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by Adrienne Youdim, MD, FACP

Obesity is excess body weight, defined as a body mass index (BMI) of ≥ 30 kg/m 2 . Complications include cardiovascular disorders (particularly in people with excess abdominal fat), diabetes mellitus, certain cancers, cholelithiasis, fatty liver, cirrhosis, osteoarthritis, reproductive disorders in men and women, psychologic disorders, and, for people with BMI ≥ 35, premature death. Diagnosis is based on body mass index. Treatment includes lifestyle modification (eg, in diet, physical activity, and behavior) and, for certain patients, drugs or bariatric (weight-loss) surgery.

Prevalence of obesity in the US is high in all age groups (see Table: Changes in Prevalence of Obesity According to NHANES); 35.7% of adults are obese. Prevalence is highest among blacks (49.5%), compared with Mexican Americans (40.4%) and whites (34.3%). Black and Mexican-American men in higher income groups are more likely to be obese than those in lower income groups. However, women in higher income groups, regardless of ethnic group, are less likely to be obese, and most obese adults are not in lower income groups.

Changes in Prevalence of Obesity According to NHANES

Age Group





2–5 yr





6–11 yr





12–19 yr





20–74 yr





NHANES = National Health and Nutrition Examination Surveys.

In the US, obesity and its complications cause as many as 300,000 premature deaths each year, making it second only to cigarette smoking as a preventable cause of death.


Causes of obesity are probably multifactorial and may include genetic predisposition. Ultimately, obesity results from a long-standing imbalance between energy intake and energy expenditure, including energy utilization for basic metabolic processes and energy expenditure from physical activity.

Genetic factors

Heritability of BMI is about 66%. Genetic factors may affect the many signaling molecules and receptors used by parts of the hypothalamus and GI tract to regulate food intake (see Pathways Regulating Food Intake). Genetic factors can be inherited or result from conditions in utero (called genetic imprinting). Rarely, obesity results from abnormal levels of peptides that regulate food intake (eg, leptin) or abnormalities in their receptors (eg, melanocortin-4 receptor).

Genetic factors also regulate energy expenditure, including BMR, diet-induced thermogenesis, and nonvoluntary activity–associated thermogenesis. Genetic factors may have a greater effect on the distribution of body fat, particularly abdominal fat (see Metabolic Syndrome), than on the amount of body fat.

Environmental factors

Weight is gained when caloric intake exceeds energy needs. Important determinants of energy intake include portion sizes and the energy density of the food. High-calorie foods (eg, processed foods), diets high in refined carbohydrates, and consumption of soft drinks, fruit juices, and alcohol promote weight gain. Diets high in fresh fruit and vegetables, fiber, and complex carbohydrates, with water as the main fluid consumed, minimize weight gain. A sedentary lifestyle promotes weight gain.

Regulatory factors

Prenatal maternal obesity, prenatal maternal smoking, and intrauterine growth restriction can disturb weight regulation and contribute to weight gain during childhood and later. Obesity that persists beyond early childhood makes weight loss in later life more difficult.

About 15% of women permanently gain ≥ 20 lb with each pregnancy.

Insufficient sleep (usually considered < 6 to 8 h/night) can result in weight gain by changing the levels of satiety hormones that promote hunger.

Drugs, including corticosteroids, lithium, traditional antidepressants (tricyclics, tetracyclics, monoamine oxidase inhibitors [MAOIs]), benzodiazepines, anticonvulsants, thiazolidinediones (eg, rosiglitazone, pioglitazone), β-blockers, and antipsychotic drugs, can cause weight gain.

Uncommonly, weight gain is caused by one of the following disorders:

  • Brain damage caused by a tumor (especially a craniopharyngioma) or an infection (particularly those affecting the hypothalamus), which can stimulate consumption of excess calories

  • Hyperinsulinism due to pancreatic tumors

  • Hypercortisolism due to Cushing syndrome, which causes predominantly abdominal obesity

  • Hypothyroidism (rarely a cause of substantial weight gain)

Eating disorders

At least 2 pathologic eating patterns may be associated with obesity:

  • Binge eating disorder is consumption of large amounts of food quickly with a subjective sense of loss of control during the binge and distress after it (see Binge Eating Disorder). This disorder does not include compensatory behaviors, such as vomiting. Prevalence is 1 to 3% among both sexes and 10 to 20% among people entering weight reduction programs. Obesity is usually severe, large amounts of weight are frequently gained or lost, and pronounced psychologic disturbances are present.

