Obesity in adults: Prevalence, screening, and evaluation

INTRODUCTION — The morbidity and mortality associated with being considered overweight or having obesity have been known to the medical profession for more than 2000 years [1]. According to the World Health Organization (WHO), obesity is defined as “excess or abnormal fat accumulation that presents a risk to health” [2]. The Centers for Disease Control and Prevention (CDC) defines it as “weight that is considered higher than what is considered healthy for a given height is described as overweight or obesity” [3]. Both use body mass index (BMI) to further define these terms, with a BMI of 25 to 29.9 kg/m² considered overweight and ≥30 kg/m² as obesity. However defined (by WHO, CDC, or other), obesity is a chronic disease that is increasing in prevalence in adults, adolescents, and children and is now considered to be a global epidemic. Screening for obesity can identify high-risk patients who may not otherwise receive counseling about health risks, lifestyle changes, obesity treatment options, and risk factor reduction. Evaluation of a patient who is overweight or with obesity should include both clinical and laboratory studies; the combined information is used to characterize the type and severity of obesity, determine health risk, and provide a basis for selecting therapy.

The prevalence of obesity, the approach to screening, and the clinical evaluation of obesity in adults will be reviewed here. The health risks associated with obesity and approach to treatment are discussed in detail separately. (See "Overweight and obesity in adults: Health consequences" and "Obesity in adults: Overview of management".)

PREVALENCE — Obesity, a chronic disease that is increasing in prevalence globally, is a major contributor to poor health in most countries [4].

United States — Estimates of the prevalence of obesity in the United States using consistent, standardized methods have been ongoing since 1960 [5,6]. Prevalence data have been collected in two ways: annual telephone surveys conducted by state Departments of Health in collaboration with the Centers for Disease Control and Prevention (the Behavioral Risk Factor Surveillance System [BRFSS]), and directly measured height and weight obtained in field surveys by the National Center for Health Statistics as the National Health and Nutrition Examination Survey (NHANES).

Data from BRFSS consistently report obesity prevalence rates lower than those found in the NHANES surveys. This is due to self-report bias (eg, people under-reporting their weight, over-reporting their height, or both) during telephone surveys. Thus, it is essential to identify the data source when evaluating obesity prevalence information.

Based upon data collected for NHANES from 1988 to 1994, 1999 to 2000, and 2017 to 2018, the age-adjusted overall prevalence of obesity in the United States increased progressively from 22.9 to 30.5 to 42.4 percent (figure 1) [7]. In 2017 to 2018, the prevalence of obesity was the same in adult males and females [8].

The age-adjusted prevalence of class III obesity (body mass index [BMI] ≥40 kg/m²), sometimes referred to as severe obesity, has risen from 5.7 percent to 9.2 percent between 2007 and 2018 [7,8].

In 2018, the self-reported prevalence of obesity in United States adults ranged from 23 percent in Colorado to almost 40 percent in Mississippi and West Virginia [9]. The prevalence of self-reported obesity was generally higher in the Midwest and the South than in the Northeast and the West. Although the regional prevalence of self-reported obesity generally parallels the results obtained by more objective surveys, the reported degree of obesity is lower [10-12].

Projections using extrapolated data from BRFSS and NHANES suggest that by 2030, almost half of all United States adults will be obese, and almost one-fourth will be severely obese [13].

Canada — The prevalence of obesity is rising in Canada. In 1985, fewer than 10 percent of people in nine of the provinces were obese; by 1990, this was true for only three of the provinces. By 1994, five provinces had obesity rates between 15 to 19 percent [14]. By 2016, more than 22 percent of males and 20 percent of females in all of Canada were obese, a steady increase [15,16].

Global — In 2015, roughly 604 million adults had obesity worldwide [17]. Since 1980, the prevalence of obesity has doubled in more than 70 countries and has continued to increase in most other countries as well. Rates of increase were similar between males and females in all age groups and were highest during early adulthood. Additionally, in 2015, at all socioeconomic levels and for all age groups, the prevalence of obesity was higher for females than males. The prevalence of obesity increased the greatest from 1980 to 2015 for males age 25 to 29 in low- to middle-income countries, from 11.1 to 38.3 percent.

SCREENING

Our approach — We suggest that all adult patients be screened for overweight and obesity by measuring BMI as part of their routine physical examination. We also recommend measuring waist circumference in those with BMI between 25 and 35 kg/m². This is consistent with guidelines for the screening and evaluation of overweight and obesity [18-25].

