Obesity is excess body weight.
The body mass index (BMI) is used to define overweight and obesity. BMI is weight (in kilograms) divided by height (in meters squared—see also Overview of Nutrition: Fat Versus Lean: Body Composition). Overweight is usually defined as a BMI of 25 to 29.9. For Asians, the BMI for normal and overweight is slightly lower. Obesity is defined as a BMI of 30 or higher. Obesity is considered to be severe if BMI is 40 or higher.
BMI does not distinguish between muscle (lean) and fat tissue. Thus, based on BMI alone, some people may be labeled obese when their percentage of body fat is very low. For example, some people, such as body builders, have a high BMI because they have a large amount of muscle (which weighs more than fat), even though they have very little fat. Such people are not considered obese.
Obesity has become increasingly common throughout the world. In the United States, obesity is very common. Almost 36% of adults are obese, and about 17% of children and adolescents are overweight or obese. Also, severe obesity has become more common.
Obesity is much easier to prevent than treat. Once people gain excess weight, the body resists losing weight. For example, when people diet or reduce the number of calories they consume, the body compensates by increasing appetite and reducing the number of calories burned during rest.
Obesity results from a combination of factors, including the reduced opportunity for physical activity, the increased availability of high-calorie foods, and the presence of genes that make obesity more likely. But ultimately, obesity results from consuming more calories than the body needs over a long period of time.
Excess calories are stored in the body as fat. The number of calories needed varies from person to person, depending on age, sex, activity level, and metabolic rate. A person's resting (basal) metabolic rate—the amount of calories the body burns while at rest—is determined mostly by how much muscle (lean) tissue a person has. The more muscle people have, the higher their metabolic rate.
In developed countries, lack of physical activity contributes greatly to the increase in obesity. Opportunities for physical activity have been engineered away by technological advances, such as elevators, cars, and remote controls. More time is spent doing sedentary activities, such as using the computer, watching television, and playing video games. Also, people's jobs have become more sedentary as office or desk jobs have replaced manual labor. Sedentary people use fewer calories than more active people and thus require fewer calories in the diet. If caloric intake is not reduced accordingly, people gain weight.
The diet in developed countries is energy dense. That is, it consists of foods that have a large number of calories in a relatively small amount (volume). Most of these foods contain more processed carbohydrates (such as high-fructose corn syrup), more fat, and less fiber. Fats, by nature, are energy dense. Fat has 9 calories per gram, but carbohydrates and proteins have 4 calories per gram.
Convenience foods, such as energy-dense snacks offered at vending machines and fast food restaurants, contribute to the increase in obesity. High-calorie beverages, including soda, juices, many coffee drinks, and alcohol, also contribute significantly. For example, a 12-ounce soda or bottle of beer has 150 calories, and a 12-ounce coffee beverage (containing dairy and sugar) or fruit smoothie can have 500 or more calories. Larger portion sizes at restaurants and in packaged foods and beverages encourage people to overeat. Also, restaurant and packaged foods are often prepared in ways that add calories. As a result, people may consume more calories than they realize.
Obesity tends to run in families. However, families share not only genes but also environment, and separating the two influences is difficult. Genes can affect how quickly the body burns calories at rest and during exercise. They can also affect appetite and thus how much food is consumed. Genes may have a greater effect on where body fat accumulates, particularly fat around the waist and in the abdomen, than on how much body fat accumulates.
Many genes influence weight, but each gene has only a very small effect. Obesity rarely results when only one gene is abnormal.
Rarely, mutations in the following genes result in obesity:
Certain characteristics can increase the risk of becoming overweight or obese. They include the following:
Pregnancy and menopause:
Gaining weight during pregnancy is normal and necessary. However, pregnancy can be the beginning of weight problems if women do not return to their prepregnancy weight. About 15% of women permanently gain 20 pounds or more with each pregnancy. Having several children close together may compound the problem. Breastfeeding can help women return to their prepregnancy weight.
If a pregnant woman is obese or smokes, weight regulation in the child may be disturbed, contributing to weight gain during childhood and later.
