Have you ever started a new fitness routine, only to find yourself constantly ravenous? Have you ever finished a tough morning workout, and been surprised by your lack of appetite that day? The effect our workouts have on our appetite has long been a source of fascination, not just for gym goers and athletes but also for scientists.
Just recently, a study published in the journal Nature shed some additional light on the possible links between how we work out and how we eat. And interestingly, it can all be traced back to a single molecule.
Meet the New “Anti-Hunger” Hormone
Conducted by a group of scientists from Stanford Medicine, Baylor University, and the University of Copenhagen, this multi-phase study showed that at least some of the effects of exercise on appetite involve a single molecule, which is a mix of lactate and phenylalanine. Appropriately named lac-phe by the researchers, this molecule increased in the blood of mice after exercise more than any other molecule that was tested.
This came as a surprise to the researchers, since much of the previous research on appetite has focused on leptin and ghrelin, the two main hormones that control your hunger signals. Ghrelin, which is produced when your stomach is empty, sends signals to your brain that it’s time to eat; leptin, which is produced in the small intestines and suppresses appetite, lets your brain know when you’ve had your fill. But lac-phe quickly became a hot source of interest to the researchers, who continued the study by testing how lac-phe affects the appetites of animals on its own.
To do this, they fed lac-phe to obese mice and saw that their food intake dropped more than 30 percent. Finally, the researchers bred mice into two groups—the first group could produce lac-phe and the second could not. They had both groups of mice perform intense workouts—in this case, running on a treadmill, for several weeks—and observed their eating habits. The results clearly showed that the mice who could not produce lac-phe in response to their workouts ate significantly more food, and gained 25 percent more weight than the mice who could produce the molecule.
The results showed that this molecule—which scientists are now referring to as the “anti-hunger” hormone—made the difference between overeating after a workout and not.
Related: What’s the Best Time of Day to Exercise?
Intense Workouts for Appetite Control
Knowing how lac-phe affects appetite is one thing, but how can we as humans take advantage of this newfound knowledge? Luckily, the group of researchers also explored the role of lac-phe in humans, testing the blood of eight young men before and after three different types of exercise—biking leisurely, weight lifting, and then sprinting on a stationary bike. The results showed that the levels of lac-phe in the men’s blood spiked the most after the sprints and the least after the prolonged, leisurely bike ride, suggesting that intense workouts could produce more lac-phe, and surprises appetite, better than more leisurely physical activity.
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If you’ve noticed that you lose your appetite the day of an intense workout, lac-phe might explain why. And according to Kevin Libby, nutritionist and founder of PH2 nutrition, this is something fitness and nutrition professionals have observed for years. “If you’re training for something and it requires a lot of endurance or intensity, that type of exercise can be interpreted as a perceived threat to your body,” explains Kevin Libby. “It decreases appetite to help you escape the threat,” he continues. According to Libby, long training sessions, like if you’re an athlete training for something, “can make food sound like the worst thing in the world.”
That said, he also points out that certain workouts can also increase your appetite. For example, “After a HIIT workout, your appetite can go through the roof, especially for carbs like rice, potatoes, and pasta” he explains. why? “During a HIIT workout, you’re consuming a ton of oxygen and you’re burning through all your glucose and glycogen, so your body really wants to replace all that post-workout,” says Libby.
According to Libby, despite the fact that this study gives us some more insight into how fitness affects our appetite. As he explains it: “This is also very person-dependent. Not everyone’s appetite will react the same way to exercise.”
Related: Don’t Have Time to Work Out? Why ‘Exercise Snacking’ Could Be an Easy Solution
How to Avoid Post-Workout Cravings
If you notice you’re ravenous after a workout or when starting a workout routine, “To some degree, that’s totally normal,” says Libby. That’s because you’re burning calories when you work out and if you’re not eating enough to replace those calories, it’s going to start to break down muscle. “You need to give yourself proper energy to increase performance,” he explains.
This study suggests that adding in more intestines forms of exercise—for example, boxing, long-distance running, or an intense spin class—might help control your appetite response. If you’re struggling with post-workout hunger, Libby also recommends focusing on avoiding calorically-dense, nutrient-depleted foods like processed foods or fast food. It’s going to have a ton of unhealthy fats in them, and refined processed carbs.
Instead, “Have a good post-workout meal that is low on the glycemic index,” says Libby. That means protein, healthy fats and slow-burning carbohydrates like sweet potato, butternut squash and brown rice. Another option is to do your workout later in the day instead of first thing in the morning. “This can help decrease appetite since you have some meals in and your body isn’t in a fasted state,” says Libby.
Up next: New Research Says Intense Exercise Can Reduce Cravings—Start With These 4 Workouts
- Kevin Libby, nutritionist and founder of PH2 Nutrition
- Li, VL, He, Y., Contrepois, K. et al. An exercise-inducible metabolite that suppresses feeding and obesity. Nature 606, 785–790 (2022). https://doi.org/10.1038/s41586-022-04828-5