Have you ever started a new fitness routine only to find yourself feeling starved all the time? Have you ever finished a hard morning workout and been surprised by your loss of appetite that day? The effect of our exercise on our appetite has long been a source of fascination, not only for gym goers and athletes, but also for scientists.
A study was recently published in the journal Nature shed some additional light on the possible connections between our training and diet. And interestingly, it all boils down to a single molecule.
Meet the new “anti-hunger” hormone
This multi-phase study, conducted by a group of scientists from Stanford Medicine, Baylor University and the University of Copenhagen, showed that at least some of the effects of exercise on appetite are due to a single molecule, which is a mixture of lactate and phenylalanine is. This molecule, appropriately named Lac-Phe by the researchers, increased more in the blood of mice after exercise than any other molecule tested.
This came as a surprise to researchers since much of the appetite research to date has focused on leptin and ghrelin, the two main hormones that control your hunger signals. Ghrelin, produced when your stomach is empty, sends signals to your brain that it’s time to eat; Leptin, which is produced in the small intestine and suppresses appetite, lets your brain know when you’re full. But Lac-Phe quickly became a hot source of interest for the researchers, who continued the study by testing how Lac-Phe alone affects the appetites of animals.
To do this, they fed obese mice Lac-Phe and saw their food intake drop by 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 exercise sessions—in this case, running on a treadmill—for several weeks and monitored their eating habits. The results clearly showed that the mice that couldn’t produce Lac-Phe in response to their training sessions ate significantly more food and gained 25 percent more weight than the mice that could produce the molecule.
The results showed that this molecule — which scientists now dub the “anti-hunger” hormone — made the difference between overeating post-workout and not.
See also: What is the best time of day to exercise?
Intensive training for appetite control
Knowing how Lac-Phe affects appetite is one thing, but how can we as humans benefit from this newfound knowledge? Fortunately, the research group also investigated the role of Lac-Phe in humans by testing the blood of eight young men before and after three different types of exercise – leisurely cycling, weight lifting, and then sprinting on a stationary bike. The results showed that the men’s blood Lac-Phe levels increased most after the sprints and least after the long, leisurely bike ride, suggesting that intense exercise might produce more Lac-Phe and surprise appetite better than leisurely physical activity.
If you’ve noticed that you lose your appetite on 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 that fitness and nutrition experts have been watching 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 the appetite to help you escape the threat,” he continues. According to Libby, long workouts, like when you’re an athlete training for something, “can make eating sound like the worst thing in the world.”
However, 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 using a lot of oxygen and burning all your glucose and glycogen, so your body really wants to replace all of that after a workout,” Libby says.
However, according to Libby, this study gives us a better insight into how fitness affects our appetite. He explains: “It also depends very much on the person. Not every appetite will respond to exercise in the same way.”
See also: No time to exercise? Why “Exercise Snacks” Could Be an Easy Solution
How to avoid post-workout cravings
If you find yourself starving after a workout or at the start of an exercise routine, “It’s totally normal to a degree,” says Libby. That’s because you burn calories when you exercise, and if you don’t eat enough to replace those calories, muscle will begin to break down. “You have to give yourself the right energy to increase performance,” he explains.
This study suggests that adding in more gut sports—for example, boxing, long-distance running, or an intense spinning class—could help control your appetite response. Additionally, if you struggle with post-workout hunger, Libby recommends focusing on avoiding high-calorie, nutrient-poor foods like processed foods or fast food. It will contain a lot of unhealthy fats and refined carbohydrates.
Instead, “Eat a good post-workout meal with a low glycemic index,” says Libby. That means protein, healthy fats, and slow-burning carbs like sweet potatoes, 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 reduce appetite since you’re eating some meals and your body isn’t in a fasted state,” says Libby.
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 food intake and obesity. Nature 606, 785-790 (2022). https://doi.org/10.1038/s41586-022-04828-5