After menopause, many women gain weight. This weight gain may result from reduced activity. Hormonal changes may cause fat to be redistributed and accumulate around the waist. Fat in this location increases the risk of health problems (see Obesity and the Metabolic Syndrome: Metabolic Syndrome).
As people age, body composition may change as muscle tissue decreases. The result is a higher percentage of body fat and a lower basal metabolic rate (because muscle burns more calories).
Obesity is more than twice as common at age 55 as at age 20.
Sleep deprivation or lack of sleep (usually considered less than 6 to 8 hours per night) can result in weight gain. Sleeplessness results in hormonal changes that increase appetite and cravings for energy-dense foods.
Stopping smoking usually results in weight gain. Nicotine decreases appetite and increases the metabolic rate. When nicotine is stopped, people may eat more food and their metabolic rate decreases, so that fewer calories are burned. As a result, body weight may increase by 5 to 10%.
Hormonal disorders rarely cause obesity. The following are among the most common:
Two eating disorders are associated with obesity:
Many drugs used to treat common disorders promote weight gain. These drugs include some drugs used to treat psychiatric disorders including depression, some drugs used to treat seizures, some drugs used to treat high blood pressures (antihypertensives, such as beta-blockers), corticosteroids, and some drugs used to treat diabetes mellitus.
The most obvious symptom of obesity is a change in overall appearance.
Being obese increases the risk of many health problems. Virtually every organ system can be affected. These weight-related health problems can cause symptoms, such as shortness of breath, difficulty breathing during activity, snoring, skin abnormalities including acne, and joint and back pain.
Obesity increases the risk of the following:
Obstructive sleep apnea can develop if excess fat in the neck compresses the airway during sleep. Breathing stops for a few moments, as often as hundreds of times a night. This disorder is often undiagnosed. It can cause loud snoring and excessive daytime sleepiness and increases the risk of high blood pressure, abnormal heart rhythms, and metabolic syndrome.
Obesity can increase the risk of early death. The more severe the obesity, the higher the risk. In the United States, 300,000 deaths a year are attributed to obesity. It is the second most common cause of preventable death (cigarette smoking is the most common).
Obesity can lead to social, economic, and psychologic problems. For example, obese people may be underemployed or unemployed, or they may have poor body image and low self-esteem.
Obesity is diagnosed by determining the BMI (see Obesity and the Metabolic Syndrome: Obesity). The BMI does not take race, sex, and age into consideration. It also does not distinguish between lean and fat tissue. Thus, some people have a high BMI because they have excess muscle (for example, if they are body builders), but they are not considered obese. Conversely, some people have a normal weight but a high percentage of body fat, which is unhealthy. Therefore, if doctors are unsure whether a high BMI is due to muscle or excessive fat, they determine body composition (the percentage of body fat and muscle). Body composition can be determined using dual-energy x-ray absorptiometry (DEXA), which is also used to check for bone loss, or by bioelectric impedance, which can be done in a doctor's office (see Overview of Nutrition: Fat Versus Lean: Body Composition). The percentage of body fat can also be estimated by measuring skinfold thickness (usually over the triceps, at the back of the upper arm) and the circumference of the upper arm. The skinfold is the skin and layer of fat under it that is obtained by pinching the skin.
Waist circumference is measured. This measurement helps identify and quantify abdominal (visceral) obesity, which is fat that accumulates around the waist and in the abdomen (see Obesity and the Metabolic Syndrome: Diagnosis). Abdominal obesity is much more harmful than fat that is distributed throughout the body under the skin (subcutaneous fat).
Typically, blood tests are done. Blood sugar (glucose) is measured to check for prediabetes or diabetes, and cholesterol and other fat levels are measured to check for high cholesterol and other abnormal fat levels. Doctors also measure blood pressure to check for high blood pressure (hypertension). These tests help doctors determine whether people have metabolic syndrome (which includes all three disorders).
Knowing how large the waist is and whether metabolic syndrome is present helps doctors estimate the risk of serious complications better than knowing what the person's BMI is.
