Understanding Adult Obesity

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.

This fact sheet provides basic information about obesity: what it is, what causes it, how to measure it. Companion fact sheets provide more in-depth information about some aspects addressed briefly here, such as health risks of obesity and treatment options for the condition.

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.

Most people are familiar with weight-for-height tables. Doctors have used these tables for decades to determine whether a person is overweight. The tables usually have a range of acceptable weights for a person of a given height.

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.

The table printed here is from the 1990 edition of Dietary Guidelines for Americans, a pamphlet printed jointly by the U.S. Departments of Agriculture and Health and Human Services. This table has a wide range for what the pamphlet calls "healthy" or "suggested" weights.

In this table, the higher weights generally apply to men, who tend to have more muscle and bone. The lower weights more often apply to women, who have less muscle and bone. The table also shows higher weights for people age 35 and older, which some experts question.

Body mass index, or BMI, is a new term to most people. However, it is the measurement of choice for many physicians and researchers studying obesity. BMI uses a mathematical formula that takes into account both a person's height and weight. BMI equals a person's weight in kilograms divided by height in meters squared. (BMI = kg/m2). The table printed here has already done the math and metric conversions. To use the table, find the appropriate height in the left-hand column. Move across the row to the given weight. The number at the top of the column is the BMI for that height and weight.

In general, a person age 35 or older is obese if he or she has a BMI of 27 or more. For people age 34 or younger, a BMI of 25 or more indicates obesity. A BMI of more than 30 usually is considered a sign of moderate to severe obesity.

The BMI measurement poses some of the same problems as the weight-for-height tables. Doctors don't agree on the cutoff points for "healthy" versus "unhealthy" BMI ranges. BMI also does not provide information on a person's percentage of body fat. However, like the weight-for-height table, BMI is a useful general guideline.

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 hand, 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.

Doctors have developed a simple way to measure whether someone is an apple or a pear. The measurement is called waist-to-hip ratio.

To find out someone's waist-to-hip ratio, measure the waist at its narrowest point, then measure the hips at the widest point. Divide the waist measurement by the hip measurement. A woman with a 35-inch waist and 46-inch hips would do the following calculation:

35 ÷ 46 = 0.76

Women with waist-to-hip ratios of more than 0.8 or men with waist-to-hip ratios of more than 1.0 are "apples." They are at increased health risk because of their fat distribution.

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.

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.

Nevertheless, people who feel that their genes have doomed them to a lifetime of obesity should take heart. As discussed in the next section, many people genetically predisposed to obesity do not become obese or manage to lose weight and keep it off.

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.

People can't change their genetic makeup, of course, but they can change what they eat and how active they are. Some people have been able to lose weight and keep it off by:

Psychological factors also may influence eating habits. Many people eat in response to negative emotions such as boredom, sadness, or anger.

While most overweight people have no more psychological disturbance than normalweight people, about 30 percent of the people who seek treatment for serious weight problems have difficulties with binge eating. During a binge eating episode, people eat large amounts of food while feeling they can't control how much they are eating. Those with the most severe binge eating problems are considered to have binge eating disorder. These people may have more difficulty losing weight and keeping the weight off than people without binge eating problems. Some will need special help, such as counseling or medication, to control their binge eating before they can successfully manage their weight.

Some rare illnesses can cause obesity. These include hypothyroidism, Cushing's syndrome, depression, and certain neurologic problems that can lead to overeating. Certain drugs, such as steroids and some antidepressants, may cause excessive weight gain. A doctor can determine if a patient has any of these conditions, which are believed to be responsible for only about 1 percent of all cases of obesity.

Obesity is not just a cosmetic problem. It's a health hazard. Someone who is 40 percent overweight is twice as likely to die prematurely as an average-weight person. (This effect is seen after 10 to 30 years of being obese.)

Obesity has been linked to several serious medical conditions, including diabetes, heart disease, high blood pressure, and stroke. It is also associated with higher rates of certain types of cancer. Obese men are more likely than nonobese men to die from cancer of the colon, rectum, and prostate. Obese women are more likely than nonobese women to die from cancer of the gallbladder, breast, uterus, cervix, and ovaries.

Other diseases and health problems linked to obesity include:

Doctors generally agree that the more obese a person is, the more likely he or she is to have health problems.

One of the most painful aspects of obesity may be the emotional suffering it causes. American society places great emphasis on physical appearance, often equating attractiveness with slimness, especially in women. The messages, intended or not, make overweight people feel unattractive. Many people assume that obese people are gluttonous, lazy, or both. However, more and more evidence contradicts this assumption. Obese people often face prejudice or discrimination at work, at school, while looking for a job, and in social situations. Feelings of rejection, shame, or depression are common.

Doctors generally agree that people who are 20 percent or more overweight, especially the severely obese person, can gain significant health benefits from weight loss.

Many obesity experts believe that people who are less than 20 percent above their healthy weight should try to lose weight if they have any of the following risk factors.