  • Night-eating syndrome consists of morning anorexia, evening hyperphagia, and insomnia, with eating in the middle of the night. At least 25 to 50% of daily intake occurs after the evening meal. About 10% of people seeking treatment for severe obesity may have this disorder. Rarely, a similar disorder is induced by use of a hypnotic such as zolpidem.

Similar but less extreme patterns probably contribute to excess weight gain in more people. For example, eating after the evening meal contributes to excess weight gain in many people who do not have night-eating syndrome.


Complications of obesity include the following:

  • Metabolic syndrome

  • Diabetes mellitus

  • Cardiovascular disorders

  • Liver disorders (nonalcoholic steatohepatitis [fatty liver] and cirrhosis)

  • Gallbladder disease

  • Gastroesophageal reflux

  • Obstructive sleep apnea

  • Reproductive system disorders, including infertility

  • Many cancers

  • Osteoarthritis

  • Social and psychologic problems

Insulin resistance, dyslipidemias, and hypertension (metabolic syndrome) can develop, often leading to diabetes mellitus and coronary artery disease (see Metabolic Syndrome). These complications are more likely in patients with fat that is concentrated abdominally, a high serum triglyceride level, a family history of type 2 diabetes mellitus or premature cardiovascular disease, or a combination of these risk factors.

Obesity is also a risk factor for nonalcoholic steatohepatitis (which may lead to cirrhosis) and for reproductive system disorders, such as a low serum testosterone level in men and polycystic ovary syndrome in women.

Obstructive sleep apnea can result if excess fat in the neck compresses the airway during sleep. Breathing stops for moments, as often as hundreds of times a night (see Obstructive Sleep Apnea). This disorder, often undiagnosed, can cause loud snoring and excessive daytime sleepiness and increases the risk of hypertension, cardiac arrhythmias, and metabolic syndrome.

Obesity may cause the obesity-hypoventilation syndrome (Pickwickian syndrome). Impaired breathing leads to hypercapnia, reduced sensitivity to CO 2 in stimulating respiration, hypoxia, cor pulmonale, and risk of premature death. This syndrome may occur alone or secondary to obstructive sleep apnea.

Osteoarthritis and tendon and fascial disorders may result from obesity. Skin disorders are common; increased sweat and skin secretions, trapped in thick folds of skin, are conducive to fungal and bacterial growth, making intertriginous infections especially common. Being overweight probably predisposes to cholelithiasis, gout, deep venous thrombosis and pulmonary embolism, and some cancers (especially colon and breast cancers).

Obesity leads to social, economic, and psychologic problems as a result of prejudice, discrimination, poor body image, and low self-esteem. For example, people may be underemployed or unemployed.


  • BMI

  • Waist circumference

  • Sometimes body composition analysis

In adults, BMI, defined as weight (kg) divided by the square of the height (m 2 ), is used to screen for overweight or obesity. BMI of 25 to 29.9 kg/m 2 indicates overweight; BMI of 30 kg/m 2 indicates obesity (see Table: Body Mass Index (BMI)). However, BMI is a crude screening tool and has limitations in many subpopulations. Some experts think that BMI cutoffs should vary based on ethnicity, sex, and age. For example, in certain nonwhite populations, complications of obesity develop at a much lower BMI than in whites. In children and adolescents, overweight is defined as BMI at the 95th percentile, based on age- and sex-specific growth charts from the Centers for Disease Control and Prevention (CDC) at the CDC web site .

Body Mass Index (BMI)




Extremely obese








Height (in)

Body Weight (lb)
































































*BMIs less than those listed as normal are considered underweight.

Calculations are done using metric units (kg, m), but the table is presented in inches and pounds for US readers.

Asians, Japanese, and many aboriginal populations have a lower cut-off (23 kg/m 2 ) for overweight. In addition, BMI may be high in muscular athletes, who lack excess body fat, and may be normal or low in formerly overweight people who have lost muscle mass.

Waist circumference and the presence of metabolic syndrome appear to predict risk of metabolic and cardiovascular complications better than BMI does.

The waist circumference that increases risk of complications due to obesity varies by ethnic group and sex:

  • White men: > 93 cm (> 36.6 in), particularly > 101 cm (> 39.8 in)

  • White women: > 79 cm (> 31.1 in), particularly > 87 cm (> 34.2 in)

  • Asian Indian men: > 78 cm (> 30.7 in), particularly > 90 cm (> 35.4 in)

  • Asian Indian women: > 72 cm (> 28.3 in), particularly > 80 cm (> 31.5 in)

Body composition analysis

Body composition—the percentage of body fat and muscle—is also considered when obesity is diagnosed. Although probably unnecessary in routine clinical practice, body composition analysis can be helpful if clinicians question whether elevated BMI is due to muscle or excessive fat.