Doctors also check for other disorders that are common among people who are obese, such as obstructive sleep apnea, fatty liver, and depression.
The main treatment for obesity is changes in lifestyle, which includes changes in diet, increased physical activity, and changes in behavior. Some people may also need to take drugs or to have weight-loss (bariatric) surgery (Obesity and the Metabolic Syndrome: Bariatric Surgery). Losing as little of 5 to 10% of body weight can help reduce the risk or severity of weight-related health problems, such as diabetes, high blood pressure, and high cholesterol levels.
Successful weight loss requires motivation and a sense of readiness. People who are most successful have realistic goals and recognize that healthy weight loss can be achieved only with lifelong lifestyle changes rather than a magic bullet or fad diet that cannot be sustained. Seeking the support of health care practitioners such as dieticians or doctors can be beneficial. Support from family members is also crucial. Programs that require regular contact increase accountability and can increase the likelihood of success. Some examples include Overeaters Anonymous (OA), Take Off Pounds Sensibly (TOPS), community-based and work-site programs, and organized commercial programs such as Weight Watchers. Typically, weekly meetings are conducted by counselors and supplemented with instructional and guidance materials.
Changes in diet:
Healthy, balanced eating for weight loss requires reducing the number of calories consumed and choosing a wide range of foods that provide good nutrition. Reducing the number of calories consumed by 500 to 1,000 calories a day results in weight loss of 1 to 2 pounds per week, which is a healthy rate of weight loss. This approach usually means consuming 1,200 to 1,500 calories a day. Consuming a low-fat, high-fiber diet plus reducing the number of calories by about 600 calories a day and substituting some protein for carbohydrate appears to be the best way to lose weight and keep it off. Weight can be lost more rapidly with a very low calorie diet, but such diets should be supervised by a doctor and can be followed only for a short time.
The following changes in diet are recommended:
Saturated and trans fats not only contribute to weight gain and obesity but can be harmful by leading to abnormal cholesterol levels and an increased risk of coronary artery disease. Eating foods with a low glycemic index (see Table 2: Overview of Nutrition: Glycemic Index of Some Foods) and foods that contain fish oils (including deep-sea fish such as salmon and tuna) or monounsaturated fats derived from plants (such as olive oil) reduce the risk of heart disorders and diabetes.
No-fat or low-fat dairy products, which provide vitamin D, should be included to help prevent a deficiency of this vitamin.
Using meal replacements, regularly or once in a while, can help some people lose weight and keep it off.
Increasing physical activity can help people lose weight in a healthy way and keep it off. Physical activity includes not only exercise (that is, structured physical activity) but also lifestyle activities, such as taking the stairs instead of the elevator, gardening, and walking instead of driving when possible. Lifestyle activities can burn a considerable number of calories. People who do not exercise while dieting are more likely to regain the weight they lose.
Aerobic exercise, such as jogging, walking briskly (3 to 4 miles an hour), biking, singles tennis, skating, and cross-country skiing, burn more calories than less active exercises (see Exercise and Fitness: Choosing the Right Exercise). For example, vigorous walking can burn about 4 calories per minute, so that 1 hour of brisk walking per day burns about 240 calories. Running burns about 6 to 8 calories per minute (about 360 to 480 calories per hour). As a general guide, people need to walk at least 150 minutes each week to promote health. To lose weight and keep it off, people need to spend 300 to 360 minutes each week doing moderate physical activity or 150 minutes each week doing vigorous aerobic exercise (such as running or using an elliptical machine). Vigorous aerobic exercise, has other health benefits, including reducing the risk of coronary artery disease and increasing endurance (see Exercise and Fitness: Benefits of Exercise).
To get the most benefit from exercise, people should do strength training (with weights or another form of resistance) about 3 days of the week. Strength training increases the amount of muscle tissue, which increases the metabolic rate, so that the body burns more calories when at rest.
