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Dr. Robert Keith finds himself repressing a grin whenever he runs across an article purporting to reveal some startling new nutrition or fitness finding — which is often.

As an example, he points to Gretchen Reynolds's recent article on exercise in the New York Times.

Most everything she explores in the article is valid; it's just not new or startling by any measure, Keith says. As a matter of fact, he and other nutrition and fitness experts have been aware of the hard realities of diet and exercise for a long time.

The hard reality goes something like this: There are many compelling reasons to exercise— exercise is indisputably a good thing — but in terms of causing significant weight loss, it's not as effective as calorie reduction.

Simply put, if you're determined to lose lots of weight, your best strategy is to reduce calorie intake rather than depending exclusively on exercise as a weight-loss strategy.

The numbers tell the story, Keith says.

"Losing about 50 pounds over a six-month period will require you to lose about 2 pounds a week," Keith says. "Reducing a normal 2,800 calorie-a-day diet to an 1,800-calorie diet will get you there — in other words, I can reduce your caloric intake by roughly about 1,000 calories a day, and you're likely to lose the weight."

Keith readily concedes that caloric reduction is a big challenge for some people. Yet, shedding this weight through exercise alone would present most people with an even more daunting challenge.

For example, moderate exercise for about 10 minutes burns approximately 100 calories for an average-sized person. So, to create the same effects provided by an 1,800-calorie diet would require unusually strenuous exercise — the equivalent of running ten miles a day, he says.

"Yes, it can be done, but for the vast majority of people, it's just not an attractive or workable solution," says Keith. "Granted, I've known a few people who have become marathoners and achieved substantial weight loss, but generally speaking, exercise plays a lesser role than calorie reduction in most successful weight-loss strategies."

On the other hand, exercise has proven to be a valuable component of weight-loss maintenance.

"If you do the math and consider how we gain weight, you can see how exercise can serve a valuable role," says Keith.

As he observes, people do not gain large amounts of weight over a few weeks or even months. Most large weight gains typically occur over years.

"For example, consuming roughly 100 more calories a day beyond what you're able to burn will translate into a 10-pound-a-year weight gain," Keith says.

This is where the value of exercise as a weight-maintenance strategy proves especially useful, he says.

"If you're burning an extra 100 calories from short bursts of very strenuous exercise or longer stretches of more moderate exercise, you're getting rid of these extra daily calories that contribute to long-term weight gain," he says. For example, running or walking only a mile a day would do the job for weight maintenance.

The bottom line: Moderate exercise is very important in weight maintenance, but very problematic if you're depending on it exclusively to lose large amounts of weight over several months.

In fact, Keith says this emphasis on exercise as an integral part of a weight-maintenance regimen is consistent with the experiences of people who have successfully shed between 30 and 50 pounds and kept it off for one or more years.

Of course, there are plenty of other compelling reasons to exercise besides weight maintenance, Keith says, citing the effects it plays in improving cardio-vascular health and mental outlook and staving off chronic diseases such as hypertension and diabetes.

More rigorous levels of exercise also appear to produce hormonal changes in the body that promote metabolic changes which, in turn, contribute to weight maintenance.