You'll learn one basic message from this article, and I'll give it to you right up front. In the "battle of the bulge," you have two weapons--diet and exercise. Although you can jab with exercise, the real knockout blow comes from controlling the kinds of foods you eat. In other words, the power in weight loss comes from mastery of the diet, not adding more exercise, which many scientists now regard as a relatively weak weapon in the never-ending offensive against weight gain.

I'm not urging that you avoid exercise. Not by any means. We have proof that active people live longer, have less heart disease and cancer, healthier bones, less sickness, and enhanced psychological feelings of well-being.

Exercise is wonderful for your health, but don't be fooled into believing that you will lose large amounts of body fat when you begin a regular exercise program. Regular exercise is valuable for your health, but largely ineffective for weight loss. Now let me explain.

Misconceptions about exercise.

I have conducted three major studies with overweight and sedentary women, and have discovered that there are three prevalent misconceptions regarding exercise and weight loss.

Misconception 1: Starting a moderate exercise program will result in hefty decreases in body weight.

Some overweight people have been led to believe that if they start walking briskly for 30 to 60 minutes a day, significant amounts of body weight will be lost quickly. Most research does not support this idea, even when the exercise program lasts several months. For example, in a recent one-year study of 357 men and women at Stanford University, three to five 30- to 40-minute exercise sessions per week had no significant effect on body weight. The subjects did not change their dietary intake. They did experience a 5 to 8 percent improvement in heart and lung fitness. In other words, the exercise made the subjects fitter but not leaner.

In other research, when moderate amounts of aerobic exercise (two to seven hours per week) were combined with a reducing diet, exercise was found to add little to the weight loss brought on by eating less. In fact, researchers now believe that in order for aerobic exercise to have a major effect on weight loss, daily exercise sessions need to be unusually long in duration (more than one hour) and high in intensity, something that most overweight people cannot do without injury and fatigue.

Let me share with you the results of one five-week study I conducted on 21 overweight women. The women ate 1,300 calories a day in a research kitchen and were divided into an exercise group (five 45minute brisk walking sessions per week) and a nonexercise group. Both groups averaged 12 pounds of weight loss after five weeks, with the exercise program having no effect on either the total amount or type of weight lost (fat versus lean body weight, including muscles).

A 12-week study conducted at the Institute for Aerobic Research in Dallas, Texas, in which exercise subjects walked/jogged 30 minutes five times a week showed similar results. Exercise alone had no effect on weight loss, and contributed little to the weight loss effect of the reducing diet (1,200 calories a day).

Why does moderate exercise have little effect on weight loss? The primary reason is that the number of calories burned during exercise is much smaller than many people realize. For example, the extra calories you would burn from walking two miles briskly versus staying at home is only about 90. One pound of human fat contains approximately 3,500 calories. In short, it would take three months of walking three times a week to lose one pound of fat.

Counting exercise calories. When you exercise and are counting calories to burn body fat, you should use the net energy cost of exercise, not the gross (which is what is commonly found in most exercise calorie charts). What is the net energy cost of exercise? This equals the calories expended during the exercise session minus the calories expended for the resting metabolic rate and other activities that would have occupied you had you not been exercising. If these values are not subtracted, you will be disappointed when your body fat does not burn off at the rate you expected. If you exercise one day from 7:00 to 7:30 a.m. but not the next day during that time, you will still burn calories from your metabolism plus any informal activity you engage in.

For example, even though the gross energy cost of a two-mile (30minute) walk for a 150-pound overweight woman is 138 calories, the net energy cost is only 89 calories because the resting metabolism accounts for 34 calories and other informal physical movement counts for at least 15 calories.

The practical advice of all this information is that exercise alone must not be seen as a major weapon in the treatment of obesity. Instead, improvements in the quality and quantity of the diet should take the lead, with exercise relegated to an important supporting role. In 1992 the National Institutes of Health brought together obesity experts for a threeday conference and in their summary statement concluded that "weight loss that can be achieved by exercise programs alone is more limited than that which can be obtained by caloric restriction. However, exercise has beneficial effects independent of weight loss... and can be an important adjunct to other strategies."

Misconception 2: Moderate physical activity raises the resting metabolic rate after exercise, helping you lose weight.

Aerobic exercise does not cause the metabolism to stay elevated for a long time after the bout, thus burning a lot of extra calories.

In general, most researchers have found that the calories expended after aerobic exercise is small and short lived unless you've engaged in a great amount of high-intensity exercise. For example, jogging (12 minutes per mile) or cycling at moderate intensities for about 30 minutes causes the metabolism to stay elevated for 20 to 30 minutes, burning 10 to 12 extra calories. When the intensity is increased (e.g., jogging at 8.5 minutes per mile), the metabolism is increased for about 35 to 45 minutes, with 15 to 30 extra calories expended. Obviously this is too little to have any significant effect on weight loss.

Even in runners and other endurance athletes, the best studies show that their metabolism is no different than in nonathletes, and that after a hard exercise bout (e.g., 80 minutes of running), the metabolism rate may stay up for several hours, burning about 125 extra calories. This is hardly enough to urge that overweight people run long distances every day, a task that few of them could accomplish anyway, especially without injury. The same caloric deficit could be achieved by avoiding one tablespoon of oil.

Misconception 3: Moderate exercise increases the lean body weight.

Some people believe that moderate aerobic exercise can increase the lean body weight (muscle, bone, and water), improving metabolism and counteracting the effects of the reducing diet (during which both the metabolism and the lean body weight fall). In other words, even if aerobic exercise does not result in a greater quantity of total weight loss, the contention is that exercise does enhance the quality of weight loss (more fat loss, more muscle buildup). In general, most studies do not support this argument.

Dr. Joseph Donnelly of Kearney State College in Nebraska put 69 obese females on a 530-calorie liquidformula diet for three months and tested the effects of four hours per week of aerobic exercise on lean body weight and metabolism. Whether subjects exercised or not, all lost an average of 46 pounds, with three fourths of the weight loss measured as fat and one fourth as lean body weight. According to Dr. Connelly, "This study did not identify any advantage of using exercise in combination with a diet over a diet alone, with respect to metabolism, body-weight loss, or the type of weight lost."

In studies I've conducted, overweight women on 1,300-calorie diets tended to lose about two pounds a week, with 90 percent of this loss being fat, and 10 percent being lean body weight. And whether the women exercised or not, both the quantity and quality of the weight loss was similar.

Limiting fat. Many researchers have concluded that the most effective way to control body weight is to control caloric intake, especially fat calories from oils, butter, meat, and dairy fat.

No doubt, the abundant availability of tasty, calorically rich, high-fat foods is the major reason that we have twice as many overweight people today as we did in the year 1900. A consistent finding among many recent studies is that when the intake of dietary fat is high, most people tend to gain weight rather quickly. However, when the intake of dietary fat is low and carbohydrate and fiber is high (from whole grains, fruits, and vegetables), desirable body weight is more easily achieved.