Rationale — Obesity is associated with a significant increase in morbidity (including diabetes mellitus, hypertension, dyslipidemia, heart disease, stroke, sleep apnea, and cancer) and mortality. Weight loss is associated with a reduction in obesity-associated morbidity. (See "Overweight and obesity in adults: Health consequences" and "Obesity in adults: Overview of management", section on 'Importance of weight loss'.)

Without screening, many high-risk patients may not receive counseling about health risks, lifestyle changes, obesity treatment options, and risk factor reduction.

Clinicians miss opportunities to screen, diagnose, and manage obesity. As an example, in a study of 20,383 patients with BMI ≥25 kg/m² in 57 primary care practices, only 12 percent had a conversation with their primary care provider about their weight that resulted in the use of a weight-related ICD-10 code for billing. Only 6 percent received continuing care for their weight (ie, prescription of anti-obesity medication or referral to a dietician or specialist) [26]. Similarly, a 2018 report of 3008 patients with obesity found that only 55 percent had a diagnosis of obesity and even fewer had an obesity management plan documented by their clinicians [27].

Barriers to screening — A frequently cited barrier to screening is the potential risk of stigma associated with a diagnosis of obesity. Use of sensitive language around obesity (such as "unhealthy" or "excess" weight) and education about the increasing number of treatment options can improve patient engagement in their care. Using language such as “person with obesity” rather than “obese person" may also help to reduce stigma and improve patient engagement.

Measurements — All adult patients should be screened for overweight and obesity by measuring height, weight, and calculating body mass index (BMI) as part of the routine physical examination. In addition, we suggest measuring waist circumference in those with BMI between 25 and 35 kg/m² (figure 2) as abdominal adiposity (and associated risk) may not be captured in this BMI range, particularly for Asian individuals. (See 'Waist circumference' below.)

Body mass index — Measuring BMI is the generally accepted first step to determine the degree of overweight. The BMI is easy to measure, reliable, and correlated with percentage of body fat and body fat mass [28]. BMI provides a better estimate of total body fat compared with body weight alone [29].

Clinicians should be aware that BMI may overestimate the degree of adiposity in individuals who are overweight but very muscular (for example, professional athletes or bodybuilders) and underestimate it in older persons due to loss of muscle mass associated with aging. In addition, among older adults in whom there is a loss of height (eg, due to osteoporosis, kyphosis), we use the patient's current, measured height to calculate the BMI [30]. Although failing to consider height loss may suggest an artifactually high calculated BMI, there is typically an associated loss of muscle mass (and weight) with aging such that the calculated BMI reflects the degree of true adiposity.

Measurement of waist circumference in conjunction with BMI can provide additional information on risk. (See 'Waist circumference' below.)

Calculation — The BMI is the most practical first step for evaluating the degree of excess weight. It is calculated from the weight and square of the height as follows:

The BMI can also be obtained from a calculator (calculator 1).

BMI-based classifications — Body mass index (BMI) classifications are based upon risk of cardiovascular disease (CVD) [31]. The recommended classifications for BMI adopted by the National Institutes of Health (NIH) and World Health Organization (WHO) [18,32] for White, Hispanic, and Black individuals are (table 1):

●Underweight – <18.5 kg/m²

●Normal weight – ≥18.5 to 24.9 kg/m²

●Overweight – ≥25.0 to 29.9 kg/m²

●Obesity – ≥30 kg/m²

•Class I – 30.0 to 34.9 kg/m²

•Class II – 35.0 to 39.9 kg/m²

•Class III – ≥40 kg/m² (also referred to as severe, extreme, or massive obesity)

The cutoffs underestimate the risk associated with obesity, such as diabetes, in the Asian population. In some populations, the level of risk in terms of percent body fat is reached at a much lower BMI (South Asian subjects) and in others a higher BMI (Black subjects) compared with White subjects [33-35]. This was illustrated in a study comparing South Asian and White individuals [34]. The mean BMI associated with development of an adverse metabolic profile (defined by markers of glucose and lipid metabolism) was 21 kg/m² among South Asian individuals and 30 kg/m² among White individuals.

