Exercise can make you hungry. But if you pick the right workout, you’re less likely to overeat.
New research suggests up-tempo, high-energy workouts, even if they are quite short, can reduce a key appetite-upping hormone in many people.
“This evidence counters the popular assertion that exercise transiently increases appetite” and leads us to shovel in food afterward, according to a new scientific review of studies about exercise and eating in the journal Appetite. Instead, challenging physical activities might leave us less interested in food, at least for a while.
The result, over time, could mean we eat less and control our weight better if we push ourselves a bit during exercise.
How exercise changes hunger hormones
Consider what happened when Tom Hazell, an associate professor of kinesiology at Wilfrid Laurier University in Waterloo, Canada, invited nine healthy, middle-aged men and women to an exercise lab. During their visits, he assigned them various workouts on different days, including:
- Easy run for 30 minutes.
- A minute of fast, intense treadmill running followed by a minute of rest, repeated 10 times.
- 15-second, all-out bike intervals, interspersed with two minutes of rest, repeated eight times.
- Sitting quietly.
This experiment, published in April in the journal of applied physiology , grew out of earlier work, from more than a decade before, in which Hazell and his colleagues noticed that brief sessions of interval training prompted many of their volunteers to shed some body fat.
That result was a surprise, since these workouts, often called high-intensity interval training or HIIT, seemed too short to incinerate many calories. What could HIIT be doing that was affecting people’s body composition?
The role of lactic acid
The researchers drew blood before and several times after each session, checked on how hungry people felt and asked them to keep detailed food diaries the day before and day of their workouts.
Hazell and his colleagues then checked their volunteers’ blood for a variety of substances, including hormones known to either curb or goose appetite.
The most striking finding was that lactate levels had soared in people’s bloodstreams after the interval sessions, compared to the levels after the moderate run and the quiet sitting
Lactate once was thought to be an undesirable waste product produced by our muscles during exercise. Most of us believed lactic acid made us sore.
But scientist now know lactic acid is more desirable than damaging. It’s an essential signaling molecule that jump-starts many of the processes leading to beneficial effects from exercise. Pumping out lactate during exercise is, it turns out, mostly a good thing.
And the bloodstreams of the middle-aged volunteers in Hazell’s lab were awash in lactate after each of their short, intense exercise sessions.
Even more important, the higher their lactate levels, the lower the amounts of acylated ghrelin in their bloods. Acylated ghrelin is one of the primary hormones that increases appetite.
In effect, brief, intense exercise had raised people’s lactate and, in the process, lowered ghrelin.
It’s likely the lactate partially blocked the release of acylated ghrelin from the stomach, where it normally originates, said Seth McCarthy, a graduate student in Hazell’s lab, who led the new study, although that possibility needs to be confirmed.
More intensity, less food
According to food diaries, the exercisers subsequently consumed 129 fewer calories, on average, on the day of their one-minute intervals and 201 calories fewer after their repeated 15-second intervals, compared to the no-exercise day. The moderate running had no measurable effect on eating.
These differences are obviously small. They weren’t statistically significant, the scientists note, and might be accounted for just by people’s inaccurate recording of what they ate those days. The effects also varied from one person to the next, suggesting changes to ghrelin were not the only factor at work.
“There are multiple biological signals thought to play a role” in appetite, “but many other psychological, environmental, and individual factors” as well, said David Stensel, a professor of exercise metabolism at Loughborough University in Britain, who studies exercise and eating. He co-authored the scientific review in Appetite but was not involved in the lactate study.
Still, if repeated over time, a difference of even a few hundred calories a day might help us avoid gaining weight, other research shows.
Pick up the pace
The upshot is that if you’d like to keep your appetite in check after exercise, you may want to pick up the pace. Instead of strolling on the flats, walk or run at a brisk, challenging clip up a long hill, arms swinging, breath hitching. Or pedal a stationary bike at your top, all-out speed during several 15-second spurts.
You don’t need a heart rate monitor or scientific exactitude (although, if you enjoy those things, aim for a heart rate above about 80 percent of your maximum, which in broad terms would 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 to be performed every day or at least several times each week,” Stensel said.
Additional research is needed, too, into other, possible explanations for how exercise affects appetite. In a 2017 study Stensel’s lab, an hour of fast treadmill walking resulted in increases of other hormones besides lactate, including GLP-1, which famously is also involved in the effects of drugs such as Ozempic. The increases were orders of magnitude smaller than those seen with the drug, however.
Overall, most researchers agree, sweaty exercise probably will help us mostly with weight control — staving off weight gain — rather than with weight loss.
But stable weight matters, and exercise has its own unique and irreplaceable benefits. “The most important role of exercise,” Stensel said, “is keeping people sufficiently fit, healthy and active to remain engaged with family, friends and society and hence lead interesting and fulfilling lives