The result may mean that, over time, we eat less and control our weight better if we push ourselves a little bit during exercise.
How does exercise alter hunger hormones?
Consider what happened when Tom Hazel, associate professor of kinesiology at Wilfrid Laurier University in Waterloo, Canada, invited nine healthy, middle-aged men and women into an exercise lab. During their visits, he assigned them different exercises on different days, Included:
- Easy 30-minute run.
- A minute of running on a fast, intense treadmill followed by a minute of rest, repeated 10 times.
- 15-second, bicycle intervals, interspersed with two minutes of rest, repeated eight times.
- Sit quietly.
The experiment, published in April in the Journal of Applied Physiology, stems from previous work, more than a decade ago, in which Hazel and his colleagues observed that short sessions of interval training led many volunteers to shed some body fat.
This result was a surprise, because these workouts, often called high-intensity interval training, or HIIT, seemed too short to burn many calories. What can HIIT do and affect people’s body composition?
The researchers drew blood several times before and after each session, checked how hungry they were and asked them to keep detailed food diaries on the day before and on the day of their workouts.
Hazel and his colleagues then checked the volunteers’ blood for a variety of substances, including hormones known to either suppress or suppress appetite.
The most surprising finding was higher levels of lactate in the subjects’ bloodstreams after the interval sessions, compared to levels after moderate running and quiet sitting.
Lactate was once thought to be an unwanted waste product that our muscles produce during exercise. Most of us think that lactic acid made us sore.
But scientists now know that lactic acid is more desirable than harmful. It is an essential signaling molecule that triggers many processes that lead to beneficial effects from exercise. It turns out that pumping lactate during exercise is mostly a good thing.
Lactate flooded the bloodstreams of middle-aged volunteers in Hazel’s lab after each of the short, intense exercise sessions.
More importantly, the higher their lactate levels, the lower the amounts of ghrelin-acylene in their blood. Acylated ghrelin is one of the primary hormones that increases appetite.
In fact, short, intense exercise raised people’s lactate level and, in the process, lowered ghrelin.
It’s possible that the lactate partially blocked the release of ghrelin from the stomach, where it naturally arises, said Seth McCarthy, a graduate student in Hazel’s lab who led the new study, though this possibility needs to be confirmed.
More density, less food
According to the food diary, the exercisers then consumed 129 fewer calories, on average, on the day of the one-minute interval and 201 fewer calories after the repeated 15-second intervals, than on the day of no exercise. Moderate running had no significant effect on eating.
Obviously, these differences are small. Scientists note that it was not statistically significant, and can only be explained by people’s inaccurate recording of what they ate on those days. The effects also varied from person to person, suggesting that changes in ghrelin weren’t the only factor at work.
said David Stencil, a professor of exercise metabolism at Loughborough University in Britain, who studies exercise and food intake. He co-authored the scientific review in Appetite but was not involved in the lactate study.
However, if the difference is repeated over time, a difference of a few hundred calories per day may help us avoid weight gain, other research shows.
The bottom line is, if you want to keep your appetite in check after a workout, you might want to pick up the pace. Instead of strolling on the flats, walk or jog briskly, challenging a long section of hill, swinging the arms, and stopping for breath. Or pedal on a stationary bike at full speed, top speed over several 15-second bursts.
You don’t need a heart rate monitor or scientific accuracy (although if you enjoy those things, aim for a heart rate above about 80 percent of your maximum, which would generally be 220 minus your age).
Also keep your expectations of the effects on your appetite reasonable. They are likely to be short-lived and “require exercise every day or at least a few times each week,” Stencil said.
More research is needed, too, into other possible explanations for how exercise affects appetite. In a 2017 study in Stensel’s lab, an hour of brisk walking on a treadmill increased other hormones besides lactate, including GLP-1, which is also known to affect drugs like Ozempic. However, the increases were smaller than those seen with the medication.
In general, most researchers agree that sweaty exercise is more likely to help us control weight—avoid weight gain—than lose weight.
But a stable weight is important, and exercise has its own unique and irreplaceable benefits. “The most important role of exercise is to keep people fit, healthy and active enough to stay engaged with family, friends and the community and thus lead an interesting and fulfilling life,” said Stencil.
Do you have a fitness question? e-mail YourMove@washpost.com We may answer your question in a future column.
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