Waist circumference — In addition to measuring BMI, we suggest measuring waist circumference in patients considered overweight or who have obesity to assess abdominal obesity. A waist circumference of ≥40 in (102 cm) for males and ≥35 in (88 cm) for females is considered elevated and indicative of increased cardiometabolic risk [19]. Waist circumference measurement is unnecessary in patients with BMI ≥35 kg/m² as almost all individuals with this BMI also have an abnormal waist circumference and are already at a high risk from their adiposity.

Waist circumference is a measurement of abdominal obesity and provides independent risk information that is not accounted for by BMI [36]. Patients with abdominal obesity (also called central adiposity, visceral, android, or male-type obesity) are at increased risk for heart disease, diabetes, hypertension, dyslipidemia, nonalcoholic fatty liver disease [37-41], and have higher overall mortality rates [36]. Waist circumference is used with BMI for identifying adults at increased risk for morbidity and mortality, particularly in the BMI range 25 to 35 kg/m².

The waist circumference is measured with a flexible tape placed on a horizontal plane at the level of the iliac crest as seen from the anterior view (figure 2). There is population variability in waist circumference values that predict increased risk. As an example, Japanese Americans and Indians from South Asia have more total fat and visceral fat and therefore may be at higher risk of developing type 2 diabetes for a given BMI than White individuals [33,42]. A waist circumference ≥31 in (80 cm) in Asian females and ≥35 in (90 cm) in Asian males is considered abnormal. Although dual-energy x-ray absorptiometry (DXA), computed tomography (CT), and magnetic resonance imaging (MRI) provide a more direct measurement than waist circumference for assessing the distribution of body fat, they are costly exams and are generally reserved for use as research tools.

Measurement of the waist-to-hip ratio provides no advantage over waist circumference alone and is infrequently used by clinicians. It is not recommended as part of the routine obesity evaluation by the American Heart Association (AHA)/American College of Cardiology (ACC)/The Obesity Society (TOS) guideline, although it was in the previous version [19].

EVALUATION OF PATIENTS WITH OBESITY — In patients considered overweight (body mass index [BMI] ≥25 kg/m²) or who have abdominal obesity (waist circumference greater than ≥35 in [88 cm] in females or ≥40 in [102 cm] in males), assessment of the etiology of the weight gain and its associated health risk should be undertaken. Specifically, evaluation of patients who are overweight or with obesity includes a history, physical examination, and measurement of fasting glucose (or glycated hemoglobin [A1C]), thyroid-stimulating hormone (TSH), liver enzymes, and fasting lipids (figure 3). Subsequent intervention, if necessary, is based upon historical, physical, and/or laboratory results in the setting of shared decision making with the patient. (See "Obesity in adults: Etiologies and risk factors".)

Investigating the cause — Many factors contribute to the development of obesity (table 2). However, most cases of obesity are related to behaviors such as a sedentary lifestyle and increased caloric intake. Although secondary causes of obesity are uncommon, they should be considered and ruled out [41].

To determine etiology and plan future management strategies, additional medical history should include age at onset of weight gain, events associated with weight gain, previous weight loss attempts, change in dietary patterns, history of exercise, current and past medications, and history of smoking cessation. Medications are a common cause of weight gain, in particular insulin, sulfonylureas, thiazolidinediones, glucocorticoids, and antipsychotics (table 3). Smoking cessation is also associated with weight gain. (See "Benefits and consequences of smoking cessation", section on 'Weight gain'.)

Females have more body fat as a percent of body weight than males from puberty onward and tend to gain more fat during adult life than males. In addition, females may experience modest but adverse increases in body weight and fat distribution after a first pregnancy that persist. (See "Overview of the postpartum period: Disorders and complications", section on 'Postpartum weight retention'.)

Findings from physical examination that might point to a secondary or related cause of obesity include goiter (hypothyroidism); proximal muscle weakness, moon facies, and/or purple striae (Cushing's syndrome); and acne and/or hirsutism (polycystic ovary syndrome [PCOS]). (See "Obesity in adults: Etiologies and risk factors".)

Additional testing may be required depending upon the findings on history, physical examination, and initial blood tests. This could include laboratory tests to assess the hypothalamic-pituitary axis if there are signs of disorders such as Cushing's syndrome, growth hormone deficiency, or hypothalamic obesity. (See "Obesity in adults: Etiologies and risk factors", section on 'Conditions associated with weight gain'.)

