Obesity is a substantial public health crisis in the United States and in the rest of the industrialized world. The prevalence is increasing rapidly in numerous industrialized nations worldwide. This growing rate represents a pandemic that needs urgent attention if obesity’s potential toll on morbidity, mortality, and economics is to be avoided. Research into the complex physiology of obesity may aid in avoiding this impact. (See Pathophysiology and Etiology.)

The annual cost of managing obesity in the United States alone amounts to approximately $190.2 billion per year, or 20.6% of national health expenditures, according to a study. [13] Compared with a nonobese person, an obese person incurs $2741 more in medical costs (in 2005 dollars) annually. In addition, the annual cost of lost productivity due to obesity is approximately $73.1 billion, [14] and almost $121 billion is spent annually on weight-loss products and services. [15] (See Treatment and Medication.)

In a 2016 position statement, the American Association of Clinical Endocrinologists (AACE) and the American College of Endocrinology (ACE) proposed a new name for obesity, adiposity-based chronic disease (ABCD). The AACE/ACE did not introduce the name as an actual replacement for the term obesity but instead as a means of helping the medical community focus on the pathophysiologic impact of excess weight. [16]

For information on pediatric obesity, see Obesity in Children.

Measurements of obesity

Obesity represents a state of excess storage of body fat. Although similar, the term overweight is puristically defined as an excess of body weight for height. Normal, healthy men have a body fat percentage of 15-20%, while normal, healthy women have a percentage of approximately 25-30%. [17] However, because differences in weight among individuals are only partly the result of variations in body fat, body weight is a limited, although easily obtained, index of obesity.

The body mass index (BMI), also known as the Quetelet index, is used far more commonly than body fat percentage to define obesity. In general, BMI correlates closely with the degree of body fat in most settings; however, this correlation is weaker at low BMIs.

An individual’s BMI is calculated as weight/height2, with weight being in kilograms and height being in meters (otherwise, the equation is weight in pounds ´ 0.703/height in inches2). Online BMI calculators are available.

A person’s body fat percentage can be indirectly estimated by using the Deurenberg equation, as follows:

body fat percentage = 1.2(BMI) + 0.23(age) - 10.8(sex) - 5.4

with age being in years and sex being designated as 1 for males and 0 for females. This equation has a standard error of 4% and accounts for approximately 80% of the variation in body fat.

Although the BMI typically correlates closely with percentage body fat in a curvilinear fashion, some important caveats apply to its interpretation. In mesomorphic (muscular) persons, BMIs that usually indicate overweight or mild obesity may be spurious, whereas in some persons with sarcopenia (eg, elderly individuals and persons of Asian descent, particularly from South Asia), a typically normal BMI may conceal underlying excess adiposity characterized by an increased percentage of fat mass and reduced muscle mass.

In view of these limitations, some authorities advocate a definition of obesity based on percentage of body fat. For men, a percentage of body fat greater than 25% defines obesity, with 21-25% being borderline. For women, over 33% defines obesity, with 31-33% being borderline.

Other indices used to estimate the degree and distribution of obesity include the 4 standard skin thicknesses (ie, subscapular, triceps, biceps, suprailiac) and various anthropometric measures, of which waist and hip circumferences are the most important. Skinfold measurements are the least accurate means by which to assess obesity.

Dual-energy radiographic absorptiometry (DXA) scanning is used primarily by researchers to accurately measure body composition, particularly fat mass and fat-free mass. It has the additional advantage of measuring regional fat distribution. However, DXA scans cannot be used to distinguish between subcutaneous and visceral abdominal fat deposits.

The current standard techniques for measuring visceral fat volume are abdominal computed tomography (CT) scanning (at L4-L5) and magnetic resonance imaging (MRI) techniques. A simpler technique, using bioelectrical impedance, was recently introduced. [18] However, these methods are limited to clinical research.

