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Obesity experts agree that daily exercise is essential for good health, but whether it can successfully lead to long-term weight loss is a question of much debate. What has become increasingly clear, however, is that the conventionally accepted advice — 30 minutes of moderate-intensity activity most days of the week — is probably insufficient to spur any real change in a person's body weight. A study published July 28 in the Archives of Internal Medicine adds to the burgeoning scientific consensus: when it comes to exercise for weight loss, more is better. It suggests that obese people would have to exercise at least an hour at a time to see any significant difference in their weight.

The study, led by John Jakicic at the Physical Activity and Weight Management Research Center at the University of Pittsburgh, followed nearly 200 overweight or obese women ages 21 to 45 through a two-year weight-loss program. The women were given free treadmills to use at home, regular group meetings and telephone pep talks to help keep them on track. Participants were also asked to restrict their food intake to between 1,200 and 1,500 calories per day, and were randomized to one of four physical activity intervention groups based on energy expenditure (either 1,000 calories or 2,000 calories burned per week) and exercise intensity (high vs. moderate). By the end of the 24-month intervention, the women who managed to lose at least 10% of their starting body weight (which was, on average, about 193 lbs.) — and keep it off — were exercising twice as long as health authorities typically recommend and expending more than twice as many calories through exercise as women who had no change in body weight. The biggest weight losers were active a full 68 minutes a day, five days a week (about 55 minutes a day more than they had been before the trial began), burning an extra 1,848 calories a week.

Jakicic and his colleagues originally designed their study to measure whether weight loss could really be achieved and maintained through moderate-intensity exercise, akin to "walking when you're late for a meeting," he says, or whether it was preferable to engage in shorter bursts of more vigorous-intensity activity, "like, when you're late for the bus, chasing it down." The problem was that not enough of the women stuck with their assigned exercise categories for the researchers to gather enough meaningful data. Within a few months, most of the participants had resorted to exercising as much as they chose to. That left researchers with a slightly different data set than they had planned for, but they were still able to associate women's reported physical activity with their weight loss. Indeed, exercise was more strongly associated with weight loss than any other factor, including diet. Overall, the more the women exercised, the more weight they lost.

More than half of the study participants managed to lose at least 10% of their body weight within the first six months. At the half-year mark, however, most of those women relapsed and started gaining the weight back — a discouragingly common phenomenon. "The major outcome of this paper is the maintenance issue," Jakicic says. Once a patient hits her target weight, he says, it's imperative that she stick with her exercise and diet regimen to maintain her new weight.

Still, the underlying question remains: are diet and exercise a reliable cure for obesity? Modern-day obesity researchers are skeptical — achieving thinness, they say, is not simply a matter of willpower. Research suggests that weight may largely be regulated by biology, which helps determine the body's "set point," a weight range of about 10 lbs. to 20 lbs. that the body tries hard to defend. The further you push you weight beyond your set point — either up or down the scale — some researchers say, the more your body struggles to return to it. That might help to explain why none of the women in Jakicic's study managed to lose much more than 10% of their body weight. After two years on a calorie-restricted diet, keeping up more than an hour of physical activity five days a week on average, most were still clinically overweight (though much less so than before). But what Jakicic and other obesity researchers stress is that a 10% reduction in body weight represents a tremendous boon for overall well-being, lowering blood pressure, improving heart health and reducing the risk of Type 2 diabetes. For the obese, the end goal should not be thinness, but health and self-acceptance, which are more realistic and beneficial objectives. "The women's health was absolutely improved," Jakicic says.

Jakicic, in fact, seems heartened by his findings. "I think the beauty of this study is that we now have a target" — a better idea of how much exercise is needed for weight maintenance. There is, of course, some variation in how people respond. Some of the study participants fared well with less exercise than the additional 275 minutes per week (about 55 minutes per day, five days a week) that the study's author now recommends for weight maintenance. Others needed more. But the keys to success, according to Jakicic, were embracing the weight-loss program fully, and finding a way around the daily obstacles to exercising — that's something he says many of his participants were able to achieve, regardless of their socioeconomic group. So, if you're aiming to lose weight and keep it off, his message is clear: don't slack off.

• Laura Blue