Five weeks ago, the headline to this column read "A loss of muscle: Why so many diets fail." The column used hypothetical examples of overweight women losing weight with and without augmenting the loss with exercise and explained why those who exercise have a far better chance of maintaining the weight loss.

Exercise during moderate weight loss insures that the weight lost is primarily fat. Without exercise as much as 50 percent of the weight loss can be muscle.

Losing muscle makes it more likely that the weight lost will be regained for a simple reason. Muscle isn't inert.

It requires calories. Between 50 and 75 calories per day per pound.

That means if you diet foolishly and lose 7 pounds of muscle as part of a 25-pound weight loss, you now must eat at least 350 fewer calories a day forever to maintain that weight loss.

With a restriction like that, is it any wonder why such a high percentage of diets the estimate usually ranges between 90 and 97 percent fail in the long term?

But exercising while you're dieting decreases that rate dramatically. It allows you to keep muscle mass and sometimes even add a bit while losing body fat. If you're able to maintain muscle mass, the number of calories you need to eat a day does not change even when you're five or 10 or 15 pounds lighter.

That's because what you've lost, body fat, is inert. It doesn't need calories to sustain itself.

Researchers working for the National Institute of Diabetes and Digestive and Kidney Diseases recently verified the benefit of exercise during diet by analyzing data accrued on the contestants on the reality show, "The Biggest Loser." The show takes extremely obese individuals and helps them drop weight through a combination of diet and demanding exercise.

When senior investigator Kevin Hall, Ph.D., crunched the data on 11 of the participants, he found that an average of 128 pounds was lost. Of those 128 pounds, nearly 105 of them were fat.

Muscle mass loss was 23 pounds.

Hall then applied a mathematical formula used in the health field to play the "what if?" game. If the contestants would've lost that same amount of weight without the benefit of exercise, Hall determined that the muscle mass loss would've been significantly higher, nearly 45 pounds.

Can you see how the lack of exercise during a diet dooms it afterwards? In this case of a 128-pound weight loss, the dieters who didn't exercise would have to eat about 1000 fewer calories a day to sustain the full weight loss compared to the dieters who did exercise and maintained muscle mass.

So you need to see exercise as an absolutely essential part of weight loss even if you only plan to drop five or 10 or 15 pounds.

And there are other health-related reasons to exercise during weight loss.

Those who diet are usually overweight. Those who are overweight stand a greater chance of developing type 2 diabetes, a disease that increases the incidence of heart disease. Yet a recent study performed at the University of Colorado School of Medicine, showed that even if you develop type 2 diabetes, exercise still reduces the odds of eventual heart problems.

While the findings revealed that those with type 2 diabetes have lower fitness levels and exercise less than people of the same age who don't have the disease, previous studies showed that this decrease in exercise doesn't have to be.

One study found that fitness rates of type 2 diabetics increased as much as 40 percent after as little as 12 weeks of typical exercise. With this sort of improvement, the type 2 diabetic negates the fact that the disease prematurely ages the heart and increases the likelihood of cardiovascular disease.

Amy Huebshmann, speaking for the group of researchers, said, "Type 2 diabetes has a significant negative impact on health, but that impact can be improved with as simple an intervention as regular brisk walking or other physical activity that most people with diabetes can do."

The same idea applies to those non-diabetics who are overweight and need to diet. While being overweight negatively affects health, losing the excess weight negates much of that.

In short, the reason exercise is so important is that it appears to be the constant in weight loss that lasts long term. So if you don't exercise while you diet, the chances are that you'll regain the weight rather quickly with the real threat of a few extra pounds being tacked on.