Exercise by itself won't help you lose weight.

This is not to say that exercise isn't good for you; it is, in fact, great for you. It conveys an astonishing array of health benefits.

But - and we all hate hearing this - many experts, while extolling the benefits of exercise, say the primary villain when it comes to excess weight is what's on our menu. To lose weight, we have to cut calories.

Exercise helps keep lost pounds off, but exercise alone can't do the initial job of losing it.

"I think the role of exercise in weight loss is highly overrated," says Marc Reitman, chief of the diabetes, endocrinology and obesity branch of the National Institute of Diabetes and Digestive and Kidney Diseases, or NIDDK. "I think it's really great for being healthy, but I'm a strong believer that overeating is what causes obesity. To exercise your way out of overeating is impossible."

Michael Joyner, a Mayo Clinic researcher who studies how people respond to the stress of exercise, agrees. "The key for weight loss is to generate and maintain a calorie deficit," he says. "It's pretty easy to get people to eat 1,000 calories less per day, but to get them to do 1,000 calories per day of exercise - walking 10 miles - is daunting at many levels, including time and motivation," he says.

To be sure, some people can work weight off, experts say. These include those who exercise vigorously for long periods, and professional athletes, who typically engage in high-intensity workouts.

But they are the exceptions. Those high-level workouts are "not something most people do," says Philip F. Smith, co-director of NIDDK's office of obesity research. "Walking for an hour won't do it."

Joyner agrees. "Theoretically, people can exercise enough to lose without changing what they eat, but they have to exercise a whole lot," he says.

Moreover, moderate exercise doesn't really burn all that many calories, especially when you think about a single piece of chocolate cake, which has between 200 and 500 calories. Most people burn only about 100 calories for every mile of running or walking, although this can vary depending on the person, according to Joyner. Put another way, to lose one pound, you must run a deficit of about 3,500 calories - meaning that if you burn an excess 500 calories a day, it would take a week to drop that pound.

Kevin D. Hall, an NIDDK scientist who studies how metabolism and the brain adapt to diet and exercise, agrees that a modest degree of weight loss would require large amounts of exercise. However, "high levels of physical activity seem to be very important for maintenance of lost weight," he adds, defining "high" as more than an hour of exercise daily.

In a recent study, Hall concluded that exercise "typically result[s] in less average weight loss than expected, based on the exercise calories expended," and that individual weight changes "are highly variable" even when people stick to exercise regimens.

The likely reason is that people tend to compensate for changes in food intake and non-exercise physical activities, Hall wrote. Or, as Joyner puts it: "If people replace non-exercise - but otherwise active - time with sedentary time, sometimes things cancel out."

Strength training or resistance training - lifting weights, for example - also is important for overall health, but, as with other forms of exercise, it doesn't prompt weight loss. (In fact, it may cause the reading on the scale to inch up a bit, because muscle is denser than fat.) Nevertheless, "strength training is good to maintain lean tissue," Joyner says.

And you can't count on exercise to increase your metabolism for several hours afterward.

"Exercise, if hard enough and long enough, certainly can do this," Joyner says. "But again, it depends on how much, what type and how hard. A two-mile stroll, while a good thing, will not do too much to resting metabolism."

But now the good news: Exercise remains one of the best things you can do for yourself. It enhances health in numerous ways.

It strengthens the heart and lungs. It reduces the risk of Type 2 diabetes and metabolic syndrome, a collection of symptoms that include hypertension, high blood sugar, excess body fat around the waist and abnormal cholesterol or triglyceride levels.

Weight-bearing activities, such as running, strengthen bones and muscles. Having strong bones prevents osteoporosis, helping to avert bone-breaking falls in the elderly. "For older people, exercise facilitates the capacity for them to stay engaged in life," Joyner says.

Exercise also reduces the risk of certain cancers, including breast and colon cancer. It elevates mood, and it keeps thinking and judgment skills sharp.

Overall, it helps you live longer. People who work out for about seven hours a week have a 40 percent lower risk of dying early compared with those who exercise less than 30 minutes a week, according to the Centers for Disease Control and Prevention.

"Exercise in almost any dose does so many good things for people," Joyner says.

Is one exercise more effective than another?

"I love to play soccer," Smith says. "I would do anything to play soccer, and try to play three times a week until my body can't take it. But people should exercise as much as they can tolerate and enjoy. That's what they should shoot for."

Reitman agrees. "The best exercise is the one you keep doing," he says.