I love food. As far as watching my weight goes, I’d choose to squeeze in an extra workout the next morning instead of turning down a late-night slice of pizza.

Other people are exactly the opposite; they’d much rather restrict their diets than set foot on a treadmill. Either way, it’s a common quandary for anyone trying to lose a few pounds: Should I join a gym…or go on a diet? A new study reveals that there’s no easy answer—and to reap the health benefits of weight loss, you must do both.

Researchers at the University of Missouri tested two groups of overweight, sedentary women head-to-head. One group was given a 12-week membership to Weight Watchers, the other to a local fitness club. Their goal was to measure not just overall weight lost during that time, but also body-fat loss, especially dangerous abdominal fat.

Weight Watchers members lost an average of 5% of their body weight, or about nine pounds each, while the number on the scale for the fitness group didn’t budge. This confirms previous research that, in the short-term, cutting calories is a better weight management tool than exercise alone.

But much of the weight lost by the Weight Watchers group was lean muscle mass and not fat. (This is one of the first studies to track exactly what kind of weight is typically lost on a Weight Watchers plan.)

“Without adequate exercise, your body tends to get rid of lean tissue before fat,” lead researcher Steve Ball, PhD, an exercise physiologist, tells me. “But lean tissue is associated with higher metabolism, and with less of it, your metabolism could actually slow. So there are pros and cons: Yes, these women lost weight and stuck with the program, but they really may not have made themselves much healthier.” In other words, the fitness group might be bigger weight “losers” over time.

Though the fitness club group didn’t lose weight overall, it nearly matched the Weight Watchers group in the amount of abdominal fat lost—which means that these women still decreased their risk of heart disease, diabetes, and even dementia.

Drop-out rates were twice as high in the fitness club group—one possible reason members saw fewer health benefits than expected. While the Weight Watchers subjects attended weekly weigh-ins and meetings (and received information about fitness and exercise), gym members received three sessions with a personal trainer, and then were left to complete the remainder of the visits on their own.

“We could have monitored the workouts more closely or forced the women to exercise in the lab, but we wanted to duplicate a real-life scenario,” says Ball. “The gym can be a really intimidating place, and without guidance, a lot of new members will drop out.”

That doesn’t mean that joining a gym is a bad idea, Ball notes. But getting support similar to what Weight Watchers offers in its weekly meetings, like joining with a friend or taking group exercise classes, will likely increase your chances of success. (Finding the right club won’t hurt, either.) And if weight loss is your primary goal, you’ll probably need a healthy eating plan too.

As for my pizza/exercise trade-off, I realize that I shouldn’t just work out extra hard to justify my bad eating habits. Likewise, a person shouldn’t go on a diet and then sit around all day, expecting to slim down and get healthy. You need smart food choices to jump-start your weight loss—but you also need physical activity to boost your stamina, strength, flexibility, and other markers of good health.

By Amanda MacMillan

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(PHOTO: CORBIS)