Condition Definition

Obesity and overweight are most commonly defined by BMI, which is calculated as weight in kilograms divided by height in meters squared. Overweight is defined as a BMI of 25 to 29.9 kg/m². Obese is defined as a BMI of ≥30 kg/m². The category of “obese” is further divided into subcategories of class I (BMI 30.0–34.9 kg/m²), class II (BMI 35.0–39.9 kg/m²), and class III (BMI ≥40 kg/m²).¹

Prevalence and Burden of Disease/Illness

According to the most recent National Health and Nutrition Exam Survey data, the prevalence of obesity in the United States is high, exceeding 30 percent in most age- and sex-specific groups. In 2007–2008, 32 percent of U.S. men and 36 percent of U.S. women were obese and an additional 40 percent of men and 28 percent of women were overweight.² About 1 in 20 Americans has a BMI of >40 kg/m² (class III obesity).² The prevalence of obesity and overweight has increased by 134 percent and 48 percent, respectively, since 1976–1980.³ Between 1999 and 2008, while overweight/obesity trends stabilized for women, overweight/obesity rates continued to rise for men.² In the Framingham cohort, the long-term risk for becoming overweight or obese was more than 50 and 25 percent, respectively.⁴

Using standard BMI definitions across ethnic groups, nonwhite adults have a higher prevalence of overweight and obesity than white adults. Among women, for example, the age-adjusted prevalence of obesity (BMI ≥30 kg/m²) is higher among nonHispanic black (49.6 percent) and Hispanic women (43 percent) than among nonHispanic white women (33 percent). The difference in obesity prevalence is less marked among men (37.3 percent in nonHispanic black men, 34.3 percent in Hispanic men, and 31.9 percent in nonHispanic white men).² Rates of obesity among Asian Americans (8.9 percent) are much lower compared with other racial groups. Given that the relationship between BMI and disease risk appears to vary among ethnic groups (as discussed below), differences in the prevalence of obesity cannot be directly translated into comparable differences in disease risk.

Obesity is associated with an increased risk of death, particularly in adults younger than age 65 years.^5-9 Obesity has been shown to reduce life expectancy by 6 to 20 years depending on age and race.^7,10 Ischemic heart disease, diabetes, cancer (especially liver, kidney, breast, endometrial, prostate, and colon), and respiratory diseases are the leading causes of death in persons who are obese.⁸

Whether being overweight is associated with an increased mortality risk is less clear. Some,^5,8-11 but not all,^5,6,12,13 studies have found an increased risk of death in those who are overweight. The association between overweight/obesity and mortality risk, however, varies by sex, ethnicity, and age, which may be why data are mixed. The BMI value that is associated with the lowest mortality risk varies among different ethnic subgroups. For some groups, the lowest mortality risk is a BMI that falls in the normal range, but for other ethnic groups, the lowest mortality is associated with a BMI in the overweight range. Black populations, for example, appear to have lowest mortality rates at a BMI of 26.2 to 28.5 kg/m² in women and 27.1 to 30.2 kg/m² in men.^12,14 In comparison, white women and men experience lowest mortality at a BMI of 24.5 to 25.6 kg/m² and 24.8 kg/m², respectively.^12,14 On the other hand, certain Asian populations may experience lowest mortality rates at a BMI of 23 to 24.9 kg/m².^15-18

The relationship between BMI and mortality is different in adults older than age 65 years.^19,20 In this population, waist circumference appears to have an association with mortality, but BMI does not. It is hypothesized that in the older adult population, a high BMI may be a marker of more lean mass (and thus decreased mortality risk), whereas waist circumference is a better marker of adiposity and thus more correlated with cardiovascular risk.

Being overweight or obese is associated with an increased risk of coronary heart disease (CHD),^21-23 even after adjustment for established risk factors.^21,24 In a meta-analysis of 21 cohort studies including more than 300,000 predominantly white persons, overweight increased the risk of CHD events by 17 percent and obesity increased it by 49 percent after adjustment for age, sex, physical activity, smoking, blood pressure, and cholesterol levels.²¹ Recent adjusted estimates of CHD and hypertension health risks among nonHispanic white, nonHispanic black, East Asian, and Hispanic Americans suggest that all groups have increased cardiovascular disease risk with increasing BMI, but there are significant group-specific differences in absolute risk and the level of BMI at which increased risk occurs.²⁵ In black populations, increasing BMI is less associated with increasing cardiovascular disease risk compared with whites.^26-28 Data for Latino populations suggest a lesser association of cardiovascular disease and BMI compared with whites and other higher risk subgroups.²⁵ However, increasing BMI is associated with increased cardiovascular disease risk in many Asian populations, and cardiovascular disease risk seems to begin to rise at a lower BMI level in Asian compared with white populations.^29-31

