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Obesity is the accumulation of excessive body fat.

• Obesity usually results from consuming too many calories and not burning enough calories in physical activity.
• Being obese increases the risk of many disorders, such as diabetes, high blood pressure, heart disease, and certain cancers, and can result in early death.
• Increasing activity and reducing caloric intake are essential to treating obesity, but some people also need to take drugs.
• Losing as little of 5 to 10% of body weight can help lessen weight-related problems, such as diabetes, high blood pressure, and high cholesterol levels.
• People who are very obese and who have serious weight related problems may benefit from weight-loss surgery.

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. Obesity is defined as a BMI of 30 or higher. Obesity is considered to be severe if BMI is 40 or higher.

Obesity has become increasingly common throughout the world. In the United States, obesity has increased dramatically: 34% of adults are obese, and over 17% of children and adolescents are overweight or obese. 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.

Determining Body Mass Index Height Weight (Pounds)   100 110 120 130 140 150 160 170 180 190 200 210 220 230 240 250 260 4'10" 21 23 25 27 29 31 33 36 38 40 42 44 46 48 50 52 54 4'11" 20 22 24 26 28 30 32 34 36 38 40 42 45 47 49 51 53 5'0" 20 21 23 25 27 29 31 33 35 37 39 41 43 45 47 49 51 5'1" 19 21 23 25 26 28 30 32 34 36 38 40 42 43 45 47 49 5'2" 18 20 22 24 26 27 29 31 33 35 37 38 40 43 44 46 48 5'3" 18 19 21 23 25 27 28 30 32 34 35 37 39 41 43 44 46 5'4" 17 19 21 22 24 26 27 29 31 33 34 36 38 39 41 43 45 5'5" 17 18 20 22 23 25 27 28 30 32 33 35 37 38 40 42 43 5'6" 16 18 19 21 23 24 26 27 29 31 32 34 36 37 39 40 42 5'7" 16 17 19 20 22 23 25 27 28 30 31 33 34 36 38 39 41 5'8" 15 17 18 20 21 23 24 26 27 29 30 32 33 35 36 38 40 5'9" 15 16 18 19 21 22 24 25 27 28 30 31 32 34 35 37 38 5'10" 14 16 17 19 20 22 23 24 26 27 29 30 32 33 34 36 37 5'11" 14 15 17 18 20 21 22 24 25 26 28 29 31 32 33 35 36 6'0" 13 15 16 18 19 20 22 23 24 26 27 28 30 31 33 34 35 6'1" 13 15 16 17 18 20 21 22 24 25 26 28 29 30 32 33 34 6'2" 12 14 15 17 18 19 21 22 23 24 26 27 28 30 31 32 33 6'3" 12 14 15 16 17 19 20 21 22 24 25 26 27 29 30 31 33 6'4" 12 13 15 16 17 18 19 21 22 23 24 26 27 28 29 30 32 6'5" 12 13 14 15 17 18 19 20 21 23 24 25 26 27 29 30 31 6'6" 12 13 14 15 16 17 19 20 21 22 23 24 25 27 28 29 30 Underweight: Less than 17.9 Normal: 18 to 24.9 (18 to 22.9 for Asians) Overweight: 25.1 to 29.9 (23 to 29.9 for Asians) Obese, moderate: 30 to 40 Obese, severe: More than 40

Causes

Obesity can result from certain disorders, but the increase in obesity results largely from environmental changes that have increased the availability of high-calorie foods and reduced the opportunity for physical activity.

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.

Physical Inactivity: 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.

Diet: The diet in developed countries is energy dense. That is, it consists of foods that have a large number of calories in a given amount (volume). Foods contain more processed carbohydrates (such as high-fructose corn syrup), more fat, and less fiber. Fats, by nature, are energy dense. A gram of fat has 9 calories, 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, coffee drinks, and alcohol, also contribute significantly. For example, a 12-ounce soda or bottle of beer has 150 calories; a 12-ounce coffee beverage or fruit smoothie can have 500 or more calories. An additional 500 calories per day results in a weight gain of 1 pound per week.

Genes: 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.

Many genes influence weight, but each gene has only a very small effect. Obesity rarely results when only one gene is abnormal.

Mutations in the following genes are relatively common:

• Gene for the melanocortin 4 receptor: Receptors are structures on the surface of cells that inhibit or produce an action in the cell when certain substances (such as chemical messengers) bind with them. Melanocortin 4 receptors are located mainly in the brain. They help the body regulate its use of energy. A mutation in this gene may account for obesity in 1 to 4% of children.
• Ob gene: This gene controls the production of leptin, a hormone made by fat cells. Leptin travels to the brain and interacts with receptors in the hypothalamus (the part of the brain that helps regulate appetite). The message carried by leptin is to decrease food intake and increase the amount of calories (energy) burned. A mutation in the ob gene prevents leptin production and results in severe obesity in a very small number of children. In these cases, administration of leptin reduces weight to a normal amount.

