Study Shows Warmer Workouts Reduce Appetite

If you’re hoping that exercise might keep you from gaining weight this holiday season, you may want to dial up the thermostat and do your workout indoors. According to a surprising new study, exercising in chilly temperatures could undermine dieting willpower.

By now, most of us have heard or discovered for ourselves that exercise is an unreliable means of controlling weight. After starting an exercise program, some people lose a pound or two, but others don’t lose weight, and many add body fat.

Why exercise affects people so differently in terms of weight control is uncertain. Scientists know that exercise generally increases appetite, so that many people consume more after a workout than they incinerate during it. But not all people overeat after workouts.

Noting these inconsistencies, researchers at the University of Aberdeen in Scotland and the University of Birmingham in England began to wonder whether the ambient temperature in which people exercise might affect their appetite and eating habits afterward.

It has long been known that exercising in cold water, as people do when swimming, tends to ignite appetite afterward more than a comparable amount of exercise on land. Many researchers had assumed that swimming itself was the culprit — that something about paddling prone in chilly water left people ravenous.

But the British researchers wondered if the real issue was the chilly conditions and not the physical positions assumed during swimming. So they decided to see what happened to appetite when people exercised upright in alternately chilly and pleasant room temperatures.

They began by recruiting a group of overweight, sedentary men and women, as overweight adults typically have the greatest difficulty losing weight with exercise alone.

They took their volunteers into the lab to determine their resting metabolic rate, maximum endurance capacity and blood levels of certain hormones related to appetite.

Then they asked each man and woman to walk on a treadmill at a moderate pace, representing about 60 percent of each person’s maximum aerobic capacity, for 45 minutes, wearing the same clothes each time.

On one occasion, the thermostat of the room in which the volunteers walked was set to a pleasant 68 degrees.

During a second workout, the room was cooled to 46 degrees.

On both occasions, the room’s humidity was a negligible 40 percent.

Throughout each walk, researchers tracked the volunteers’ skin and core temperatures and how much energy they were using. The scientists also periodically asked the walkers how hard or easy the exercise felt and whether they currently felt warm or chilled.

At the end of each session, the volunteers sat quietly for 45 minutes while the researchers drew blood to check for appetite hormones. Then the volunteers were directed to help themselves at a large food buffet.

They were not told that their food selections and portions would be monitored.

But they were. The scientists noted how many calories and what types — whether from carbohydrates, protein or fats — each volunteer ate after each exercise session.

And then they compared those selections. Almost all of the walkers consumed significantly more calories and, in particular, more carbohydrates after they had been walking in the cold than when they had strolled in the more temperate room.

Most of those who exercised in the cold also showed higher blood levels of a hormone called ghrelin that is known to spark hunger. There was little change in ghrelin levels after the warmer exercise.

Over all, the volunteers felt more ravenous after working out in the cold and loaded their plates with more food than when they had been warm during their workout.

But they had not burned more calories during the exercise session in the cold. In fact, they had expended significantly fewer calories than walking while warm.

This finding seems to fly in the face the idea that exercising in the cold requires lots of energy, because the body must heat itself.

In fact, the researchers conclude, warm temperatures demand more from the body, because it must dissipate any buildup of internal heat. Blood flows away from the stomach and limbs and toward the skin surface so that the excess heat can be released.

When you exercise in cooler conditions, said Daniel Crabtree, a research fellow at the University of Aberdeen, who led the study, “you don’t have to pump blood to the surface to dissipate heat.” The blood instead circulates normally, picking up and distributing biochemical signals from the stomach and elsewhere that apparently prompt the release of ghrelin, augmenting appetite and undercutting your best intentions to forgo that cupcake after exercise.

Of course, this experiment was short term and involved a small, particular group of volunteers. Based on this data, “we can’t say” whether younger, thinner or more active people’s bodies would respond similarly, Dr. Crabtree said, or whether more-intense exercise or more-extreme temperatures would cause different effects on appetite. But his study does persuasively suggest that if you want to control yourself around holiday buffets, you should work out somewhere warm first.