The unexpected link between hunger and exercising

Scientists may have figured out why some workouts make you hungry and others don't.

Burger man showing muscles
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When I played roller derby, I would binge on greasy fast food after every practice. I never questioned why exercising for two hours made me hungry — it just made sense. But after bouts (games), my team and I would spend hours in euphoric camaraderie without eating and I never felt hungry at all. I always thought that my lack of an appetite was more social than anything, but new research suggests that more intense workouts may induce the production of an appetite suppressant, The New York Times reported.

It’s important to note that the study, which was published Wednesday in the journal Nature, was done on mice — which can be a flawed method that may very well yield different results than if the same research is done on humans. So, while this sheds some light, it shouldn’t be viewed as definitive. That said, researchers found that a molecule called lac-phe may be a natural appetite suppressant that’s produced during exercise. If you’ve never heard of lac-phe, that’s because scientists just discovered it. Lac-phe — a mix of lactate and the amino acid phenylalanine — seems to be produced in response to the high levels of lactate the body makes during intense activity.

Scientists knew that lac-phe had some relationship to balancing energy levels after exercise, so for the study they fed large amounts of the molecule to obese mice, the Times reported. The mice, who were usually hearty eaters, ate 30% less than usual after being fed lac-phe. In other words, lac-phe seemed to suppress the appetites of the mice.

The researchers then compared mice that produced little-to-no lac-phe to mice that produced the usual amount. All the animals ran on treadmills five days a week. What they found was that the animals who didn’t produce lac-phe ate more and gained 25% more weight than the control mice. The point was to establish a correlation between lac-phe and appetite.

Next up, the scientists studied when the body produces the most lac-phe by looking at race horses, and then humans. The details are a bit tedious, but the conclusion the scientists came to is that the more intense an activity is, the more lac-phe the body produces to suppress appetite. This is important because, while we’ve always known that exercise plays a role in metabolism, the details have been a bit hazy. Lac-phe may help explain one of the reasons that intense exercise may contribute to weight loss.

“The results are fascinating and add a new dimension to our thinking about exercise and body-weight regulation,” Richard Palmiter, a professor of biochemistry at the University of Washington in Seattle, told the Times. It feels important to note here that, while I am excited by this research, I also fear that it will contribute negatively to our diet-obsessed culture. Already, some claim that there will soon be drugs that replace exercise — as though weight loss should be our only reason for exercising. (It’s definitely not.)

This research also helps me understand some of my own history of disordered eating. At some points of my life, I exercised intensely and compulsively, and I had zero appetite. Of course there are many factors that played into that behavior, but lac-phe may be one reason why the more I worked out, the less hungry I was. It also may explain why I was hungrier after roller derby practice than after a bout. Practice is less intense than a bout.

But why would our bodies do this, anyway? Don’t we need to replenish after a workout? Yes, definitely. Some experts think this may be an evolutionary development that helps humans respond to danger. “If you’re sprinting from a rhino or some other threat, the autonomic nervous system yells at the brain to shut down digestion and any other unneeded processes,” Jonathan Long, professor of pathology at Stanford University School of Medicine and senior author of the study, told the Times. In other words, the production of lac-phe is likely part of our fight-flight response and is not meant to become an oppressive tool of diet culture.

If you or someone you care about is suffering with disordered eating or exercise, you can reach out to the National Eating Disorders helpline. Call or text (800) 931-2237.