Type 2 diabetes is strongly associated with obesity or overweight. According to a systematic review and meta-analysis of prospective cohort studies, overweight and obese men had a respective 2.4- and 6.7-fold increased risk of type 2 diabetes compared with normal weight men.³² Overweight and obese women had a respective 3.9- and 12.4–fold greater risk of type 2 diabetes compared with normal weight women.³² A BMI of >25 kg/m² was associated with a 2.2-fold greater risk of death from diabetes, a greater association than with any other cause of death.⁸

Evidence suggests that the relationship between BMI and diabetes risk also varies by ethnicity. As with cardiovascular disease, there are significant group-specific differences in absolute risk and the level of BMI at which increased type 2 diabetes risk occurs.²⁵ For example, many nonwhite populations appear to have a higher diabetes risk at similar BMI levels than white populations, and diabetes risk can begin to increase at lower BMI levels in some ethnic groups. This has been best studied in East Asians (Chinese, Japanese, and Korean populations), and is also being increasingly recognized among South Asians and Latinos (two large subpopulations that also have a higher overall prevalence of diabetes relative to other groups).^33-36 Reacting to this trend, the World Health Organization (WHO) recently adjusted screening guidelines for Asia to recommend country-specific BMI cut-off points that may start as low as 23 kg/m² for some populations.^37-39

The incidence of many types of cancer increases with increasing BMI. In particular, endometrial, gallbladder, esophageal, and renal cancer incidence is increased in obese women and esophageal, thyroid, colon, rectal, and renal cancer incidence is increased in obese men.^40-42 The risk of dying from several types of cancer (i.e., liver, pancreas, and stomach cancer in men and uterine, kidney, and cervical cancer in women) is increased with increasing BMI.^42,43

Other diseases that have been associated with obesity include ischemic stroke,^31,44,45 heart failure,²⁴ atrial fibrillation/flutter,^46,47 dementia,⁴⁸ venous thrombosis,⁴⁹ gallstones,^50,51 gastroesophageal reflux disease,⁵² renal disease,^53,54 and sleep apnea.⁵⁵ Obesity also increases the risk of developing osteoarthritis^56,57 and is associated with functional disability.⁵⁸ In addition, maternal obesity is associated with pregnancy complications and adverse pregnancy outcomes and adversely influences fetal and neonatal health.^59-62

Some observational studies suggest that obese individuals, even those without comorbid diseases, can have a decreased quality of life compared with normal weight individuals.⁶³ Among normal weight and overweight women, quality of life (especially physical function) decreased with weight gain. In contrast, quality of life improved in overweight women who lost weight.⁶⁴ A recent meta-analysis suggests a reciprocal link between obesity and depression.⁶⁵ As a result of the increased morbidity, there is increased use of health care services and costs among the obese.^66,67 Compared with adults with a BMI of 20 to 24.9 kg/m², those with a BMI of 30 to 34.9 kg/m² and ≥35 kg/m² had 25 and 44 percent higher mean annual total (inpatient and outpatient) health service costs, respectively. There was no increase in health service costs in overweight adults (BMI 25 to 29.9 kg/m²).⁶⁷

Etiology and Natural History

Overweight and obesity ultimately result from an imbalance between energy intake and energy output. Energy balance appears to have both environmental and genetic influences.^68,69 Environmental factors that play an important role in the growing obesity epidemic include an increasingly sedentary lifestyle,⁷⁰ television watching,⁷¹ fast food consumption,⁷² and sleep deprivation.⁷³ Exposures in early development may influence the risk of developing obesity later in life. For example, maternal smoking,⁷⁴ maternal gestational diabetes,⁷⁵ and short or no exposure to breastfeeding are associated with an increased risk of childhood obesity.⁷⁶ Childhood obesity increases the risk of adult obesity.^77,78

In terms of the natural history of obesity, weight gain occurs until about the sixth decade of life, when weight appears to stabilize and then decline with age.^79-81 Having an elevated BMI in early adulthood (ages 20 to 22 years) appears to increase the risk of developing obesity within 15 years. For example, in a study of the natural history of the development of obesity in young U.S. adults, 41 percent of white, 47 percent of Hispanic, and 66 percent of black women who had a BMI of 24 to 25 kg/m² at ages 20 to 22 years became obese by ages 35 to 37 years.⁸²