Classification of obesity

Although several classifications and definitions for degrees of obesity are accepted, the most widely accepted classifications are those from the World Health Organization (WHO), based on BMI. The WHO designations include the following:

Grade 1 overweight (commonly and simply called overweight) - BMI of 25-29.9 kg/m 2

Grade 2 overweight (commonly called obesity) - BMI of 30-39.9 kg/m 2

Grade 3 overweight (commonly called severe or morbid obesity) - BMI greater than or equal to 40 kg/m2

The cut-off for each grade varies according to an individual’s ethnic background. For example, a BMI of 23 kg/m2 or higher may define grade 1 overweight and 27.5 kg/m2 or higher may define grade 2 overweight (obesity) in many Asian populations, in which the risk was shown to be high and extremely high for grade 1 and 2 overweight at these levels, respectively. Other BMI cutoffs identified as potential public health action points in these populations are 32.5 and 37.5 kg/m2. [19]

The surgical literature often uses a different classification to recognize particularly severe obesity. The categories are as follows:

Severe obesity - BMI greater than 40 kg/m 2

Morbid obesity - BMI of 40-50 kg/m 2

Super obese - BMI greater than 50 kg/m2

In children, a BMI above the 85th percentile (for age-matched and sex-matched control subjects) is commonly used to define overweight, and a BMI above the 95th percentile is commonly used to define obesity.

Comorbidities associated with obesity

Obesity is associated with a host of potential comorbidities that significantly increase the risk of morbidity and mortality in obese individuals. Although no cause-and-effect relationship has been clearly demonstrated for all of these comorbidities, amelioration of these conditions after substantial weight loss suggests that obesity probably plays an important role in their development. (See Presentation.)

Apart from total body fat mass, the following aspects of obesity have been associated with comorbidity:

Fat distribution

Waist circumference

Age of obesity onset

Intra-abdominal pressure

Fat distribution

Accumulating data suggest that regional fat distribution substantially affects the incidence of comorbidities associated with obesity. [4] Android obesity, in which adiposity is predominantly abdominal (including visceral and, to a lesser extent, subcutaneous), is strongly correlated with worsened metabolic and clinical consequences of obesity.

Waist circumference

The thresholds used in the National Cholesterol Education Program Adult Treatment Panel III definition of metabolic syndrome [20] state that significantly increased cardiovascular risk (metabolic central obesity) exists in men with waist circumferences of greater than 94 cm (37 in) and in women with waist circumferences of greater than 80 cm (31.5 in), as well as waist-to-hip ratios of greater than 0.95 in men and of greater than 0.8 in women. Circumferences of 102 cm (40 in) in men and 88 cm (35 in) in women indicate a markedly increased risk requiring urgent therapeutic intervention.

These thresholds are much lower in Asian populations. After analyzing survey results of Chinese, Malay, and Asian-Indian cohorts, Tan and colleagues concluded that a waist circumference of greater than 90 cm in men and of more than 80 cm in women were more appropriate criteria for metabolic central obesity in these ethnic groups. [21]

Age of obesity onset

An elevated BMI during adolescence (starting within the range currently considered normal) is strongly associated with the risk of developing obesity-related disorders later in life, independent of adult BMI. [22] Increases in BMI during early adulthood (age 25-40 y) are associated with a worse profile of biomarkers related to obesity than are BMI increases during later adulthood. [23] This is consistent with most emerging data regarding timing of changes in BMI and later health consequences.

Intra-abdominal pressure

Apart from the metabolic complications associated with obesity, a paradigm of increased intra-abdominal pressure has been recognized. This pressure effect is most apparent in the setting of marked obesity (BMI ≥ 50 kg/m2) and is espoused by bariatric surgeons. [24]

Findings from bariatric surgery and animal models suggest that this pressure elevation may play a role (potentially a major one) in the pathogenesis of comorbidities of obesity, such as the following [25] :

Pseudotumor cerebri

Lower-limb circulatory stasis

Ulcers

Dermatitis

Thrombophlebitis

Reflux esophagitis

Abdominal hernias

Possibly, hypertension and nephrotic syndrome

Osteoarthritis

A study by Losina et al found that obesity with knee osteoarthritis resulted in the loss of a substantial number of quality-adjusted life years. The association was most notable among black and Hispanic women. [26]

Focal glomerulosclerosis

Some reports, including those by Adelman and colleagues and by Kasiske and Jennette, suggest an association between severe obesity and focal glomerulosclerosis. [27, 28, 29] This complication, in particular, improves substantially or resolves soon after bariatric surgery, well before clinically significant weight loss is achieved.

