Many Americans are at increased health risk because they are obese. The U. S. Surgeon General, in a 1988 report on nutrition and health, estimated that one-fourth of adult Americans are overweight. Obesity is a known risk factor for chronic diseases including heart disease, diabetes, high blood pressure, stroke, and some forms of cancer. Everyone needs a certain amount of body fat for stored energy, heat insulation, shock absorption, and other functions. As a rule, women have more fat than men.
Doctors generally agree that men with more than 25 percent body fat and women with more than 30 percent body fat are obese. Precisely measuring a person’s body fat, however, is not easy. The most accurate method is to weigh a person underwater – a procedure limited to laboratories with sophisticated equipment. There are two simpler methods for estimating body fat, but they can yield inaccurate results if done by an inexperienced person or if done on someone with severe obesity.
One is to measure skinfold thickness in several parts of the body. The second involves sending a harmless amount of electric current through a person’s body (bioelectric impedance analysis). Both methods are commonly used in health clubs and in commercial weight-loss programs, but results should be viewed skeptically. Because measuring a person’s body fat is tricky, doctors often rely on other means to diagnose obesity.
Two widely used measurements are weight-for-height tables and body mass index. While both measurements have their limitations, they are reliable indicators that someone may have a weight problem. They are easy to calculate and require no special equipment. One problem with using weight-for-height tables is that doctors disagree over which is the best table to use. Many versions are available, all with different weight ranges.
Some tables take a person’s frame size, age, and sex into account; others do not. A limitation of all weight-for-height tables is that they do not distinguish excess fat from muscle. A very muscular person may appear obese, according to the tables, when he or she is not. Still, weight-for-height tables can be used as general guidelines. Doctors are concerned with not only how much fat a person has but where the fat is on the body.
Women typically collect fat in their hips and buttocks, giving their figures a “pear” shape. Men, on the other and, usually build up fat around their bellies, giving them more of an “apple” shape. This is not a hard and fast rule, though. Some men are pear-shaped and some women become apple-shaped, especially after menopause. People whose fat is concentrated mostly in the abdomen are more likely to develop many of the health problems associated with obesity. In scientific terms, obesity occurs when a person’s calorie intake exceeds the amount of energy he or she burns.
What causes this imbalance between consuming and burning calories is unclear. Evidence suggests that obesity often has more than one cause. Genetic, environmental, psychological, and other factors all may play a part. Genetic Factors Obesity tends to run in families, suggesting that it may have a genetic cause. However, family members share not only genes but also diet and lifestyle habits that may contribute to obesity. Separating these lifestyle factors from genetic ones is often difficult.
Still, growing evidence points to heredity as a strong determining factor of obesity. In one study of adults who were adopted as children, researchers found that the subjects’ adult weights were closer to their biological parents’ weights than their adoptive parents’. The environment provided by the adoptive family apparently had less influence on the development of obesity than the person’s genetic makeup.
Environmental Factors Although genes are an important factor in many cases of obesity, a person’s environment also plays a significant part. Environment includes lifestyle behaviors such as what a person eats and how active he or she is. Americans tend to have high-fat diets, often putting taste and convenience ahead of nutritional content when choosing meals. Most Americans also don’t get enough exercise.