Finally, the researchers checked for lac-phe in other exercising creatures. They first found it in the bloodstreams of racehorses at much higher levels after a hard run than before. Then they asked eight healthy young men to exercise three times: once by cycling at a leisurely pace for 90 minutes, another time lifting weights and a third with several 30-second sprints on a stationary bike. Blood levels of lac-phe peaked after each type of exercise, but they were highest after the sprints, followed by weight training. The prolonged, gentle exercise produced the least.
In other words, the more intense the exercise, the more lac-phe was produced and, at least in mice, the more appetite seemed to fall.
“The results are fascinating and add a new dimension to our thinking about exercise and body-weight regulation,” said Richard Palmiter, a professor of biochemistry at the University of Washington in Seattle and an expert in the neurobiology of behavior who was not part of the new study.
“We always knew that our current menu of molecules that appear to regulate appetite and food intake, such as leptin, ghrelin, etc., was incomplete and this new metabolite/signaling molecule is a potentially important addition to that list,” said Barry Braun, the executive director of the Human Performance Clinical Research Lab at Colorado State University in Fort Collins, who studies exercise and weight control. He was not involved in the new study.
Assuming this process does work the same in humans as in mice, the discovery of lac-phe provides a useful lesson. If we want to avoid bingeing after a workout, we may need to increase the intensity, said Jonathan Z. Long, a professor of pathology at Stanford University School of Medicine and senior author of the new study.
This idea makes intuitive and evolutionary sense, he added. “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.”