But the research also raises questions about whether eating less is what we really want most from our Thanksgiving or our exercise.
Exercise intensity affects appetite
The effects of exercise on appetite are powerful but strange. Exercise takes energy. Appetite, by pushing to eat, helps to supply it. So it’s intuitive that exercise makes us hungry. And, often, it is. In many studies, people who exercise moderately, such as walking, end up feeling peckish and ready to snack.
But not when they push each other. Most people “don’t feel hungry after a hard workout,” Long said.
Why, however, and how? Long, himself an avid runner and scientist, wondered if molecules circulating in our blood after exercise might be involved. These molecules would likely migrate to the brain or other organs and trigger processes there that stimulate or dampen hunger.
To find out, he and more than two dozen colleagues looked deep inside mice before and after they raced to exhaustion on tiny treadmills. For a study published this summer in Nature, scientists used a process called mass spectrometry to enumerate every change in the levels of any molecule involved in metabolism in the blood of animals after exercise.
They found plenty. But one in particular exploded profusely after the animal run. It was an obscure molecule that scientists had yet to name or type. Now working on the chemical composition of the molecule, the researchers have discovered that it is a mixture of lactate, a substance produced in abundance by cells during intense exercise, and phenylalanine, an amino acid. The scientists dubbed it lac-phe and realized from their data that the more lactate the mice produced during exercise – that is, the harder they ran – the more lac-phe there was in their blood.
A molecule that suppresses appetite after exercise
Next, they investigated whether lac-phe affected hunger, by 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 raised mice unable to produce lac-phe and ran them hard on treadmills, the animals subsequently stuffed themselves, compared to mouse runners with high levels of lac-phe. Without the molecule, intense exercise stimulates the appetite.
Finally, they checked for lac-phe increases in people’s bloodstream after gently 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 created more of the appetite-suppressing molecule than easier exercise.
The study created a scientific stir and prompted some commentators to speculate in other articles that lac-phe could possibly be purified for pharmaceutical purposes, to blunt people’s appetites, without the need for intensive training beforehand.
Exercise Won’t Help You “Gain” Food
But most exercise scientists believe that the effects of movement on hunger go far beyond the actions of a single molecule. Exercise also acutely influences various hormones that help regulate the amount of food we eat, studies show. In general, moderate or easy activities increase hormone levels that make you want to eat more, especially one called acetylated ghrelin (or simply ghrelin).
“Exercise-induced ghrelin suppression is consistent across all of our studies using intense exercise,” said Tom Hazell, a professor of kinesiology at Wilfried Laurier University in Waterloo, Ont., who has extensively studied the exercise and eating behavior.
In a new, as 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, a-t -he declares. The results echo those of his group’s previous work, which also found that ghrelin drops shortly after intense training and remains low for two hours.
Interestingly, the ghrelin levels of people in some of the studies in his group followed, in reverse, those of their blood lactate, just like in the lac-phe study. The more their lactate levels rose, indicating intense exertion, the more their ghrelin tended to drop, which can alleviate hunger.
A bewildering variety of other body processes and parts also play a role in exercise and appetite, including our brain. In some recent animal studies, for example, strenuous exercise temporarily altered the firing of specialized hunger neurons, increasing activity in those that seem to suppress appetite and increasing it in others that control hunger. hunger. This process has not yet been observed in humans.
How all of these systems and processes interact, and whether they vary between men and women, old and young, heavy and thin, or mice and us, also remains a mystery.
Perhaps more fundamentally, “it’s a bad idea to think of exercise as a way to ‘gain’ food,” said Glenn Gaesser, professor of exercise physiology at Arizona State University in Phoenix. , which studies physical activity and weight control.
For one thing, exercise burns few calories. “In one of our studies,” he said, “we asked subjects to eat two donuts,” for a total of 520 calories. “It took less than five minutes to consume the donuts, but nearly an hour or more to burn them off” with exercise.
More importantly, exercise has its own priceless rewards, as does the Thanksgiving buffet, and arming one to stop you from digging the other could tarnish the pleasures of both.
Still, if you want to slip into a Turkey Day workout and consume a little less, “a vigorous-intensity workout like high-intensity interval training would be the way to go,” Hazell says.
Do you have a fitness question? E-mail YourMove@washpost.com and we may answer your question in a future column.
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