Introductionepidemiology

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The association between obesity and type 2 (non-insulin-dependent) diabetes mellitus has been recognized for several decades. It has now been shown that obesity is also associated with cardiovascular disease (CVD) and stroke. Population-based follow-up studies have revealed this concealed link, bringing the importance of obesity as an independent risk factor for cardiovascular morbidity and mortality to the forefront. However, at first, it was assumed that only severe obesity was as powerful a risk indicator of CVD and stroke as other, established risk factors such as hypercholesterolaemia and high blood pressure.

Retrospectively, it is now possible to identify why these studies failed to highlight the impact of excess body fat on cardiovascular morbidity. First, in analysing epidemiological data, it is common to adjust for the effect of some variables believed to distort the results, and in the case of obesity, adjustments were made for comorbidities such as dys-lipidaemia, hypertension, insulin resistance and impaired glucose tolerance. However, these adjustments are biologically and clinically implausible, since obesity without such comorbidities is a rare condition. Furthermore, severe non-orthogonality is introduced in the statistical analyses since these comorbidities are highly correlated. Another problem with these early studies was that obesity, defined as increased body fat mass, was treated as one homogeneous entity. Human obesity has repeatedly been subjected to subdivisions with the clinical intuition that this is not one single disease, but rather a symptom of several underlying conditions, to some extent similar to diseases such as anaemia-polyglobulinaemia, where red blood cells vary in amount and quality for a number of underlying reasons.

Human obesity is characterized by a wide variation in the distribution of excess body fat, and the distribution of fat affects the risks associated with obesity as well as the kinds of comorbidities that result. In the 1920s the idea emerged, under Kret-schmer's influence (1), that the pychnic type of body build was associated with abdominal obesity, gout, apoplexia and impaired glucose tolerance. Vague extended these observations further and labelled obesity types android (male-type) and gynoid (female-type), and noted that, although gender-specific in general, women might have android obesity and vice versa (2). Nevertheless, the android type of obesity carries a greater risk for disease in both men and women.

In addition to the pioneering attempts by Kret-schmer and Vague to categorize obesity, recent developments have confirmed the higher prevalence of dyslipidaemia, insulin resistance and hypertension in abdominal, central obesity in comparison with the more peripherally distributed, glu-teofemoral obesity (3,4), The techniques for the assessment of adipose tissue in these studies were

International Textbook of Obesity. Edited by Per Bjorntorp. © 2001 John Wiley & Sons, Ltd.

simpler than those employed by Vague (2), and included the ratio of waist and hip circumferences (WHR). Central obesity is thus more strongly associated with comorbidities in various systems than is peripheral obesity. This is particularly evident when intra-abdominal, visceral fat depots are enlarged (5,6).

The results of prospective epidemiological studies presented a major breakthrough in the significance of this type of categorization of obesity. These studies showed that WHR contributes independently to the risk of type 2 diabetes, CVD and stroke in both men and women, and is as powerful a predictor as other established risk factors for these diseases. Moreover, these studies showed that general obesity, measured as body mass index (BMI, weight (kg)/height2 [m2)), was not necessarily a part of this health hazard (7,8).

Subsequent studies have indicated other health consequences of central obesity such as cancer of the endometrium (9), breast (10) and ovaries (9) in women, and the prostate in men (11). The respiratory function when measured in obese subjects reveals many abnormalities, and one of the most extreme is the Pickwickian hypoventilation syndrome. Obstructive sleep apnoea is also common in obesity, and about 50% of subjects with this disorder are moderately to severely abdominally obese (12). Cholelithiasis and obesity has been documented in several studies (11), and hepatic steatosis occurs in about 68-94% of obese individuals (13).

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