New study explains why you can’t eat after a hard run

Have you ever envisioned the perfect meal during a long run or race, only to have your appetite vanish when it’s time to refuel? There is evidence to suggest that appetite may be suppressed after a hard workout, due to alterations in hunger hormones and metabolism. In a recent study in Nature, researchers at Stanford University, Baylor College of Medicine in Houston and elsewhere identified a metabolite molecule that’s released into the blood during exercise, and that’s associated with appetite suppression.

The researchers analyzed blood samples from mice, racehorses, and humans after intense exercise—for humans, this was a treadmill running test to exhaustion. They noticed an increase in the circulation of a metabolite called N-lactoyl-phenylalanine (Lac-Phe) in both animals and humans. 

The study also examined the effect of different exercise modalities on circulating Lac-Phe. A second group of volunteers underwent three different exercise trials: a sprint exercise trial on a stationary bike, an endurance exercise trial on a stationary bike, and a resistance exercise trial consisting of leg extensions. All three types of exercise produce a substantial increase in the Lac-Phe metabolite, but the increase was most pronounced with the sprint exercise. 

Prior to this study, the Lac-Phe metabolite was not well studied. It is formed through an enzymatic reaction between the amino acid phenylalanine and lactate, an end-product of anaerobic respiration that occurs during intense exercise. Its function, however, was largely unknown.

In this study, when the scientists administered Lac-Phe to obese mice fed a high-fat diet, they consumed less food than usual over the next 12 hours. Over 10 days, their food consumption and weight dropped, and their glucose homeostasis improved. Interestingly, Lac-Phe did not have a similar effect in lean mice eating a low-fat diet. 

Does this mean intense exercise will lead to weight loss? Not exactly. The study has identified a potential mechanism by which exercise can contribute to weight loss in obese mice. Further studies will be needed to investigate whether these effects on appetite translate to humans. But it may start to explain why that post-race feast sometimes loses its appeal.