Defining overweight and obesity

Obesity is now so common within the world's population that it is beginning to replace undernutrition and infectious diseases as the most significant contributor to ill health. Major advances in the understanding of overweight and obesity have confirmed that they constitute an important medical condition. A better understanding of the genetic contribution to both weight gain and the intra-abdominal distribution of fat (central obesity) is identifying certain ethnic groups and susceptible families who are specifically at risk. In addition, there is increasing awareness that overweight and obesity are key factors in the development of other chronic diseases, in particular type 2 diabetes and coronary heart disease, and contribute to the high mortality rates of such diseases. Obesity can no longer be regarded simply as a cosmetic problem affecting certain individuals, but an epidemic that requires effective measures for its prevention and management.

In clinical practice, body fat is most commonly and simply estimated by using a formula that combines weight and height. The underlying assumption is that most variation in weight for persons of the same height is due to fat mass. The formula most frequently used in epidemiological studies is body mass index (BMI) which is weight in kilograms divided by the square of the height in metres. BMI is strongly correlated with densitometry measurements of fat mass adjusted for height in middle-aged adults. The main limitation of BMI is that it does not distinguish fat mass from lean mass. The succeeding chapters on the health risks of overweight and obesity confirm that measurements of body circumference are important because excess visceral (intra-abdominal) fat is a potential risk for chronic diseases independent of total adiposity. Waist circumference and the ratio of waist circumference to hip circumference are practical measures for assessing upper body fat distribution, although neither provides a precise estimate of visceral fat. Measurement of skinfold thickness with callipers provides a more precise assessment of body fat, especially if taken at multiple sites. Skinfolds are useful in the estimation of fatness in children for whom standards have been published. However, the measurements are more difficult to make in adults (particularly in the very obese), are subject to considerable variation between observers, require accurate calipers and do not provide any information on abdominal and intramuscular fat. In general, they are not superior to simpler measures of height and weight.

Measurement of bioimpedance is based on the principle that lean mass conducts current better than fat mass because it is primarily an electrolyte solution. A measurement of the resistance to a weak current (impedance) applied across the extremities provides an estimate of body fat when combined with height and weight and an empirically derived equation. Although the devices are simple and practical to use, they neither measure fat nor predict biological outcomes more accurately than the simpler anthropometric measurements. Table 1.1 lists methods that may be used to characterize obesity.

Defining a 'healthy weight' for a particular society presents problems. There are methodological problems that derive from a definition based on total mortality rates. People frequently lose weight as a consequence of illness, unrecognized at the time of survey, that is ultimately fatal. This gives an appearance of a higher mortality among those with lower weights: reverse causation. The effect can be minimized by either excluding persons with diagnoses that might effect weight and/or those who report recent weight loss , or excluding those who die

Table 1.1

Practical clinical methods for assessment of an obese subject.

during the first years of follow-up. A second major concern is the confounding factors that may distort the association between body weight and mortality: cigarette smoking is of particular importance. The Nurses Health Study, which prospectively studied 116000 women in the United States during a 17-year period, reveal a U-shaped relationship between mortality and BMI in an overall age-adjusted analysis. However, the relationship becomes a simple positive association when reverse causation is accounted for and the analysis limited to those who had never smoked.

There is a close relationship between BMI and the incidence of several chronic conditions caused by excess fat: type 2 diabetes, hypertension, coronary heart disease and cholelithiasis. This relationship is approximately linear for a range of BMI indexes less than 30: American women with a BMI of 26 have a 2-fold risk of coronary heart disease compared to women with a BMI of less than 21 and an 8-fold increased risk of developing type 2 diabetes. The equivalent figures for American men are 1.5-fold increase and 4-fold increase. The risk of hypertension is doubled in both men and women with a BMI of 26. All risks are greatly increased for those subjects with a BMI > 29, independent of gender.

  More about weight loss

Order a custom essay on Balloons now!