Changes in behavior:
Ultimately, for weight loss to be effective and long-lasting, people must change their behavior. Weight-loss programs that help people change their behavior are the most effective. To change behavior, people need certain skills, such as
Problem solving involves identifying and planning ahead for situations that make unhealthy eating more likely (such as going out to dinner or traveling) or that reduce the opportunity for physical activity (such as driving cross country).
To manage stress, people can learn to identify stressful situations and develop ways to manage the stress that do not involve eating—for example, by going for a walk, meditating, or taking deep breaths.
To monitor themselves, people may keep a food log, including the number of calories in the foods, and weigh themselves regularly. They may record where and when they eat, what their mood is when they eat, and who is with them. With this information, they can observe and record patterns of behavior and eating and may be able to avoid situations that lead to weight gain or unhealthy eating.
Contingency management involves providing rewards (other than food) for behavior that contributes to weight loss or maintenance. For example, if people walk more or eat less of certain foods, they may reward themselves by getting new clothes or going to a movie. Rewards may also come from other people—for example, praise from family members or members of a support group.
To control stimuli that can trigger unhealthy eating, people can learn to identify obstacles to healthy eating and an active lifestyle. Then they can develop strategies to overcome them. For example, people may avoid going by a fast food restaurant on their way to work or not keep sweets in the house. To develop an active lifestyle, they may take up an active hobby (such as gardening), walk more, make a habit of taking the stairs instead of elevators, or park at the far end of parking lots (resulting in a longer walk).
Internet resources, applications for mobile devices, and other technological devices may also help people develop an active lifestyle and maintain weight loss. Applications can help people set a weight-loss goal, monitor their progress, track food consumption, and record physical activity.
For people who are obese or overweight and have weight-related disorders, drugs can be useful. Drugs are most effective when used with changes in diet, increased physical activity, and structured programs that include changes in behavior. Some weight-loss drugs are intended to be used for a short time. Others are intended to be used for a long time.
Weight-loss drugs that are currently available include
These drugs are used if people have a BMI over 30 or if people have a BMI over 27 and have complications such as high blood pressure or diabetes. When people take weight-loss drugs, they usually lose about 5 to 10% of their body weight.
Orlistat limits the breakdown and absorption of fats in the intestine, producing, in effect, a low-fat diet. Orlistat is available over the counter as well as by prescription. It results in unabsorbed fat in the digestive tract. This fat can cause bloating, gas and loose stools, but these problems tend to resolve during the second year of treatment. Orlistat should be taken with meals. Orlistat can interfere with the absorption of the fat-soluble vitamins: A, D, E, and K. If not enough vitamin D is absorbed, some people develop osteoporosis, making fractures more likely. People who take orlistat should take a vitamin supplement that contains these nutrients. The supplement should be taken at least 2 hours before or after taking orlistat.
Phentermine reduces appetite by affecting chemical messengers in the part of the brain that controls appetite. It is available by prescription only and should be taken only for a short time (3 months or less). It can increase blood pressure and heart rate and cause insomnia, anxiety, and constipation.
Phentermine plus topiramate (a drug used to treat seizures and migraines) is available by prescription only. This combination results in weight loss for up to 2 years. However, it can cause birth defects, so women of childbearing age should take it only if they are using birth control and are tested monthly for pregnancy. These drugs can cause problems with sleep and concentration and can increase heart rate.
Lorcaserin may be available soon. It is given by prescription only. It suppresses appetite by affecting certain receptors in the brain.
The combination of fenfluramine and phentermine (often called fen-phen) was the most effective drug treatment. However, fenfluramine was removed from the market because heart valve problems occurred in people who took this combination.
Some over-the-counter diet aids, including medicinal herbs, claim to enhance weight loss by increasing metabolism or by increasing a feeling of fullness. These supplements have not been shown to be effective and may contain harmful additives or stimulants (such as ephedra, caffeine, guarana, and phenylpropanolamine) and should be avoided.
Many new drugs for the treatment of obesity are being developed and will probably change the way obesity is treated in the future.
Last full review/revision April 2013 by Adrienne Youdim, MD