Rationale for Screening

Screening for overweight/obesity would be beneficial if persons with increased weight have an elevated disease risk and if interventions to reduce weight successfully decrease that disease risk. However, the harms of screening must also be considered. The act of obtaining BMI, as noted in a previous USPSTF statement, is “not associated with any direct physical harm.”⁸³ Other methods of measuring obesity, such as waist circumference, WHR, or percent body fat, are still quite inexpensive and similarly not associated with any direct physical harm.⁸³

Possible secondary harms might include labeling stigma, as well as potential financial cost to patients in the form of higher insurance premiums, or reinforcement of poor self-esteem. However, there are no data about how often these potential secondary harms actually result from screening for obesity.

Screening Strategies

Measurements that can be used to estimate body fat and quantify health risks include BMI, waist circumference, WHR, bioimpedance, and dual-energy x-ray absorptiometry (DXA).³ Measuring height and weight to calculate BMI in a clinical setting is a low-cost, relatively quick, and reasonably reliable way to screen for obesity. Reference charts and BMI calculators are available to allow clinicians to look up a patient's BMI from his/her height and weight without manual calculation. The previous evidence report found good-quality evidence that BMI identifies adults with increased risk of future morbidity and mortality. As such, we did not systematically address the question of the relative value of different measures to screen for excess body fat.⁸⁴ Since that last evidence report, however, data from large (more than 10,000 persons) prospective studies have been published suggesting that WHR offers independent predictive value for mortality in addition to BMI.^85-93 WHR has an added benefit in that its cut-off points are similar even in different populations, simplifying interpretation.^94-96

Of the central adiposity measures, waist circumference is probably the most reproducible and the simplest to measure, and is independently associated with risk. As such, waist circumference is emerging as the most useful measure to add to screening recommendations.^{86,94,95,97-99} The bulk of the recent identified literature supports waist circumference as having an independent association with morbidity and mortality, especially in many higher-risk populations, such as South Asians or Mexicans, who might have a higher prevalence of obesity-associated morbidity such as diabetes.^36,98 It also appears to be more sensitive in detecting persons who are at increased cardiometabolic risk, even in the normal BMI categories.^86,97,99-105

For waist circumference, the National Heart, Lung, and Blood Institute (NHLBI) has defined cut-off points for abdominal obesity as >88 cm in women and >102 cm in men.¹⁰⁶ However, WHO has recommended lower cut-off points for Asian populations of >80 cm in women and >90 cm in men, meant to correspond to the lower cut-off points defined by NHLBI.^107,108 A review and meta-analysis of waist circumference and WHR variation in cut-off points among different ethnic groups supports a lower waist circumference cut-off point for East Asian populations, consistent with WHO's guidelines, and that South Asian populations in particular may need similar or possibly even slightly lower cut-off points (>80 cm in women and >85 cm in men).⁹⁸ In Latino populations, data are mixed, likely in part due to cultural practices as well as genetics and body type variation within the overall categorization of “Latino” or “Hispanic.” Black populations may have similar cut-off points to whites, but data in that population are not sufficient and require further study, as different components of risk exist in that population. Pacific Islander and Middle Eastern populations are not adequately studied to identify different cut-off points.⁹⁸ There are also increasing populations of adults in the United States of mixed ethnicity, and disease risk for them is complex and largely unstudied.

Interventions/Treatment

Clinical interventions to achieve and maintain weight reduction include behavioral-based interventions to induce lifestyle change (dietary restriction, increased physical activity, or both), pharmacotherapy, and surgery. Behavioral-based clinical interventions optimally will combine information on safe physical activity and healthy eating for weight loss with cognitive and behavioral management techniques to help participants make and maintain lifestyle changes.¹ Several medications are currently approved in the United States for the management of obesity, including weight loss and maintenance of weight loss, in conjunction with a reduced calorie diet: orlistat, phentermine, and diethylpropion. These medications are recommended for obese patients with an initial BMI of ≥30 kg/m² or ≥27 kg/m² in the presence of other risk factors (e.g., diabetes, dyslipidemia, or controlled hypertension).

Orlistat decreases fat digestion by inhibiting pancreatic lipases. Ingested fat is not completely hydrolyzed, resulting in increased fecal fat excretion. The recommended prescription dose is 120 mg three times a day (tid) with each main meal containing fat. The patient should be on a nutritionally balanced, reduced-calorie diet that contains approximately 30 percent of calories from fat. A lower dose of 60 mg is available as an over-the-counter medication. Per the U.S. Food and Drug Administration (FDA), the safety and effectiveness of orlistat beyond 4 years have not been determined at this time. Orlistat is contraindicated in patients with chronic malabsorption syndrome or cholestasis and in patients with known hypersensitivity to orlistat or to any component of this product.

