But the research also raises questions about whether eating less is what we really want most from our Thanksgiving or from our exercise.
The effect of exercise on appetite is strong but strange. Exercise requires energy. Appetite, driving food, helps to satisfy it. So it seems intuitive that exercise will make us hungry. And it often is. In many studies, people who exercise moderately, such as by walking, are later hungry and ready to sneeze.
But not when they push themselves. Most people “don’t feel hungry after a hard workout,” Long said.
But why and how? Long, himself both an avid runner and scientist, wondered if the molecules circulating in our bloodstreams after exercise might be involved. These molecules would presumably migrate to the brain or other organs and trigger processes there that drive or weaken hunger.
To find out, he and more than two dozen colleagues looked deep inside mice before and after they ran to exhaustion on tiny treadmills. In a study published this summer in Nature, scientists used a process called mass spectrometry to quantify any change in the levels of any molecule involved in metabolism in the animals’ blood after exercise.
They found plenty. But one in particular shot up in abundance after the animals escaped. It was an obscure molecule that scientists had not previously named or typed. Now working on the chemical composition of the molecule, scientists have discovered that it is a mixture of lactate, a substance produced in abundance by cells during strenuous exercise, and phenylalanine, an amino acid. The researchers called it lac-phe, and from their data, they realized that the more lactate the mice pumped out during exercise – meaning the harder they ran – the more lac-phe appeared in their blood.
A molecule that suppresses appetite after exercise
They then decided to test whether lac-phe affected hunger by injecting it into inactive mice, which usually enjoy their food. The animals immediately “halved their food intake in 12 hours,” Long said. Similarly, when they bred mice that were incapable of producing lak-phe and had them race on treadmills, the animals then stuffed themselves, compared to runner mice with high levels of lak-phe. Without the molecule, intense exercise stimulated appetite.
Finally, they checked the increase in lak-phe in people’s blood after mild cycling, weightlifting or high-intensity interval running. “We found that sprinting produces the highest level of ‘lac-phe,’ Long said, ‘followed by strength training followed by cardio.’
In other words, intense exercise produced more of the appetite-suppressing molecule than easier exercise.
The study caused a scientific stir and led some commentators to speculate in other articles that lac-phe could eventually be purified for pharmaceutical use to dull people’s appetites, without the need for heavy training first.
Exercise won’t help you “earn” food
But most exercise scientists believe that the effects of exercise on hunger go well beyond the actions of a single molecule. Research shows that exercise also has a strong effect on various hormones that help regulate the amount of food you eat. In general, moderate to light activity increases hormone levels that make you want to eat more, especially acetylated ghrelin (or just ghrelin).
“Exercise-induced ghrelin suppression is consistent across all our studies using high-intensity exercise,” said Tom Hazell, a professor of kinesiology at Wilfried Laurier University in Waterloo, Ontario, who has studied exercise and eating behavior extensively.
He said in a new, still unpublished study from his lab, nine middle-aged participants had significantly lowered ghrelin levels almost immediately after training that included repeated, high-intensity 15-second sprint intervals. The results echo those of his group’s previous work, which also found ghrelin to drop sharply shortly after a hard workout and stay low for up to two hours.
Interestingly, human ghrelin levels in some of his group’s studies were the opposite of blood lactate levels, similar to the lac-phe study. The more their lactate levels rose, indicating heavy exertion, the more their ghrelin tended to drop, which can suppress hunger.
An amazing variety of other body processes and parts also influence exercise and appetite, including our brain. For example, in some recent animal studies, intense exercise temporarily altered the firing of specialized hunger neurons, increasing the activity of those that seem to reduce appetite and increasing it in others that keep hunger in check. This process has not yet been observed in humans.
It also remains a mystery how all these systems and processes interact and whether they differ between men and women, old and young, fat and thin, mice and us.
Perhaps most fundamentally, “it’s a bad idea to think of exercise as a way to ‘make money’ from 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 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 eat the donuts, but almost an hour or more to burn them off,” thanks to exercise.
More importantly, exercise has its invaluable benefits, as does the Thanksgiving Buffet, and using one as a weapon to keep you from kicking the other can dilute the enjoyment of both.
Still, if you want to slip into your Turkey Day workout as well as consume a little less, “an intense workout like high-intensity interval training would be a good option,” Hazell said.
Do you have a fitness question? E-mail Your [email protected] and we can answer your question in a future column.
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