Background: Certain characteristics can increase the risk of becoming overweight or obese. They include the following:

• Certain racial and ethnic backgrounds, such as African Americans, Hispanics, and Pacific Islanders
• A lower socioeconomic group
• A lower education level
• Obesity during childhood (see Problems in Adolescents: Obesity), which tends to persist into adulthood.

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. Having several children close together may compound the problem. Breastfeeding can help women return to their prepregnancy weight.

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).

Aging: 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).

Lifestyle: 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 Some Trade Names
NICORETTE
NICOTROL
decreases appetite and increases the metabolic rate. When nicotine Some Trade Names
NICORETTE
NICOTROL
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%.

Hormones: Hormonal disorders rarely cause obesity. The following are among the most common examples:

• Cushing's syndrome is caused by excessive levels of cortisol in the body. The syndrome can result from a benign tumor in the pituitary gland (pituitary adenoma) or from a tumor in the adrenal gland or elsewhere, such as in the lungs. Cushing's syndrome typically causes fat to accumulate in the face, making it look full (called moon face), and behind the neck (called a buffalo hump).
  Polycystic ovary syndrome (see Menstrual Disorders and Abnormal Vaginal Bleeding: Polycystic Ovary Syndrome) affects about 5 to 10% of women. Affected women tend to be overweight or obese. Levels of testosterone and other male hormones are increased, causing fat to accumulate in the waist and abdomen, which is more harmful than the fat that is distributed throughout the body.

Eating Disorders: Two eating disorders are associated with obesity:

• Binge eating disorder is characterized by binging, by eating large amounts of food during a short amount of time, and usually by a feeling of guilt or remorse or sense of being out of control. Most affected people do not "purge" (for example, by vomiting or using laxatives or diuretics). The disorder is diagnosed when bingeing episodes occur at least twice a week for 6 or more months (see Eating Disorders: Binge Eating Disorder).
• Night-eating syndrome involves not eating much during the day, consuming a lot of food or calories in the evening, and awakening to eat in the middle of the night.

Drugs: Many drugs used to treat common disorders promote weight gain. These drugs include those used to treat psychiatric disorders including depression, those used to treat seizures, some antihypertensives (such as beta-blockers), corticosteroids, and some drugs used to treat diabetes mellitus.

Symptoms

The most obvious and only true symptom of obesity is a change in overall appearance. However, being obese also 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:

• High cholesterol levels
• High blood pressure
• Metabolic syndrome
• Coronary artery disease
• Heart failure
• Diabetes or a high blood sugar level (insulin resistance or prediabetes)
• Cancer of the breast, uterus, ovaries, colon, prostate, kidneys, or pancreas
• Gallstones and other gallbladder disorders
• A low testosterone level, erectile dysfunction, and reduced fertility in men
• Menstrual disorders, infertility, and increased risk of miscarriage in women
• Skin abnormalities, including acne and facial hair in women
• Varicose veins
• Fatty liver, hepatitis, and cirrhosis
• Blood clots (deep venous thrombosis and pulmonary embolism)
• Asthma
• Obstructive sleep apnea
• Kidney disorders, including nephrotic syndrome
• Arthritis, gout, low back pain, and other joint disorders
• Depression and anxiety

Obesity doubles or triples 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.

Diagnosis

Obesity is diagnosed by determining the BMI (see Obesity and the Metabolic Syndrome: Obesity). The BMI 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, body composition, especially percentage of body fat, is also important. Body composition can be measured using dual-energy x-ray absorptiometry (DEXA), which is also used to check for bone loss, or bioelectric impedance, which can be done in a doctor's office.

Waist circumference is measured. This measurement helps identify and quantify abdominal (visceral) obesity, which is fat that accumulates in the midsection (see Obesity and the Metabolic Syndrome: Metabolic Syndrome). Abdominal obesity is much more harmful than fat that is distributed throughout the body under the skin (subcutaneous fat).

Treatment

The main treatment for obesity is changing lifestyle, which includes changes in diet, increased physical activity, and behavioral modification to help with weight loss and maintenance. Some people may also need to take drugs or to have weight-loss (bariatric) surgery.

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 physicians can be beneficial. Programs that require regular contact increase accountability and can increase success rate. 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 by instructional and guidance materials.

