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Obesity is
severe excess body fat. Complications include cardiovascular disorders,
diabetes mellitus, many cancers, cholelithiasis, fatty liver and
cirrhosis, osteoarthritis, psychologic disorders, and premature death.
Diagnosis is based on body mass index (BMI—calculated from height
and weight) and waist circumference. Blood pressure, fasting blood
glucose, and lipid levels should be measured. Treatment includes exercise,
dietary and behavior modification, and sometimes drugs or surgery.
Prevalence of obesity in the US is high and is increasing. Age-adjusted prevalence was 22.9% in 1988 to 1994, increasing to 30.5% in 1999 to 2000. Prevalence of overweight (less severe excess body fat) increased from 55.9 to 64.5% during this period. Prevalence is more than twice as high at age 55 than at age 20. Obesity is twice as common among women with a lower socioeconomic status as among those with a higher status. Prevalence among black and white men does not differ significantly, but it is higher among black women than white women. More than half of black women aged ≥ 40 yr are obese; > 80% are overweight. Obesity and its complications cause as many as 300,000 premature deaths each year, making it second to cigarette smoking as a cause of death.
Etiology
Almost all cases of obesity result from chronic overeating plus inadequate exercise and a genetic predisposition. Genetic, metabolic, and other determinants usually play minor roles.
Genetic determinants:
Heritability of BMI is about 33%. Genetic factors may affect the many signaling molecules and receptors used by parts of the hypothalamus and GI tract to regulate food intake (
see Sidebar 1: Obesity and the Metabolic Syndrome: Pathways Regulating Food Intake ). 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 play a larger role in determining body fat distribution, particularly abdominal fat (see Obesity and the Metabolic Syndrome: Metabolic Syndrome), than the amount of body fat.
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Sidebar 1
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Environmental
determinants:
Overweight results much more often from excess caloric intake than from slow metabolism. Diets high in fat and refined carbohydrates promote weight gain; those high in fresh fruit and vegetables, fiber, and complex carbohydrates minimize weight gain. A sedentary lifestyle promotes weight gain.
Regulatory determinants:
Maternal obesity, maternal smoking, intrauterine growth restriction, drugs, and, rarely, brain damage and endocrine disorders can disturb weight regulation. About 15% of women permanently gain ≥ 20 lb with each pregnancy. Infant or childhood obesity makes weight loss in later life more difficult.
Drugs, including corticosteroids, lithium , traditional antidepressants (tricyclics, tetracyclics, and monoamine oxidase inhibitors [MAOIs]), benzodiazepines, and antipsychotic drugs, often cause weight gain.
Rarely, brain damage caused by a tumor (especially a craniopharyngioma) or an infection (particularly affecting the hypothalamus) can stimulate consumption of excess calories. Hyperinsulinism due to pancreatic tumors may result in weight gain. Hypercortisolism due to Cushing's syndrome produces predominantly abdominal obesity. Hypothyroidism is rarely a cause of substantial weight gain.
Psychologic and
behavioral determinants:
Psychologic and behavioral factors are believed to be limited largely to two pathologic eating patterns: binge eating disorder and night-eating syndrome. Similar but less extreme patterns, classified as eating disorders not otherwise specified, probably contribute to excess weight gain in more people.
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 Mood Disorders). 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.
The night-eating
syndrome consists of morning anorexia, evening hyperphagia, and insomnia. 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. However, nocturnal eating contributes to excess weight gain in many other people.
Complications
Insulin resistance, dyslipidemias, and hypertension develop, ultimately predisposing to diabetes mellitus and coronary artery disease. Complications are more likely if fat is concentrated abdominally. Obesity is also a risk factor for nonalcoholic fatty liver, which may lead to cirrhosis.
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 Sleep Apnea: Obstructive Sleep Apnea). This disorder, often undiagnosed, can cause loud snoring and excessive daytime sleepiness.
In the obesity-hypoventilation syndrome (Pickwickian syndrome), impaired breathing leads to hypercapnia, reduced sensitivity to CO2 in stimulating respiration, hypoxia, cor pulmonale, and risk of premature death. This syndrome may occur alone or secondary to obstructive sleep apnea.
Degenerative arthritis, particularly affecting weight-bearing joints, 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, polycystic ovary syndrome, gout, deep venous thrombosis and pulmonary embolism, and many cancers.
Obesity leads to social, economic, and psychologic problems as a result of prejudice, discrimination, poor body image, and low self-esteem.