Routine genetic testing is not recommended, as monogenic disorders that include obesity are rare and usually present in childhood. Measurement of metabolic rate is not recommended, as it is not widely available and may be misinterpreted. (See "Obesity: Genetic contribution and pathophysiology", section on 'Heritable factors'.)

Assessing obesity-related health risk — Assessment of an individual's overall risk status includes determining the degree of overweight (BMI) and the presence of abdominal obesity (waist circumference), cardiovascular risk factors, sleep apnea, nonalcoholic fatty liver disease, symptomatic osteoarthritis, and other obesity-related comorbidities. The coexistence of several diseases, including established coronary heart disease (CHD), other atherosclerotic disease, type 2 diabetes mellitus, and sleep apnea, places patients in a very high-risk category for subsequent mortality. (See "Overweight and obesity in adults: Health consequences", section on 'Mortality'.)

Assessment and management of these diseases is an important part of the evaluation and management of the adult who is considered overweight or who has obesity. Further, the relationship between BMI and risk allows identification of BMI categories that can be used to guide selection of weight loss therapy [19,20].

●Weight history – The age of onset of obesity is of some importance in determining health risk. Children with a low birth weight and those whose weight rises more rapidly in the first 10 years are at high risk for diabetes as adults [43]. The risk for any given degree of obesity seems to be greater in patients whose obesity begins before the age of 40 years, probably because of the longer time period over which comorbid conditions, such as diabetes mellitus and hypertension, can develop [44]. Children with obesity at age 7 years and continue with obesity throughout puberty will almost certainly have it as adults [45]. (See "Definition, epidemiology, and etiology of obesity in children and adolescents".)

Weight gain after age 18 years is also important. Even very modest weight gain (≥5 kg) after age 18 years in females and after age 20 years in males increases the risk of CHD and type 2 diabetes at all levels of initial BMI [44]. (See "Overweight and obesity in adults: Health consequences".)

●Cardiovascular risk factors – Cardiovascular risk factors should be identified, including hypertension, dyslipidemia (reduced levels of high-density lipoprotein [HDL] or elevated levels of low-density lipoprotein [LDL]), elevated triglycerides, impaired fasting glucose or diabetes, obstructive sleep apnea, and cigarette smoking (figure 3) [32]. These risk factors should be managed to mitigate cardiovascular disease (CVD) risk independently of weight loss efforts. (See "Prevention of cardiovascular disease events in those with established disease (secondary prevention) or at very high risk".)

●Other comorbidities – Obesity is also associated with other disorders that do not increase cardiovascular risk but are associated with significant morbidity. Examples include symptomatic osteoarthritis, cholelithiasis, nonalcoholic fatty liver disease, PCOS, depression, and impaired quality of life. Attenuation of these comorbidities may be achieved through weight loss. (See "Overweight and obesity in adults: Health consequences".)

Candidates for weight-loss interventions — Because of known health risks associated with excess body weight [31], people with BMI >25 kg/m² are candidates for weight-loss interventions. Nevertheless, there are few data to support specific targets, and the approach described below is based upon expert consensus. The goal of therapy is to prevent, treat, or reverse the complications of obesity, including decrements in quality of life. (See "Obesity in adults: Overview of management".)

●For patients with BMI between 24.9 and 30 kg/m² who do not have risk factors for CVD or other obesity-related comorbidities, counseling regarding prevention of further weight gain is important. This includes advice on dietary habits and physical activity. (See "Obesity in adults: Role of physical activity and exercise", section on 'Exercise as prevention of overweight/obesity'.)

●Patients with BMI ≥30 kg/m² or between 24.9 and 30 kg/m² and with one or more risk factors for CVD (diabetes, hypertension, dyslipidemia), or with obstructive sleep apnea or symptomatic osteoarthritis, should be counseled about weight-loss interventions (diet, physical activity, behavioral modification) [19].

Specific recommendations for the treatment of obesity are reviewed separately. (See "Obesity in adults: Overview of management" and "Obesity in adults: Dietary therapy" and "Obesity in adults: Drug therapy" and "Obesity in adults: Behavioral therapy".)

SOCIETY GUIDELINE LINKS — Links to society and government-sponsored guidelines from selected countries and regions around the world are provided separately. (See "Society guideline links: Obesity in adults".)

