The National Institutes of Health considers obesity a killer disease, and for good reason. A person who is obese--defined as 20% above ideal weight based on population statistics considering age, sex, and build--is at higher risk for diabetes, digestive disorders, heart disease, kidney failure, hypertension, stroke, and cancers of the female reproductive organs and the gallbladder. The body is enormously strained to support the extra weight--miles of blood vessels are needed to nourish the additional pounds.

Obesity refers specifically to extra pounds of fat. The proportion of fat in a human body ranges from 5% to more than 50%, with normal for males falling between 12% and 23% and for females between 16% and 28%. An elite athlete may have a body fat level as low as 4%. Fat distribution also affects health. Excess poundage above the waist is linked to increased risk of heart disease, diabetes, hypertension, and lipid disorders.

Both heredity and the environment contribute to obesity. We inherit genes that control metabolism, but the fact that identical twins reared in different households can grow into adults of vastly different weights indicates that environment influences weight too. Studies comparing body mass index between adult identical twins reared apart indicate that weight is about 70% influenced by genes and 30% by the environment.

A safe goal for weight loss is 1 pound of fat per week. A pound of fat contains 3,500 calories of energy, so that pound can be shed by an appropriate combination of food restriction and exercise. This might mean eating 500 calories less per day or exercising off 500 calories each day. Actually more than a pound of weight will drop because water is lost as well as fat.

Calorie cutting should apply to the energy-providing nutrients (carbohydrates, proteins, and fats) but never to the vitamins and minerals. A rule of thumb is to leave the proportion of protein calories about the same or slightly increased, cut fat calories in half, and cut carbohydrates by a third. Choose foods that you like, and distribute them into three or four balanced meals of 250 to 500 calories each.

Diet plans abound. Many are simply variations on the preceding suggestions, with a gimmick added, such as a cup of chili or an ice cream cone each day. Other diets emphasize a particular food, such as bananas, pineapple, or rice. Avoid diets that are based on erroneous but impressive-sounding scientific principles, such as the immune power diet that predicts that the effects of certain food extracts on isolated white blood cells will be echoed in the body, or diets that suggest that the order in which foods are eaten influences how many calories are absorbed. Also avoid diets very low in carbohydrates (these deplete energy), diets high in protein (these strain the kidneys), and diets with less than 1,200 calories per day, which is starvation level.

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