise,” said Tom Hazell, a professor of kinesiology at Wilfried Laurier University in Waterloo, Ontario, who has studied exercise and eating behaviors extensively.
In a new, as-yet-unpublished study from his lab, nine middle-aged participants experienced significantly reduced ghrelin levels almost immediately after training with repeated, intense 15-second sprint intervals, he said. The results mirror those of his group earlier work, who also found that ghrelin plummeted quickly after a hard workout, staying low for as long as two hours.
Interestingly, in some of his group’s studies, people’s ghrelin levels inversely followed those of their blood lactate, much as they did in the lac-phe study. The more their lactate levels rose, indicating strenuous exercise, the more their ghrelin tended to drop, which can suppress hunger.
A bewildering variety of other bodily processes and parts also play a role in exercise and appetite, including our brain. In a recent animal studiesFor example, vigorous exercise temporarily altered the firing of specialized neurons dedicated to hunger, increasing activity in those that seem to reduce appetite and increasing it in others that control hunger. This process has not yet been seen in humans.
It also remains a mystery how all these systems and processes interact and whether they vary between men and women, old and young, heavy and slim, or mice and us.
Perhaps most fundamentally, “It’s a bad idea to think of exercise as a way to ‘earn’ food,” says Glenn Gaesser, a professor of exercise physiology at Arizona State University in Phoenix who studies physical activity and weight management. .
For starters, exercise burns few calories. “In one of our studies,” he said, “we had subjects eat two donuts,” for a total of 520 calories. “It took less than five minutes to consume the donuts, but almost an hour or more to burn them off” with exercise.
More importantly, exercise has its own priceless rewards, just like the Thanksgiving buffet, and weaponizing one to avoid digging into the other can dull the pleasures of both.
Still, if you want to do a turkey day workout and also consume a little less, “a powerfulintensity workout like high-intensity interval training would be the way to go,” Hazell said.
Do you have a fitness question? E-mail YourMove@washpost.com and maybe we’ll answer your question in a future column.