Pickwickian syndrome

The so-called Pickwickian syndrome is a combined syndrome of obesity-related hypoventilation [6] and sleep apnea. It is named after Charles Dickens’s novel The Pickwick Papers, which contains an obese character who falls asleep constantly during the day.

The hypoventilation in Pickwickian syndrome results from severe mechanical respiratory limitations to chest excursion, caused by severe obesity. The sleep apnea may be from obstructive and/or central mechanisms. Obstructive sleep apnea is common among men with collar size greater than 17 in (43 cm) and women with collar size greater than 16 in (41 cm).

Increased and decreased sleep duration

Sleep duration of less than 5 hours or more than 8 hours was associated with increased visceral and subcutaneous body fat, in a study of young African Americans and Hispanic Americans. [30] This association relates mostly to decreased leptin hormone and increased ghrelin hormone levels. [31]

COVID-19

A study reported that out of 178 adult patients hospitalized with coronavirus disease 2019 (COVID-19), at least one underlying condition was found in 89.3%, the most common being hypertension (49.7%), obesity (48.3%), chronic lung disease (34.6%), diabetes mellitus (28.3%), and cardiovascular disease (27.8%). Moreover, obesity was the most prevalent underlying condition among patients aged 18-64 years. [32]

In addition, report by Lighter et al, based on data from a large academic hospital system in New York City, indicated that in persons under age 60 years, obesity increases the risk of hospitalization for COVID-19 two-fold, with such patients also being more likely to require intensive care. [33, 34]

A study by Kass et al indicated that among patients with COVID-19 admitted to the intensive care unit (ICU), there is a greater tendency for younger individuals to be obese, with age inversely correlated with BMI. [35, 36]

A relationship between obesity and progression to severe COVID-19 status was also seen in studies by Cai et al and Gao et al, with the Cai study indicating that the odds ratio (OR) of persons with obesity progressing to severe disease is 3.4, and the Gao study finding the progression to severe or critical COVID-19 status to be threefold greater in patients with obesity. [37, 38, 39]

The Centers for Disease Control and Prevention (CDC) includes obesity in the list of conditions that increase the likelihood of severe illness in persons with COVID-19. [40]

A study from England, by Szatmary et al, suggested that overweight or obese young men with COVID-19 may be at particular risk for developing pancreatitis. [41, 42]

Additional comorbidities

Overweight and obese individuals are at increased risk for the following health conditions:

Metabolic syndrome

Type 2 diabetes

Hypertension

Dyslipidemia

Coronary heart disease

Osteoarthritis

Stroke

Depression

Non-alcoholic fatty liver disease (NAFLD)

Infertility (women) and erectile dysfunction (men)

Risk of stillbirth [43, 44]

Gall bladder disease

Obstructive sleep apnea

Gastroesophageal reflux disease (GERD)

Some cancers (eg, endometrial, breast, and colon) [7, 8, 9]

Asthma

A study by Abdullah et al indicated that not only the severity of a patient’s obesity but its duration as well is associated with the individual’s risk of developing type 2 diabetes mellitus. Based on a more than four decade follow-up of 5132 participants in the Framingham Offspring Study, the investigators found a significant rise in type 2 diabetes risk as obese-years increased. [45]

Research indicates that the likelihood of developing type 2 diabetes is not the same in all persons with overweight or obesity, with some of these individuals being genetically inclined toward an adiposity profile that lowers their chances for the disease. Fourteen genetic variants have been identified that investigators say lead to subcutaneous storage of excess fat rather than accumulation of the fat around organs such as the liver, thus reducing the diabetes risk. [46]

A Korean study, by Evangelista et al, found a higher prevalence of general and abdominal obesity in persons with some stages of chronic kidney disease (CKD) than in those without CKD. The greatest prevalence of these forms of obesity was found in patients with stage 2 CKD. The investigators also reported that general and abdominal obesity were not associated with stage 4 or 5 CKD. [47]