Why are we so peckish after some workouts but
uninterested in eating after others?
In a new study
published June 15 in Nature, an international team of scientists suggest the
answer lies in part in the actions of a single molecule produced after exercise
that blunts hunger. The molecule — found in the
bloodstreams of mice, humans,
and racehorses — turned up in much greater profusion after strenuous workouts
than easy ones, suggesting that exercising hard could be a key to controlling
how much we eat afterward.
اضافة اعلان
The relationship
between fitness and eating is famously prickly. Studies have shown that people
who start working out without also managing their caloric intake typically drop
few if any pounds over time, and may gain weight. Plenty of factors play into
that outcome, including someone’s current fitness, body mass, diet, gender,
genetics, metabolic rate, and even the timing of exercise. Some experiments —
although not all — suggest morning sessions may burn more fat than the same
exertions later in the day.
Appetite
matters, too. If you feel ravenous in the hours after a workout, you easily can
wind up consuming more calories than you burned. But what causes us to feel
hungry — or not — after we exercise has been a bit of a mystery. For decades,
scientists have known various substances, like the hormones leptin and ghrelin,
travel to the brain and prompt us to be more or less interested in eating.
Studies show
exercise alters the levels of these substances, but so do diet and sleeping
habits. Some researchers began to wonder whether there might be some kind of
exercise-specific reaction that influences appetite.
So scientists
from Stanford University School of Medicine,
Baylor College of Medicine, the
University of Copenhagen and other institutions used newly developed techniques
to hunt for molecules that appeared in greater numbers in the bloodstream after
exercise. They began with mice, setting them on tiny treadmills to run at
increasing speeds until they were exhausted. They drew blood before and after
and then compared the levels of thousands of molecules in the rodents’ blood.
One stood out,
increasing more than any other molecule. It had been noted before in a few
studies of metabolism and exercise, but its chemistry and biological role
remained unknown. The scientists found that this new molecule — a mix of
lactate and the amino acid phenylalanine — was created apparently in response
to the high levels of lactate released during exercise. The scientists named it
lac-phe.
The researchers
speculated that lac-phe might have something to do with energy balance after
exercise, since the cells in the blood and elsewhere that create it are broadly
involved in energy intake and body mass. Maybe, they thought, it affects
appetite. To find out, they gave a form of lac-phe to obese mice, which typically
eat with gusto. But their kibble intake dropped more than 30 percent. They were
apparently less hungry with the extra lac-phe.
The researchers
then turned back to exercise. They bred mice that produced little if any
lac-phe and had them run all-out on treadmills five times a week for several
weeks. After each run, the animals were allowed as much high-fat kibble as they
wanted. Normally, running helps mice stave off weight gain, even on a
high-calorie diet. But the animals unable to produce much lac-phe ballooned,
eating more kibble, and gaining about 25 percent more weight than the control
group.
Lac-phe, it
seems, had been a key to how strenuous exercise helped the mice avoid weight
gain. Without it, that same exercise resulted in overeating.
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, 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 people 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.”
His study does
not tell us, though, how lac-phe might be interacting with our brain cells to
affect appetite or how strenuous exercise needs to be to goose lac-phe
formation or how long the molecule’s effects might linger. Also, the human
exercisers were healthy young men, meaning we do not know if lac-phe exists or operates
in the same ways in everyone else.
Still, if you want to be
less hungry after you exercise, you may want to pick up the pace. Throw in some
hills during your next walk or sprint to the far street corner. “What the data
says is that intensity matters” for exercise and appetite control, Long said.
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