INFORMATION FOR PATIENTS — UpToDate offers two types of patient education materials, "The Basics" and "Beyond the Basics." The Basics patient education pieces are written in plain language, at the 5^th to 6^th grade reading level, and they answer the four or five key questions a patient might have about a given condition. These articles are best for patients who want a general overview and who prefer short, easy-to-read materials. Beyond the Basics patient education pieces are longer, more sophisticated, and more detailed. These articles are written at the 10^th to 12^th grade reading level and are best for patients who want in-depth information and are comfortable with some medical jargon.

Here are the patient education articles that are relevant to this topic. We encourage you to print or e-mail these topics to your patients. (You can also locate patient education articles on a variety of subjects by searching on "patient info" and the keyword(s) of interest.)

●Basics topics (see "Patient education: Weight loss treatments (The Basics)")

●Beyond the Basics topics (see "Patient education: Losing weight (Beyond the Basics)" and "Patient education: Weight loss surgery and procedures (Beyond the Basics)")

SUMMARY AND RECOMMENDATIONS

●We suggest that all adult patients be screened for overweight and obesity (Grade 2B). This is done by measuring body mass index (BMI) (calculator 1) at the routine physical examination. In addition, we suggest measuring waist circumference in those with BMI between 25 and 35 kg/m² (Grade 2C) (figure 2) as abdominal adiposity (and associated risk) may not be captured in this BMI range. Waist circumference measurement is unnecessary in patients with BMI ≥35 kg/m². (See 'Measurements' above.)

●BMI classifications are based upon risk of cardiometabolic disease (table 1). The classification is used for identifying adults at increased risk for morbidity and mortality due to obesity. However, the relationship between percent body fat and BMI is different among different patient populations. In White, Hispanic, and Black individuals, overweight is defined as a BMI of 25 to 29.9 kg/m² and obesity as a BMI of ≥30 kg/m². The cutoffs underestimate the risk associated with obesity in the Asian population. (See 'BMI-based classifications' above.)

●Waist circumference is a measurement of abdominal obesity and provides independent risk information that is not accounted for by BMI. Patients with abdominal obesity (also called central adiposity, visceral, android, or male-type obesity) have an increased risk for overall mortality, heart disease, diabetes, hypertension, dyslipidemia, and nonalcoholic fatty liver disease. A waist circumference of ≥40 in (102 cm) for males and ≥35 in (88 cm) for females is considered elevated and indicative of increased cardiometabolic risk. A waist circumference ≥31 in (80 cm) in Asian females and ≥35 in (90 cm) in Asian males is considered abnormal. (See 'Waist circumference' above.)

●In individuals with BMI ≥25 kg/m² or a waist circumference ≥35 in (88 cm) (females) or ≥40 in (102 cm) (males), we suggest further evaluation to assess overall risk status (figure 3) (Grade 2B).

Assessment of an individual's overall risk status includes determining the degree of overweight (BMI) and the presence of abdominal obesity (waist circumference), cardiometabolic risk factors, obstructive sleep apnea, symptomatic osteoarthritis, and other comorbidities. The evaluation includes a history, physical examination, and measurement of fasting glucose (or glycated hemoglobin [A1C]), thyroid-stimulating hormone (TSH), liver enzymes, and fasting lipids (figure 3). Subsequent intervention, if necessary, is based upon overall risk assessment. The coexistence of several diseases, including established coronary heart disease (CHD), other atherosclerotic disease, type 2 diabetes mellitus, and sleep apnea places patients in a very high-risk category for subsequent mortality. (See 'Evaluation of patients with obesity' above.)

Many factors contribute to the development of obesity (table 2). However, most cases of obesity are related to behaviors such as a sedentary lifestyle and increased caloric intake. Although secondary causes of obesity are uncommon, they should be considered and ruled out, typically with history and physical examination. (See 'Investigating the cause' above.)

●All individuals who would benefit from weight loss should receive counseling on diet, physical activity, behavioral modification, and goals for weight loss. Individuals who do not need to lose weight can be counseled regarding prevention of weight gain. The relationship between BMI and risk allows identification of several levels that can be used to guide further selection of therapy. (See 'Candidates for weight-loss interventions' above.)

●Specific recommendations for the treatment of obesity are reviewed separately. (See "Obesity in adults: Overview of management" and "Obesity in adults: Dietary therapy" and "Obesity in adults: Drug therapy" and "Obesity in adults: Behavioral therapy".)

ACKNOWLEDGMENT — The UpToDate editorial staff acknowledges George Bray, MD, who contributed to an earlier version of this topic review.