Everyone knows what it means to be overweight. From early age on, children can tell the difference between "fat" and "thin" people. They also get the clear message that fat people are apt to be less popular. Body weight is one of the most basic issues of human life. Self-esteem, acceptance among peers-- and perhaps lifelong success or failure—are, unfortunately, all tied to our physical appearance.

No one wants to be fat. Thin is in, especially today. The prevalence of slender, even skinny models in advertising is ample evidence of our society's attitudes about body weight. While being overweight is certainly unhealthy, the fear and loathing attached to body fat can also be detrimental when it leads to eating disorders such as anorexia. But consternation over the impact of obsession with thinness on our national psyche, especially where young people are concerned, should not obscure the obvious fact that too many Americans are overweight. Everywhere you look, people are fighting the "Battle of the Bulge." And it is certainly a positive trend that many of us seem more health-conscious these days.

Medically speaking, not all overweight people are obese. Obesity is defined as weight that exceeds 15 percent of normal weight for height and body type. "Morbid" obesity exceeds 20 percent of optimum weight. The long-term health implications are well known, in fact, obesity is considered an outright disease. Life expectancy may be decreased in overweight and obese individuals.(1) An obese person is at high risk for a number of serious health problems, including heart disease, high blood pressure, stroke, varicose veins, psychological stress, osteoarthritis, high cholesterol, and diabetes.

The detrimental health effects of obesity are more than just a matter of weighing too much. Body composition--the amount of fat in the body compared to the amount of lean muscle--is also important.

While obesity is clearly a health threat, we should not lose sight of the fact that we need fat to stay healthy. Body fat performs many vital functions: fat provides readily accessible energy during short periods of fasting and it serves a structural component of organs, the nervous system, the brain, and muscles. We require fat in the diet to absorb the fat-soluble vitamins: vitamins A, D, E, and K.

The distribution of body weight is a key part of the equation. Excess abdominal fat in particular has been shown to increase the risk of some diseases. "Apple" shaped women--women with 30 inch or larger waists and more or less equal waist and hip measurements--appear more susceptible to heart disease and diabetes than "pear" shaped women.(2) The most harmful fat is the deep "visceral" fat that cushions the abdominal organs. Visceral fat is linked to higher levels of LDL cholesterol--the plaque-forming cholesterol--and lower levels of the good HDL cholesterol. Excess fat puts an extra burden on the liver, perhaps making it less able to metabolize cholesterol effectively.(3) Too much visceral fat also favors high blood sugar and increases the risk of diabetes.(3)

Body weight and composition are to a large degree determined by the "basal metabolic rate" (BMR), the amount of energy the body burns while at rest. The BMR is a measure of how efficiently the body converts calories into heat. It comes down to the basic laws of thermodynamics: energy consumed as calories must equal energy used up. If not, we gain weight.

Why is exercise so important? Yes, we burn calories through physical activity. But there is more to it than that. Exercise builds lean muscle. As the ratio of lean muscle to body fat increases, so does the BMR. The higher our BMR, the more calories we burn. A safe and effective weight-loss program is one that combines calorie cutting with exercise to maintain lean muscle mass.

Body function