Eric Ravussin, a physiologist at the Pennington Biomedical Research Center in Baton Rouge, La., conducted a small, randomized, controlled clinical trial on the issue. Ravussin divided three dozen overweight but healthy men and women into three groups. One group reduced their calorie intake by 25 percent. Another group cut calories by half as much (12.5 percent) while increasing energy output through exercise by 12.5 percent; and the third group made no diet or exercise changes.
The researchers examined weight loss, body composition, and measures of superficial and deep fat.
What they found was that it doesn't matter whether people lose weight by diet or by exercise or a combination, although exercise has the important benefit of improving cardiovascular health.
"So long as the energy deficit is the same, body weight, fat mass, and abdominal fat will all decrease the same way," says Ravussin. In other words, "...basically, it's the net calorie deficit - expending more energy than you consume - that counts", he explains.
At the same time, William J. Evans, director of the Nutrition, Metabolism and Exercise Laboratory at the University of Arkansas for Medical Sciences vouches that it is a good idea to include exercise in any weight control program.
"When you exercise during weight loss, you lose more fat from the visceral stores - around the waist because this fat is very metabolically active and turns over more rapidly," he says. Evidence exists to show that this fat is used first by the body.
In another study published in Journal of the American Dietetic Association, researchers found that old-fashioned calorie-cutting and exercise really can keep the pounds off for the long haul, according to a review of dozens of clinical trials.
In an analysis of 80 weight-loss studies, researchers found that approaches that focused on trimming calories -- with or without exercise -- were most effective at keeping the pounds off over four years.
The results were not dramatic. On average, participants in these studies shed 11 to 19 pounds at most, then typically gained a little bit back over time.
However, the findings show that diet and exercise changes can work over the long haul, if people keep them up and have realistic expectations, the study authors report ."Although there is some regain of weight, weight loss can be maintained," write the researchers, led by Marion J. Franz, a registered dietitian and health consultant with Minneapolis-based Nutrition Concepts by Franz Inc.
The studies Franz and her colleagues analyzed ranged in their weight-loss tactics. In some, participants were given only general advice on cutting pounds. In others, they received exercise advice or actual help with boosting their physical activity levels, but no help with diet.
Among trials that focused on diet, some emphasized calorie reduction alone, and some used a combination of diet and exercise. In certain studies, participants were given meal replacements or weight-loss medications like orlistat (Xenical) to enhance their diet changes.
In general, Franz's team found, diet-focused trials were most successful. Advice-only and exercise-only studies produced "minimal" weight loss, the researchers write.