The percentage of body fat can be estimated by measuring skinfold thickness (usually over the triceps) or determining mid upper arm area (see Undernutrition:Physical examination).

Bioelectrical impedance analysis (BIA) can estimate percentage of body fat simply and noninvasively. BIA estimates percentage of total body water directly; percentage of body fat is derived indirectly. BIA is most reliable in healthy people and in people with only a few chronic disorders that do not change the percentage of total body water (eg, moderate obesity, diabetes mellitus). Whether measuring BIA poses risks in people with implanted defibrillators is unclear.

Underwater (hydrostatic) weighing is the most accurate method for measuring percentage of body fat. Costly and time-consuming, it is used more often in research than in clinical care. To be weighed accurately while submerged, people must fully exhale beforehand.

Imaging procedures, including CT, MRI, and dual-energy x-ray absorptiometry (DEXA), can also estimate the percentage and distribution of body fat but are usually used only for research.

Other testing

Obese patients should be screened for common comorbid disorders, such as obstructive sleep apnea, diabetes, dyslipidemia, hypertension, fatty liver, and depression. Screening tools can help; for example, for obstructive sleep apnea, clinicians can use an instrument such as the STOP-BANG questionnaire (see Table: STOP-BANG Risk Score for Obstructive Sleep Apnea) and often the apnea-hypopnea index (total number of apnea or hypopnea episodes occurring per hour of sleep—see Diagnosis). Obstructive sleep apnea is often underdiagnosed, and obesity increases the risk.


Untreated, obesity tends to progress. The probability and severity of complications are proportional to the absolute amount of fat, the distribution of the fat, and absolute muscle mass. After weight loss, most people return to their pretreatment weight within 5 yr, and accordingly, obesity requires a lifelong management program similar to that for any other chronic disorder.


  • Dietary management

  • Physical activity

  • Behavioral interventions

  • Drugs (eg, phentermine, orlistat, lorcaserin, phentermine/topiramate)

  • Bariatric surgery

Weight loss of even 5 to 10% improves overall health, helps reduce risk of developing cardiovascular complications (eg, hypertension, dyslipidemia, insulin resistance) and helps lessen their severity, and may lessen the severity of other complications and comorbid disorders such as obstructive sleep apnea, fatty liver, infertility, and depression.

Support from health care practitioners, peers, and family members and various structured programs can help with weight loss and weight maintenance.


Balanced eating is important for weight loss and maintenance.

Strategies include

  • Eating small meals and avoiding or carefully choosing snacks

  • Substituting fresh fruits and vegetables and salads for refined carbohydrates and processed food

  • Substituting water for soft drinks or juices

  • Limiting alcohol consumption to moderate levels

  • Including no- or low-fat dairy products, which are part of a healthy diet and help provide an adequate amount of vitamin D

Low-fat (particularly very low saturated fat), high-fiber diets with modest calorie restriction (by 600 kcal/day) and substitution of some protein for carbohydrate appear to have the best long-term outcome. Foods with a low glycemic index (see Table: Glycemic Index of Some Foods) and marine fish oils or monounsaturated fats derived from plants (eg, olive oil) reduce the risk of cardiovascular disorders and diabetes.

Use of meal replacements can help with weight loss and maintenance; these products can be used regularly or intermittently.

Diets that are overly restrictive are unlikely to be maintained or to result in long-term weight loss. Diets that limit caloric intake to < 50% of basal energy expenditure (BEE—see Overview of Nutritional Support), described as very low calorie diets, can have as few as 800 kcal/day. A very low calorie diet may be indicated for obese patients; however, such diets must be supervised by a physician, and after weight is lost, intake must be increased gradually to prevent patients from regaining weight.

Physical activity

Exercise increases energy expenditure, BMR, and diet-induced thermogenesis. Exercise also seems to regulate appetite to more closely match caloric needs. Other benefits associated with physical activity include

  • Increased insulin sensitivity

  • Improved lipid profile

  • Lower BP

  • Better aerobic fitness

  • Improved psychologic well-being

  • Decreased risk of breast and colon cancer

  • Increased life expectancy

Strengthening (resistance) exercises increase muscle mass. Because muscle tissue burns more calories at rest than does fat tissue, increasing muscle mass produces lasting increases in BMR. Exercise that is interesting and enjoyable is more likely to be sustained. A combination of aerobic and resistance exercise is better than either alone. Guidelines suggest physical activity of 150 min/wk for health benefits and 300 to 360 min/wk for weight loss and maintenance. Developing a more physically active lifestyle can help with weight loss and maintenance.