Dietary Change: Healthy 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 1000 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 1200 to 1500 calories a day. Weight can be lost more rapidly with a very low calorie diet, but such diets should be supervised by a doctor. The following changes in diet are recommended:

• Eating 5 or more servings of fruits and vegetables a day
• Eating lean protein—for example, fish or chicken breast or vegetable protein, such as soy
• Switching to no-fat dairy products
• Eating whole grains
• Eliminating high-calorie beverages
• Limiting consumption of restaurant and fast food
• Switching from harmful fats, such as saturated fat and trans fat, to good fats, such as monounsaturated fats (found in olive and canola oils) and polyunsaturated fats (in deep-sea fish and vegetable oils) and limiting the amount of fat consumed.

Saturated and trans fat 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.

Physical Activity: Increasing physical activity is essential to healthy weight loss and weight maintenance. 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. Also, physical activity helps people maintain weight loss. People who do not exercise while dieting are more likely to regain the weight they lose.

As a general guide, people need to walk at least 150 minutes each week to promote health and to maintain weight. For weight loss, 60 to 90 minutes of physical activity per day is needed. 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 burns 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.

To get the most benefit from exercise, people should do strength training (with weights or another form of resistance) every 48 to 72 hours, 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.

Behavioral Modification: 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. Some of the skills involved in behavioral modification include problem solving, stress management, and self-monitoring.

Did You Know... Losing as little as 5 to 10% of body weight can greatly reduce weight-related health risks.

Drugs: 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 behavioral modification. Some weight-loss drugs are intended to be used for a short time. Others are intended to be used for a long time. Seven weight-loss drugs are currently available by prescription: orlistat Some Trade Names
XENICAL
, sibutramine Some Trade Names
MERIDIA
, phentermine Some Trade Names
IONAMIN
, benzphetamine Some Trade Names
DIDREX
, diethylpropion Some Trade Names
TENUATE
TEPANIL
, mazindol Some Trade Names
MAZANOR
SANOREX
, and phendimetrazine Some Trade Names
BONTRIL
PRELU-2
.

Orlistat Some Trade Names
XENICAL
limits the breakdown and absorption of fats in the intestine, producing, in effect, a low-fat diet. It is also currently available over the counter. It can cause bloating, gas and loose stools. Orlistat Some Trade Names
XENICAL
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 and bone fractures. People who take orlistat Some Trade Names
XENICAL
should take a vitamin supplement that contains these nutrients. The supplement should be taken at least 2 hours before or after taking orlistat Some Trade Names
XENICAL
.

Sibutramine Some Trade Names
MERIDIA
, phentermine Some Trade Names
IONAMIN
, benzphetamine Some Trade Names
DIDREX
, diethylpropion Some Trade Names
TENUATE
TEPANIL
, mazindol Some Trade Names
MAZANOR
SANOREX
, and phendimetrazine Some Trade Names
BONTRIL
PRELU-2
are believed to reduce appetite by affecting chemical messengers in the part of the brain that controls appetite. Some of these drugs may also increase the metabolic rate so that more calories are burned.

The combination of fenfluramine and phentermine Some Trade Names
IONAMIN
(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 nonprescription 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, 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.

Spotlight on Aging In the United States, the percentage of older people who are obese has been increasing. Obesity in older people is a concern because excess weight increases the risk of certain health problems that tend to become more common as people age: diabetes, cancer, abnormal levels of fats (lipids) in the blood (dyslipidemia), high blood pressure, heart failure, coronary artery disease, and joint disorders. Several age-related changes contribute to gaining weight: Decreased physical activity: Some reasons for decreased activity are related to aging. They include retirement, loss of capacity for exercise, development of disorders that make movement painful (such as arthritis), and problems with balance. Other factors may also limit physical activity. For example, people may not want to walk because there are no sidewalks, there is too much traffic, or the neighborhood seems unsafe. Loss of muscle tissue: Muscle tissue is lost partly because levels of growth hormone and sex hormones (estrogen in women and testosterone in men) decrease. But the main reason older people lose lean tissue is physical inactivity. The less muscle tissue people have, the fewer calories their body burns when resting and the easier it is to gain weight. Increased body fat: When the amount of muscle tissue decreases, the percentage of fat in the body increases. Fat tissue burns fewer calories. Also, the higher percentage of fat means that older people with a normal body mass index (BMI), which is based only on weight and height, may have a higher risk of weight-related health problems than expected. Waist circumference predicts health risks better than BMI in older people. Shifting of body fat to the waist: With aging, body fat tends to shift to the waist. Fat that accumulates around the waist and abdomen (as opposed to the hips and thighs) increases the risk of health problems. Older people are at greater risk of undernutrition than younger people. Therefore, when they try to lose weight, they should be sure to consume a healthy and balanced diet. Whether weight loss in older people has health risks is controversial. Doctors help older people devise weight-loss strategies based on their individual circumstances. Treatment may include diet, exercise, drugs, and surgery. In older people, weight loss is best supervised by a doctor.

Last full review/revision August 2008 by Adrienne Youdim, MD