Obesity: definitions and phenotypes
المؤلف:
Holt, Richard IG, and Allan Flyvbjerg
المصدر:
Textbook of diabetes (2024)
الجزء والصفحة:
6th ed , page 250-252
2025-12-08
19
Obesity is defined as an abnormal or excessive accumulation of adipose tissue that may impair health. Adipose tissue is present throughout the body. As weight is gained, fats (lipids) are stored:
• In different adipose depots – subcutaneous, visceral, or intra- abdominal (e.g. omental and peri- renal).
• Ectopically – in tissues such as liver, muscle, heart, and pancreas and even in the vasculature and bone marrow.
The triad of obesity, type 2 diabetes, and non- alcoholic fatty liver disease (NAFLD) is well recognized (Figure 1). NAFLD, defined by the presence of steatosis in ≥5% of hepatocytes after excluding other causes of fatty liver, includes a spectrum of conditions, ranging from hepatic steatosis to non- alcoholic steatohepati tis and liver fibrosis, which may progress to cirrhosis and hepatocellular carcinoma. The term metabolic (dysfunction)- associated fatty liver disease (MAFLD) is increasingly used, as it emphasizes the role of the metabolic dysfunction common (but not necessarily due) to obesity and diabetes.
Fig1. The prevalences of type 2 diabetes, obesity, and non- alcoholic fatty liver disease. Generally accepted epidemiological evidence suggests that, among people with non-alcoholic fatty liver disease (global prevalence: 5%–6%), 82% live with obesity, and 44% with type 2 diabetes; among those with type 2 diabetes (global prevalence in 2019: 9%, 463 million), 37% are estimated to have non-alcoholic fatty liver disease, and approximately 60% have obesity; for obesity (global prevalence: 35%, 650 million), 33% may have non-alcoholic fatty liver disease and 12% diabetes.
Adipose tissue is not just an energy storage organ, but an active endocrine tissue secreting hormones such as leptin and adiponectin, as well as many adipocytokines that increase systemic and paracrine inflammation. Operationally, body mass index (BMI), defined as weight in kg/(height in m)2, is the most convenient population- level measure that estimates overweight and obesity. A BMI ≥30 kg/m2 is considered to define obesity in people of white North European ancestry, while a BMI ≥25 kg/m2 defines overweight (now termed pre- obesity). Different thresholds have been proposed for Asian (≥23 kg/m2 pre- obesity, ≥25 kg/m2 obesity) and Japanese populations (≥25 kg/m2) [16]. BMI has limitations when defining the clinical severity of excess weight, since at any given BMI there may be a threefold variation in % body fat. Additionally, many factors, such as age, sex, ethnicity, muscle mass, illness, and weight loss, can alter the relationship between BMI and body fat. BMI does not give information about fat distribution. Adipose tissue physiology differs considerably depending on its location, with visceral (intra- abdominal, peri- renal, and ectopic fat) most closely related to obesity complications. Epidemiologically many obesity complications, especially cardiometabolic and diabetes, correlate better with visceral fat accumulation than BMI. While waist circumference or other anthropometric ratios (waist- to- hip and waist- to- height) are commonly used as surrogates of visceral fat accumulation, imaging techniques with computer- assisted tomography or magnetic resonance imaging (MRI) are the gold standard.
Severity of obesity has been, rather arbitrarily, classified into BMI bands (Table 1). The recognition that fat distribution is of funda mental importance in determining the pathogenicity of obesity has led to the inclusion of waist circumference to improve the definition of risk within each BMI band (Table 1). More recent classifications (both of which include the presence of type 2 diabetes) are clinically rather than anthropometrically based. The Edmonton Obesity Staging System is based on the presence of obesity- related risk factors or complications (Table 2) that correlate better than BMI with mortality, while the King’s Obesity Staging Criteria (Table 3), albeit based on arbitrary criteria, can guide clinical management and assess responses to treatment.
Table1. Classification of obesity and risk based on anthropometric measures.
Table2. Edmonton staging system for obesity.
Table3. King’s obesity staging criteria.
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