Diagnosis
In adults, overweight or obesity is determined by BMI, defined as weight (kg) divided by height (m)2. BMI of 25 kg/m2 to 29.9 kg/m2 indicates overweight; BMI ≥ 30 kg/m2 indicates obesity (see Table 1: Obesity and the Metabolic Syndrome: Body Mass Index (BMI)  ). BMI is age- and race-specific; its use is limited in children and the elderly. In children and adolescents, overweight is BMI ≥ 95th percentile based on age- and sex-specific CDC growth charts. Asians, Japanese, and many aboriginal populations have a lower cut-off (23 kg/m2) for overweight. Large muscle mass without excess body fat may result in a high BMI.
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Table 1
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Body Mass Index (BMI)
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Normal*
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Over-weight
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Obese
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Extremely Obese
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BMI
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19 – 24
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25 – 29
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30 – 34
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35 – 39
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40 – 47
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48 – 54
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Height (inches)
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Body Weight (pounds)
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60 – 61
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97 – 127
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128 – 153
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153 – 180
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179 – 206
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204 – 248
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245 – 285
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62 – 63
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104 – 135
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136 – 163
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164 – 191
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191 – 220
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218 – 265
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262 – 304
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64 – 65
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110 – 144
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145 – 174
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174 – 204
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204 – 234
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232 – 282
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279 – 324
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66 – 67
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118 – 153
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155 – 185
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186 – 217
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216 – 249
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247 – 299
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297 – 344
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68 – 69
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125 – 162
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164 – 196
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197 – 230
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230 – 263
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262 – 318
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315 – 365
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70 – 71
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132 – 172
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174 – 208
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209 – 243
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243 – 279
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278 – 338
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334 – 386
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72 – 73
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140 – 182
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184 – 219
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221 – 257
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258 – 295
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294 – 355
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353 – 408
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74 – 75
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148 – 192
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194 – 232
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233 – 272
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272 – 311
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311 – 375
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373 – 431
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76
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156 – 197
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205 – 238
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246 – 279
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287 – 320
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328 – 385
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394 – 443
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*BMIs less than those listed as normal are considered underweight.
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In whites, a waist circumference > 93 cm (> 36.3 in), particularly > 101 cm (> 39.4 in), in men or > 79 cm (> 30.8 in), particularly > 87 cm (> 33.9 in), in women is a risk factor for complications of obesity.
Body
composition analysis:
Body composition—the percentage of body fat and muscle—is also considered when obesity is diagnosed. The percentage of body fat can be estimated by measuring skinfold thickness or determining mid upper arm area (see Undernutrition: Physical examination). The appropriate percentage is based on the patient's demographic group. Ranges are higher for women and the elderly.
Skinfold thickness estimates body fat stores. On average, about 50% of adipose tissue is beneath the skin. This figure can vary in the elderly because of age-related atrophic changes. Skinfold thickness (consisting of a double layer of skin and subcutaneous fat) can be measured with a special caliper at subscapular, posterior triceps (triceps skinfold [TSF]), lower thoracic, iliac, and abdominal sites. A single measurement of the TSF is acceptable; this area is easily accessible and usually edema-free. Typical ranges for TSF are 0.5 to 2.5 cm (average, 1.2 cm) in healthy men and 1.2 to 3.4 cm (average, 2.0 cm) in healthy women. TSF varies with age. In the elderly, the subscapular region is more reliable.
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 people with a limited number of 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, a person 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 sleep apnea with an instrument such as the Epworth Sleepiness Scale. If the respiratory distress index is > 6, a polysomnographic sleep study should be done. Fasting blood glucose and plasma lipids should be measured routinely in obese patients.
Prevention
Exercise, healthful eating, and behavior changes, which improve general health, are recommended. They can control weight even in healthy people and help prevent obesity and diabetes mellitus. Also, exercise decreases the risk of cardiovascular disorders; dietary fiber decreases the risk of colon cancer and cardiovascular disorders.
Prognosis
Untreated, obesity tends to progress. After weight loss, most people return to their pretreatment weight within 5 yr. The probability and severity of complications are proportional to the severity of obesity and, independently of sex, to the waist circumference. In men, mortality and morbidity are worse, probably because abdominal adiposity is greater. However, most people treated for obesity are women, who are less likely to develop its complications.
Treatment
Weight loss of even 5 to 10% seems to improve health, increase longevity, and decrease risk of complications. In obstructive sleep apnea, a much greater weight loss is required.
Support from health care practitioners, peers, and family members and various structured programs can help with weight loss and weight maintenance.
Weight loss requires dietary modification and increased physical activity, usually with behavioral therapy. Sometimes drugs or surgery is required.
Diet:
Low-fat and healthful diets, modest calorie restriction (to 1000 to 1400 kcal/day), and the substitution of some protein for carbohydrate appear to have the best long-term outcome. Fresh fruits and vegetables and fiber should be substituted for refined carbohydrates and processed food, and water for soft drinks or juices. Foods with a low glycemic index (see Table 1: Nutrition: General Considerations: 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.
