But the research also raises questions about whether eating less is what we want most from our Thanksgiving or from our exercise.
Exercise intensity affects appetite
The effects of exercise on appetite are powerful but strange. Exercise requires energy. Appetite, by prompting eating, helps to supply it. So it makes intuitive sense that exercise makes us hungry. And often it does. In many studiespeople who exercise moderately, for example by walking, end up hungry afterward and ready to eat.
But not when they push themselves. Most people “don’t feel hungry after a hard workout,” Long said.
Why, however, and how? Long, an avid runner as well as a scientist, wondered if molecules circulating in our bloodstreams after exercise might be involved. Presumably, these molecules would migrate to the brain or other organs and activate processes there that drive or attenuate hunger.
To find out, he and more than two dozen colleagues looked inside mice before and after they ran to exhaustion on tiny treadmills. For to study Published this summer in Nature, the scientists used a process called mass spectrometry to enumerate each change in the levels of any molecule involved in metabolism in the animals’ bloodstreams after exercise.
They found a lot. But one in particular went off in profusion after the animals ran. It was an obscure molecule that scientists had not previously named or typed. Now working on the chemical composition of the molecule, the researchers found that it was a mixture of lactate, a substance produced abundantly by cells during strenuous exercise, and phenylalanine, an amino acid. The scientists named it lac-phe and realized from their data that the more lactate the mice pumped out during exercise—that is, the more 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 affected hunger, injecting it into inactive mice, which normally enjoy their food. The animals immediately “cut their food intake in half over a 12-hour period,” Long said. Similarly, when they bred mice unable to produce lac-phe and had them run hard on treadmills, the animals then gorged themselves, compared to running mice with high lac-phe levels. Without the molecule, intense exercise stimulated the appetite.
Finally, they checked for increases in lac-phe in people’s bloodstreams after light cycling, weight lifting, or running through high-intensity intervals. “We found that sprinting produced the highest levels” of lac-phe, Long said, “followed by weight training and then cardio.”
In other words, the intense exercise created more of the appetite-suppressing molecule than the easier exercise.
The study created a scientific stir and led some commentators to speculate on other documents that lac-phe could eventually be purified for pharmaceutical use, to appease people’s appetites, without the need for hard training first.
Exercise Won’t Help You ‘Gain’ Food
But most exercise scientists think that the effects of movement on hunger extend well beyond the actions of a single molecule. Exercise also acutely influences several hormones that help regulate how much we eat, studies show. In general, moderate to easy activities increase levels of hormones that make you want to eat more, particularly one called acetylated ghrelin (or just ghrelin).
“Exercise-induced suppression of ghrelin is consistent across all of our studies using intense exercise,” said Tom Hazell, a professor of kinesiology at Wilfried Laurier University in Waterloo, Ontario, who has extensively studied exercise and eating behavior.
In a new, as-yet-unpublished study from his lab, nine middle-aged participants ended up with significantly reduced levels of ghrelin almost immediately after a workout involving repeated, intense 15-second intervals of sprinting, he said. Results echo your group’s previous results to work, which also found that ghrelin plummeted shortly after an intense workout and stayed low for two hours.
Interestingly, the ghrelin levels of people in some of his group’s studies followed their blood lactate levels inversely, just as in the lac-phe study. The more her lactate levels rose, indicating intense exertion, the more her ghrelin tended to fall, which can quell hunger.
An astonishing variety of other bodily processes and parts also influence exercise and appetite, including our brains. In some recent animal studiesFor example, intense exercise temporarily altered the firing of specialized neurons dedicated to hunger, increasing activity in those that seem to decrease appetite and elevating it in others that keep hunger in check. This process has yet to be seen in people.
It also remains a mystery how all these systems and processes interact and whether they vary between male and female, old and young, heavy and thin, 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 exercise. weight control.
For one thing, exercise burns few calories. “In one of our studies,” she 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 using one as a weapon to keep you from messing with the other could dull the pleasures of both.
Still, if you want to get in a Turkey Day workout and also consume a little less, “a vigorous-intensity training like high-intensity interval training would be the way to go,” Hazell said.
Do you have a fitness question? Email [email protected] and we can answer your question in a future column.
