April 17, 2010

HOMEOSTASIS QUO:

Weighing the Evidence on Exercise (GRETCHEN REYNOLDS, 4/12/10, NY Times Magazine)

[A]s many people have found after starting a new exercise regimen, working out can have a significant effect on appetite. The mechanisms that control appetite and energy balance in the human body are elegantly calibrated. “The body aims for homeostasis,” Braun says. It likes to remain at whatever weight it’s used to. So even small changes in energy balance can produce rapid changes in certain hormones associated with appetite, particularly acylated ghrelin, which is known to increase the desire for food, as well as insulin and leptin, hormones that affect how the body burns fuel.[...]

Thankfully there has lately been some more encouraging news about exercise and weight loss, including for women. In a study published late last month in The Journal of the American Medical Association, researchers from Harvard University looked at the weight-change histories of more than 34,000 participants in a women’s health study. The women began the study middle-aged (at an average of about 54 years) and were followed for 13 years. During that time, the women gained, on average, six pounds. Some packed on considerably more. But a small subset gained far less, coming close to maintaining the body size with which they started the study. Those were the women who reported exercising almost every day for an hour or so. The exercise involved was not strenuous. “It was the equivalent of brisk walking,” says I-Min Lee, a researcher at Harvard Medical School and Brigham and Women’s Hospital and the lead author of the study. But it was consistently engaged in over the years. “It wasn’t something the women started and stopped,” Lee says. “It was something they’d been doing for years.” The women who exercised also tended to have lower body weights to start with. All began the study with a body-mass index below 25, the high end of normal weight. “We didn’t look at this, but it’s probably safe to speculate that it’s easier and more pleasant to exercise if you’re not already heavy,” Lee says.

On the other hand, if you can somehow pry off the pounds, exercise may be the most important element in keeping the weight off. “When you look at the results in the National Weight Control Registry,” Braun says, “you see over and over that exercise is one constant among people who’ve maintained their weight loss.” About 90 percent of the people in the registry who have shed pounds and kept them at bay worked out, a result also seen in recent studies. In one representative experiment from last year, 97 healthy, slightly overweight women were put on an 800-calorie diet until they lost an average of about 27 pounds each. Some of the women were then assigned to a walking program, some were put on a weight-training regimen and others were assigned no exercise; all returned to their old eating habits. Those who stuck with either of the exercise programs regained less weight than those who didn’t exercise and, even more striking, did not regain weight around their middles. The women who didn’t exercise regained their weight and preferentially packed on these new pounds around their abdomens. It’s well known that abdominal fat is particularly unhealthful, contributing significantly to metabolic disruptions and heart disease.

Scientists are “not really sure yet” just how and why exercise is so important in maintaining weight loss in people, Braun says. But in animal experiments, exercise seems to remodel the metabolic pathways that determine how the body stores and utilizes food.

Posted by Orrin Judd at April 17, 2010 6:42 AM
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