Sympathomimetic drugs block the reuptake of norepinephrine and serotonin into nerve terminals, thereby leading to early satiety and reduced food intake. The only currently approved sympathomimetic drugs, phentermine and diethylpropion, are for short-term use (usually interpreted as up to 12 weeks). The use of these short-term drugs in the treatment of obesity was not included in this systematic evidence review.

Sibutramine is a sympathomimetic weight loss drug that was previously approved for longer-term use. However, it was voluntarily removed from the market by Abbott Laboratories at the request of the FDA on October 8, 2010. The FDA recommended against continued prescribing and use of sibutramine because it concluded that the drug may pose unnecessary cardiovascular risks to patients. The FDA's recommendation was based on new data from the Sibutramine Cardiovascular Outcomes trial, a trial of persons older than age 55 years with cardiovascular disease. The FDA concluded that the risk for adverse cardiovascular events from sibutramine outweighed any benefit from the modest weight loss observed with the drug.

Metformin is primarily a medication used to treat diabetes, but has been used off label to promote weight loss and prevent diabetes in high-risk persons. Metformin decreases hepatic glucose production, decreases intestinal absorption of glucose, and increases peripheral glucose uptake and utilization. The mechanism by which metformin reduces weight is not clear. Metformin might enhance glucagon-like peptide (GLP-1) secretion.^109-111 GLP-1 has been shown to slow gastric emptying and reduce food intake.^112,113 There is no fixed dosage regimen for the management of hyperglycemia in patients with type 2 diabetes. Dosage must be individualized on the basis of both effectiveness and tolerance, while not exceeding the maximum recommended daily dose. The maximum recommended daily dose of metformin is 2,550 mg in adults. It should be taken in divided doses with meals. Metformin is contraindicated in patients with renal disease or renal dysfunction, known hypersensitivity to metformin, or acute or chronic metabolic acidosis.

Another medication that is used off label for weight loss is zonisamide, an antiepileptic agent.¹¹⁴

We did not include this medication in our systematic evidence review. There are also several novel antiobesity drugs in development. Lorcaserin, a selective 5-hydroxytryptamine receptor agonist, was voted against by an FDA advisory panel on September 16, 2010 because of concerns over both safety and efficacy. Qnexa, a combination of phentermine and topiramate, an antiepilepsy and migraine drug, was rejected by the FDA on October 28, 2010 because of safety concerns. Contrave, a combination of naltrexone (an opioid receptor antagonist) and bupropion (a dopamine and norepinephrine reuptake inhibitor), was rejected by the FDA on January 31, 2011, who cited the need for a large-scale study of the cardiovascular effects of the drug before it could be approved.¹¹⁵ A combination of bupropion and zonisamide is currently being studied in phase III trials.¹¹⁴

Current Clinical Practice

Despite the ease of determining BMI, surveys have indicated that only 38 to 66 percent of overweight or obese patients have received diagnoses of overweight or obesity, and less than half of obese patients report that their physicians have advised them to lose weight and/or provided specific information about how to lose weight.^116,117 According to the most recent data from the U.S. National Ambulatory Medical Care Survey, almost 50 percent of clinic visits lack complete height and weight data needed to screen for obesity using BMI.¹¹⁸ Of those visits where BMI was determined to be ≥30 kg/m², 70 percent of patients were not given a diagnosis of obesity and 63 percent did not receive any counseling for weight reduction.¹¹⁸ Even among those who suffer from obesity-related comorbidities, only 52 percent were screened for obesity, 34 percent were diagnosed with obesity, and 46 percent were counseled about their obesity.¹¹⁸ When overweight American adults were surveyed, only 24.4 percent of obese Americans were referred by their physician to a dietician or nutritionist and 11 percent were recommended to a formal diet program; less than 10 percent of those who were overweight were referred for these nutritional services.¹¹⁹ Close to 10 percent of obese adults were prescribed a weight loss medication.¹¹⁹ However, many who are prescribed weight loss medications may not meet approved indications and/or may have contraindications.¹²⁰ For example, a Swedish survey found that 6 percent of patients prescribed orlistat did not meet the BMI requirement (≥30 kg/m² with no cardiovascular risk factors or ≥27 kg/m² with cardiovascular risk factors).¹²⁰