Behavioral interventions

Clinicians can recommend various behavioral interventions to help patients lose weight. They include

  • Support

  • Self-monitoring

  • Stress management

  • Contingency management

  • Problem solving

  • Stimulus control

Support may come from a group, a buddy, or family members. Participation in a support group can improve adherence to lifestyle changes and thus increase weight loss. The more frequently people attend group meetings, the greater the support, motivation, and supervision they receive and the greater their accountability, resulting in greater weight loss.

Self-monitoring may include keeping a food log (including the number of calories in foods), weighing regularly, and observing and recording behavioral patterns. Other useful information to record includes time and location of food consumption, the presence or absence of other people, and mood. Clinicians can provide feedback about how patients may improve their eating habits.

Stress management involves teaching patients to identify stressful situations and to develop strategies to manage stress that do not involve eating (eg, going for a walk, meditating, deep breathing).

Contingency management involves providing tangible rewards for positive behaviors (eg, for increasing time spent walking or reducing consumption of certain foods). Rewards may be given by other people (eg, from members of a support group or a health care practitioner) or by the person (eg, purchase of new clothing or tickets to a concert). Verbal rewards (praise) may also be useful.

Problem solving involves identifying and planning ahead for situations that increase the risk of unhealthy eating (eg, travelling, going out to dinner) or that reduce the opportunity for physical activity (eg, driving across country).

Stimulus control involves identifying obstacles to healthy eating and an active lifestyle and developing strategies to overcome them. For example, people may avoid going by a fast food restaurant or not keep sweets in the house. For a more active lifestyle, they may take up an active hobby (eg, gardening), enroll in scheduled group activities (eg, exercise classes, sports teams), walk more, make a habit of taking the stairs instead of elevators, and 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 with adherence to lifestyle changes and weight loss. Applications can help patients set a weight-loss goal, monitor their progress, track food consumption, and record physical activity.


Drugs (eg, orlistat, phentermine, phentermine/topiramate, lorcaserin) may be used if BMI is ≥ 30 or if BMI is 27 in patients who have complications (eg, hypertension, insulin resistance). Usually, drug treatment results in modest (5 to 10%) weight loss.

Orlistat inhibits intestinal lipase, decreasing fat absorption and improving blood glucose and lipids. Because orlistat is not absorbed, systemic effects are rare. Flatus, oily stools, and diarrhea are common but tend to resolve during the 2nd yr of treatment. A dose of 120 mg po tid should be taken with meals that include fat. A vitamin supplement should be taken at least 2 h before or after taking orlistat. Malabsorption and cholestasis are contraindications; irritable bowel syndrome and other GI disorders may make orlistat difficult to tolerate. Orlistat is available OTC.

Phentermine is a centrally acting appetite suppressant for short-term use ( ≤ 3 mo). Usual starting dose is 15 mg once/day, and dose may be increased to 30 mg once/day, 37.5 mg once/day, or 15 mg bid. Common side effects include elevated BP and heart rate, insomnia, anxiety, and constipation. Phentermine should not be used in patients with preexisting cardiovascular disorders, poorly controlled hypertension, hyperthyroidism, or a history of drug abuse or addiction. Twice/day dosing may help control appetite better throughout the day.

The combination of phentermine and topiramate (used to treat seizures and migraines) results in weight loss for up to 2 yr. The starting dose of the extended-release form (phentermine 3.75 mg/topiramate 23 mg) can be increased to 7.5 mg/46 mg after 2 wk, then gradually increased to a maximum of 15 mg/92 mg if needed to maintain weight loss. Because birth defects are a risk, the combination (which is pregnancy category X) should be given to women of reproductive age only if they are using contraception and are tested monthly for pregnancy. Other potential adverse effects include sleep problems, cognitive impairment, and increased heart rate. Long-term cardiovascular effects are unknown, and postmarketing studies are ongoing.

Lorcaserin suppresses appetite via selective agonism of serotonin 2C (5-HT 2C ) brain receptors. The usual and maximum dose of lorcaserin is 10 mg po q 12 h.

Lorcaserin and phentermine/topiramate should be stopped if patients do not have documented weight loss after 12 wk of treatment.

Most OTC weight-loss drugs are not recommended because they have not been shown to be effective. Examples of such drugs are brindleberry, l -carnitine, chitosan, pectin, grapeseed extract, horse chestnut, chromium picolinate, fucus vesiculosus, and ginkgo biloba. Some (eg, caffeine, ephedrine, guarana, phenylpropanolamine) have adverse effects that outweigh their advantages. Also, some of these drugs contain substances, sometimes unlisted, that may have adverse effects.


