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 potent but odd. Exercise requires energy. Appetite, by driving eating, helps supply it. So, it makes intuitive sense that exercise would make us hungry. And, often, it does. In many studies, people who work out moderately, by, for instance, walking, end up peckish afterward and ready to nosh.
But not when they push themselves. Most people “don’t feel hungry after a hard workout,” Long said.
Why, though, and how? Long, himself an avid runner as well as a scientist, wondered whether molecules circulating in our bloodstreams after exercise might be involved. These molecules presumably would migrate to the brain or other organs and jump-start processes there that drive or dim 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 study published this summer in Nature, the scientists used a process called mass spectrometry to enumerate every change in the levels of any molecule involved in metabolism in the animals’ bloodstreams after exercise.
They found plenty. But one in particular shot up in profusion after the animals ran. It was an obscure molecule scientists had not previously named or typed. Working out the molecule’s chemical makeup now, the researchers found it was a mix of lactate, a substance produced abundantly by cells during strenuous exercise, and phenylalanine, an amino acid. The scientists dubbed it lac-phe and realized from their data that the more lactate mice pumped out during exercise — meaning the harder they ran — the more lac-phe turned up in their blood.
A molecule that dulls the appetite after exercise
Next, they set out to see if lac-phe affected hunger, injecting it into inactive mice, which normally enjoy their chow. The animals immediately “reduced their food intake by half over a 12-hour period,” Long said. Similarly, when they bred mice unable to produce lac-phe and had them race hard on treadmills, the animals afterward stuffed themselves, compared to mouse runners with high levels of lac-phe. Without the molecule, intense exercise stimulated appetites.
Finally, they checked for lac-phe increases in the bloodstreams of people after they either gently cycled, lifted weights or sprinted 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, intense exercise created more of the molecule that suppresses appetite than easier exercise did.
The study created a scientific stir and prompted some commentators to speculate in other papers that lac-phe eventually might be purified for pharmaceutical use, to blunt people’s appetites, without any need for a hard workout first.
Exercise won’t help you ‘earn’ food
But most exercise scientists think the effects of movement on hunger extend well beyond the actions of any single molecule. Exercise also acutely influences various hormones that help regulate how much we eat, studies show. In general, moderate or easy activities boost levels of hormones that make you want to eat more, particularly one called acetylated ghrelin (or just ghrelin).
“The exercise-induced suppression of ghrelin is consistent across 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, still-unpublished study from his lab, nine middle-aged participants wound up with meaningfully reduced ghrelin levels almost immediately after a workout involving repeated, intense 15-second sprint intervals, he said. The results echo those of his group’s earlier work, which also found ghrelin plummeting soon after a hard workout and staying low for as long as two hours.
Interestingly, people’s ghrelin levels in some of his group’s studies tracked, in reverse, to those of their blood lactate, much as in the lac-phe study. The more their lactate levels rose, indicating hard exertion, the more their ghrelin tended to fall, which can tamp down hunger.
A boggling variety of other bodily processes and parts likewise play into exercise and appetite, including our brains. In some recent animal studies, for instance, intense exercise temporarily altered the firing of specialized neurons devoted to hunger, upping activity in those that seem to lower appetite and raising it in others that keep hunger in check. This process has not yet been seen in people.
It also remains a mystery how all of 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 about 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 control.
For one thing, exercise burns few calories. “In one of our studies,” he said, “we had subjects eat two doughnuts,” for a total of 520 calories. “It took less than five minutes to consume the doughnuts, but close to an hour or more to burn them off” with exercise.
Even more important, exercise has its own inestimable rewards, as does the Thanksgiving buffet, and weaponizing one to keep you from digging in at the other could dull the pleasures of both.
Still, if you would like to slip in a Turkey Day workout and also consume a little less, “a vigorous-intensity workout like high-intensity interval training would be the way to go,” Hazell said.
Do you have a fitness question? Email [email protected] and we may answer your question in a future column.
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