What is obesity?

To most people, the term "obesity" means to be very overweight. Health professionals define "overweight" as an excess amount of body weight that includes muscle, bone, fat, and water. "Obesity" specifically refers to an excess amount of body fat. Some people, such as bodybuilders or other athletes with a lot of muscle, can be overweight without being obese.

Body Mass Index

BMI uses a mathematical formula based on a person's height and weight. BMI equals weight in kilograms divided by height in meters squared (BMI = kg/m2). The BMI table that follows has already calculated this information.

Although the BMI ranges shown in the table are not exact ranges of healthy and unhealthy weight, they are useful guidelines. A BMI of 25 to 29.9 indicates a person is overweight. A person with a BMI of 30 or higher is considered obese.

Like the weight-to-height table, BMI does not show the difference between excess fat and muscle. BMI, however, is closely associated with measures of body fat. It also predicts the development of health problems related to excess weight. For these reasons, BMI is widely used by health care providers.

Find your weight on the bottom of the graph. Go straight up from that point until you come to the line that matches your height. Then look to find your weight group.

Causes of Obesity

In scientific terms, obesity occurs when a person consumes more calories than he or she burns. What causes this imbalance between calories in and calories out may differ from one person to another. Genetic, environmental, psychological, and other factors may all play a part.

Genetic factors

Obesity tends to run in families, suggesting a genetic cause. Yet families also share diet and lifestyle habits that may contribute to obesity. Separating these from genetic factors is often difficult. Even so, science shows that heredity is linked to obesity.

In one study, adults who were adopted as children were found to have weights closer to their biological parents than to their adoptive parents. In this case, the person's genetic makeup had more influence on the development of obesity than the environment in the adoptive family home.

Environmental factors

Genes do not destine people to a lifetime of obesity, however. Environment also strongly influences obesity. This includes lifestyle behaviors such as what a person eats and his or her level of physical activity. Americans tend to eat high-fat foods, and put taste and convenience ahead of nutrition. Also, most Americans do not get enough physical activity.

Although you cannot change your genetic makeup, you can change your eating habits and levels of activity. Try these techniques that have helped some people lose weight and keep it off:

Psychological factors

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

Most overweight people have no more psychological problems than people of average weight. Still, up to 10 percent of people who are mildly obese and try to lose weight on their own or through commercial weight loss programs have binge eating disorder. This disorder is even more common in people who are severely obese.

During a binge eating episode, people eat large amounts of food and feel that they cannot control how much they are eating. Those with the most severe binge eating problems are also likely to have symptoms of depression and low self-esteem. These people may have more difficulty losing weight and keeping it off than people without binge eating problems.

If you are upset by binge eating behavior and think you might have binge eating disorder, seek help from a health professional such as a psychiatrist, psychologist, or clinical social worker.

Other causes of obesity

Some illnesses can lead to obesity or a tendency to gain weight. These include hypothyroidism, Cushing's syndrome, depression, and certain neurological problems that can lead to overeating. Also, drugs such as steroids and some antidepressants may cause weight gain. A doctor can tell whether there are underlying medical conditions that are causing weight gain or making weight loss difficult.

Consequences of Obesity

Health Risks

Obesity is more than a cosmetic problem; it is a health hazard. Approximately 280,000 adult deaths in the United States each year are related to obesity. Several serious medical conditions have been linked to obesity, including type 2 diabetes, heart disease, high blood pressure, and stroke. Obesity is also linked to higher rates of certain types of cancer. Obese men are more likely than non-obese men to die from cancer of the colon, rectum, or prostate. Obese women are more likely than non-obese women to die from cancer of the gallbladder, breast, uterus, cervix, or ovaries.

Other diseases and health problems linked to obesity include:

Health care providers generally agree that the more obese a person is, the more likely he or she is to develop health problems.

Psychological and social effects

Emotional suffering may be one of the most painful parts of obesity. American society emphasizes physical appearance and often equates attractiveness with slimness, especially for women. Such messages make overweight people feel unattractive.

Many people think that obese individuals are gluttonous, lazy, or both, even though this is not true. As a result, obese people often face prejudice or discrimination in the job market, at school, and in social situations. Feelings of rejection, shame, or depression are common.

Who should lose weight?

Health care providers generally agree that people who have a BMI of 30 or more can improve their health through weight loss. This is especially true for people who are severely obese.

Preventing additional weight gain is recommended if you have a BMI between 25 and 29.9, unless you have other risk factors. Obesity experts recommend you try to lose weight if you have two or more of the following:

Fortunately, a weight loss of 5 to 10 percent can do much to improve health by lowering blood pressure and cholesterol levels. In addition, recent research has shown that a 5- to 7-percent weight loss can prevent type 2 diabetes in people at high risk for the disease.

How is obesity treated?

The method of treatment depends on your level of obesity, overall health condition, and motivation to lose weight. Treatment may include a combination of diet, exercise, behavior modification, and sometimes weight-loss drugs. In some cases of severe obesity, gastrointestinal surgery may be recommended. Remember, weight control is a life-long effort.

For more information on health risks, treatment options, and binge eating, click on these links to WIN publications:

Additional Reading

Allison DB, Fontaine KR, Manson JE, Stevens J, VanItallie TB. Annual deaths attributable to obesity in the United States. Journal of the American Medical Association; 1999;282(16):1530-1538.

National Heart, Lung, and Blood Institute. Clinical Guidelines on the Identification, Evaluation, and Treatment of Overweight and Obesity in Adults. Department of Health and Human Services, National Institutes of Health; 1998. NIH Publication No. 98-4083.

National Task Force on Prevention and Treatment of Obesity. Overweight, obesity, and health risk. Archives of Internal Medicine. 2000;160(7):898-904.

Partnership for Healthy Weight Management. Weight Loss: Finding a Weight Loss Program that Works for You. 2000. Phone: 1-888-8-PUEBLO. Website: www.consumer.gov/weightloss/brochures.htm.

Partnership for Healthy Weight Management. Setting Goals for Healthy Weight Loss. 1999. Phone: 1-888-8-PUEBLO. Website: www.consumer.gov/weightloss/brochures.htm.

The President’s Council on Physical Fitness and Sports, Department of Health and Human Services. Exercise and Weight Control. Website: www.fitness.gov/Reading_Room/reading_room.html.

U.S. Department of Agriculture and U.S. Department of Health and Human Services. Dietary Guidelines for Americans. 2000. Phone: 1-888-878-3256. Website: www.usda.gov/cnpp or www.health.gov/dietaryguidelines.

Weight-control Information Network

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The Weight-control Information Network (WIN) is a national service of the National Institute of Diabetes and Digestive and Kidney Diseases of the National Institutes of Health, which is the Federal Government's lead agency responsible for biomedical research on nutrition and obesity. Authorized by Congress (Public Law 103-43), WIN provides the general public, health professionals, the media, and Congress with up-to-date, science-based health information on weight control, obesity, physical activity, and related nutritional issues.

WIN answers inquiries, develops and distributes publications, and works closely with professional and patient organizations and Government agencies to coordinate resources about weight control and related issues.

Publications produced by WIN are carefully reviewed by both NIDDK scientists and outside experts. This fact sheet was also reviewed by Thomas Wadden, Ph.D., Director, Weight and Eating Disorders Program, University of Pennsylvania, and Goulda Downer, Ph.D., President, Metroplex Health and Nutrition Services.

This e-text is not copyrighted.WIN encourages unlimited duplication and distribution of this fact sheet.

NIH Publication No. 01-3680
October 2001