The result, over time, could mean that we eat less and control our weight better if we push ourselves a little 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 workshop. During their visits, he assigned them various workouts on different days, Included:
- Easy run for 30 minutes.
- One minute of fast, intense running on the treadmill followed by one minute of rest, repeated 10 times.
- 15-second bike intervals, interspersed with two minutes of rest, repeated eight times.
- Sitting in silence.
This experiment, published in April in the Journal of Applied Physiology, grew out of earlier work, more than a decade earlier, in which Hazell and his colleagues noticed that short interval training sessions prompted many of their volunteers to lose body fat.
That result came as 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 researchers drew blood before and several times after each session, checked how hungry people felt, and asked them to keep detailed food diaries the day before and the day of their workouts.
Hazell and his colleagues then checked their volunteers’ blood for a variety of substances, including hormones known to curb the goose’s appetite.
The most surprising finding was that lactate levels had soared in people’s bloodstreams after the interval sessions, compared to levels after moderate running and quiet sitting.
Lactate was once thought to be an unwanted waste product produced by our muscles during exercise. Most of us believed that lactic acid was bad for us.
But scientists now know that lactic acid is more desirable than harmful. It is an essential signaling molecule that initiates many of the processes that lead to beneficial effects from exercise. Pumping up lactate during exercise is, it turns out, mostly a good thing.
And the bloodstreams of middle-aged volunteers in Hazells’ lab were flooded with lactate after each of their short, intense training sessions.
More importantly, the higher their lactate levels, the lower the amount of acylated ghrelin in their blood. Acylated ghrelin is one of the primary hormones that increases appetite.
In fact, the short, intense exercise had raised people’s lactate and, in the process, lowered ghrelin.
It’s likely that lactate partially blocked the release of acylated ghrelin from the stomach, where it normally originates, said Seth McCarthy, a graduate student in Hazells’ lab who led the new study, though that possibility needs to be confirmed.
More intensity, less food
According to the food diaries, the athletes subsequently consumed an average of 129 fewer calories on the day of their one-minute intervals and 201 fewer calories after the repeated 15-second intervals, compared to the day without exercise. Moderate running had no measurable effect on eating.
These differences are obviously small. They weren’t statistically significant, the scientists note, and could only be explained by people’s inaccurate recording of what they ate in those days. The effects also varied from person to person, suggesting that ghrelin modifications weren’t the only factor at play.
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, professor of exercise metabolism at Loughborough University in Britain, who studies exercise and eating. He co-authored the scientific journal Appetite but was not involved in the lactate study.
However, if repeated over time, a difference of even a few hundred calories a day could help us avoid gaining weight, other research shows.
The upshot is that if you’d like to keep your appetite in check after exercise, you might want to pick up the pace. Instead of strolling on the flat, walk or jog briskly and hard up a long hill, arms swinging, breath in suspense. Or pedal a stationary bike at maximum speed during several 15-second sprints.
You don’t need a heart rate monitor or scientific accuracy (although, if you’re into those things, aim for a heart rate above about 80 percent of your maximum, which in general terms would be 220 minus your age).
Also keep your expectations about the effects on appetite reasonable. They are likely to be short-lived and require exercise to be done every day or at least several times a week, Stensel said.
More research into other possible explanations for how exercise affects appetite is also needed. In a 2017 study at Stensels’ lab, an hour of brisk walking on a treadmill resulted in a rise in other hormones besides lactate, including GLP-1, which is also famously involved in the effects of drugs like Ozempic. However, the increases were orders of magnitude smaller than those seen with the drug.
Overall, most researchers agree that sweaty exercise will likely help us primarily with weight control by avoiding weight gain rather than weight loss.
But stable weight is important, and exercise has its own unique and irreplaceable benefits. The most important role of exercise, Stensel said, is to keep people fit, healthy and active enough to stay engaged with family, friends and society and thus lead interesting and fulfilling lives.
Have a fitness question? E-mail YourMove@washpost.com and we may answer your question in a future column.
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