Diets that require atypical eating habits should be avoided. They are unlikely to be maintained, and weight increases when the patient resumes previous poor eating habits. Calorie restriction to < 1200 kcal/day cannot be sustained, but such diets are sometimes needed to achieve rapid short-term weight loss (eg, before surgery or for obstructive sleep apnea). Diets of < 800 kcal do not produce greater weight loss and are less well tolerated.
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 include increased insulin sensitivity, improved plasma lipid profile, reduced blood pressure, better aerobic fitness, and improved psychologic well-being. Strengthening (resistance) exercises increase muscle mass. Because muscle tissue burns more calories at rest than fat tissue, increasing muscle mass produces lasting increases in BMR. Exercise that is interesting and enjoyable is more likely to be sustained.
Behavioral
therapy:
Behavioral therapy aims to improve eating habits and physical activity level. Rigid dieting is discouraged in favor of healthy eating. Common-sense measures include avoiding high-calorie snacks, choosing healthful foods when dining out, eating slowly, and substituting a physically active hobby for a passive one. Social support, cognitive therapy, and stress management may help, particularly during the lapses usually experienced during any long-term weight loss program.
Drugs:
Drugs are indicated if BMI is > 30 or if BMI is > 27 and there are complications (eg, hypertension, insulin resistance). Most weight loss due to drug treatment is modest (5 to 10%) and occurs during the first 6 mo; drugs are more useful for maintaining weight loss. Premenopausal women taking systemically acting drugs for weight control should use contraception.
Sibutramine is a centrally acting appetite suppressant that produces dose-related weight loss. The usual starting dose is 10 mg po once/day; the dose can be decreased to 5 mg or increased to 15 mg. Common adverse effects are headache, dry mouth, insomnia, and constipation; the most common serious one is hypertension. Cardiovascular disorders, particularly poorly controlled hypertension, are contraindications.
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 second year 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.
OTC weight-loss drugs are not recommended. Some (eg, caffeine, ephedrine , guarana, phenylpropanolamine) may be marginally effective, but their adverse effects outweigh their advantages. Others (eg, brindleberry, l‑carnitine, chitosan, pectin, grapeseed extract, horse chestnut, chromium picolinate, fucus vesiculosus, ginkgo biloba) have not been shown to be effective and may have adverse effects.
Surgery:
Surgery is indicated if exercise, diet, and behavioral therapy are ineffective in patients who are very obese (BMI > 40) or have serious complications. Weight loss (usually 40 to 60 kg) is proportional to the severity of obesity; losses appear to be maintained for the long term. The Roux-en-Y gastric bypass is most effective. Adjustable gastric bands placed via a laparoscope, a reversible procedure, are also effective.
Weight loss after surgery is rapid at first, slowing gradually over 2 yr. Many complications of obesity resolve; mood, self-esteem, body image, activity levels, and interpersonal and vocational effectiveness improve. If experienced surgeons perform surgery, preoperative and operative mortality is usually < 1% and operative complications are < 10%. Chronic complications depend on the procedure and may include vomiting, diarrhea, and dumping syndrome (see Gastritis and Peptic Ulcer Disease: Surgery). Vitamin and iron deficiencies may occur but are rare if the diet is nutritionally balanced and supplements are taken.
Special Populations
Obesity is a particular concern in children and the elderly.
Children:
Childhood obesity is even more worrisome than adult obesity. For obese children, complications are more likely because they are obese longer. About 20 to 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 and musculoskeletal complications can develop early. Respiratory, metabolic, and hepatic complications may also develop. Some musculoskeletal complications, such as slipped capital femoral epiphyses, occur only in children. The risk of cardiovascular, respiratory, and other obesity-related complications increases during adulthood.
The risk of obesity persisting into adulthood is low if obesity first develops during infancy, 25% if between 6 mo and 5 yr, > 50% if after 6 yr, and > 80% if during adolescence and a parent is obese.
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. 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 benefit public health the most. Such measures should be implemented in the family, schools, and primary care programs.
The
elderly:
With aging, body fat increases and is redistributed to the abdomen, and muscle mass is lost, largely because of physical inactivity. The risk of complications depends on body fat distribution (increasing with a predominantly abdominal distribution), history of obesity, and associated sarcopenia. Increased waist circumference better predicts morbidity and mortality risk in the elderly than BMI. For the elderly, mortality risk is greater when BMI decreases; thus, increased physical activity is preferable to dietary restriction unless mobility is restricted. Physical activity also improves muscle strength, endurance, and overall well-being. Activity should include strengthening and endurance exercises. Use of weight-loss drugs has not been evaluated in the elderly; surgery is best avoided.
Last full review/revision November 2005
Content last modified November 2005
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