Surgery is the most effective treatment for extremely obese patients (see Bariatric Surgery).

Special Populations

Obesity is a particular concern in children and the elderly.


For obese children, complications are more likely to develop because they are obese longer. More than 25% of children and adolescents are overweight or obese. Risk factors for obesity in infants are low birth weight and maternal obesity, diabetes, and smoking.

After puberty, food intake increases; in boys, the extra calories are used to increase protein deposition, but in girls, fat storage is increased.

For obese children, psychologic complications (eg, poor self-esteem, social difficulties, depression) and musculoskeletal complications can develop early. Some musculoskeletal complications, such as slipped capital femoral epiphyses, occur only in children. Other early complications may include obstructive sleep apnea, insulin resistance, hyperlipidemia, and nonalcoholic steatohepatitis. Risk of cardiovascular, respiratory, metabolic, hepatic, and other obesity-related complications increases when these children become adults.

Risk of obesity persisting into adulthood depends partly on when obesity first develops:

  • During infancy: Low risk

  • Between 6 mo and 5 yr: 25%

  • After 6 yr: > 50%

  • During adolescence if a parent is obese: > 80%

In children, preventing further weight gain, rather than losing weight, is a reasonable goal. Diet should be modified, and physical activity increased. Increasing general activities and play is more likely to be effective than a structured exercise program. Participating in physical activities during childhood may promote a lifelong physically active lifestyle. Limiting sedentary activities (eg, watching TV, using the computer or handheld devices) can also help. Drugs and surgery are avoided but, if complications of obesity are life threatening, may be warranted.

Measures that control weight and prevent obesity in children may have the largest public health benefits. Such measures should be implemented in the family, schools, and primary care programs.

The elderly

In the US, the percentage of obese elderly people has been increasing.

With age, body fat increases and is redistributed to the abdomen, and muscle mass is lost, largely because of physical inactivity, but decreased androgens and growth hormone (which are anabolic) and inflammatory cytokines produced in obesity may also play a role.

Risk of complications depends on

  • Body fat distribution (increasing with a predominantly abdominal distribution)

  • Duration and severity of obesity

  • Associated sarcopenia

Increased waist circumference, suggesting abdominal fat distribution, predicts morbidity (eg, hypertension, diabetes mellitus, coronary artery disease) and mortality risk better in the elderly than does BMI. With aging, fat tends to accumulate more in the waist.

For the elderly, physicians may recommend that caloric intake be reduced and physical activity be increased. However, if elderly patients wish to substantially reduce their caloric intake, their diet should be supervised by a physician. Physical activity also improves muscle strength, endurance, and overall well-being and reduces the risk of developing chronic disorders such as diabetes. Activity should include strengthening and endurance exercises.

Regardless of whether caloric restriction is considered necessary, nutrition should be optimized.

Weight-loss drugs are often not studied specifically in the elderly, and possible benefits may not outweigh the adverse effects. However, orlistat may be useful for obese elderly patients, particularly those with diabetes mellitus or hypertension. Surgery can be considered in healthy elderly patients with good functional status.


Regular physical activity and healthy eating improve general fitness, can control weight, and help prevent obesity and diabetes mellitus. Even without weight loss, exercise decreases the risk of cardiovascular disorders. Dietary fiber decreases the risk of colon cancer and cardiovascular disorders. Sufficient and good-quality sleep, management of stress, and moderation of alcohol intake are also important.

Key Points

  • Obesity increases the risk of many common health problems and causes up to 300,000 premature deaths each year in the US, making it second only to cigarette smoking as a preventable cause of death.

  • Excess caloric intake and too little physical activity contribute the most to obesity, but genetic susceptibility and various disorders (including eating disorders) may also contribute.

  • Screen patients using BMI and waist circumference and, when body composition analysis is indicated, by measuring skinfold thickness or using bioelectrical impedance analysis.

  • Screen obese patients for common comorbid disorders, such as obstructive sleep apnea, diabetes, dyslipidemia, hypertension, fatty liver, and depression.

  • Encourage patients to lose even 5 to 10% of body weight by changing their diet, increasing physical activity, and using behavioral interventions if possible.

  • Try treating patients with orlistat, phentermine, phentermine/topiramate, or lorcaserin if BMI is ≥ 30 or if BMI is ≥ 27 and they have complications (eg, hypertension, insulin resistance); however, for extreme obesity, surgery is most effective.

  • Encourage all patients to exercise, to eat healthily, to get enough sleep, and to manage stress.

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