What aspects of body fat are particularly hazardous and how do we measure them?

Abstract

Numerous techniques are available to estimate body composition and fat distribution, and the method to use will depend on the aim of the study, economic resources, availability, time, and sample size. 6–8 Multi-compartment models, such as underwater weighing, dilution techniques and dual-energy X-ray absorptiometry (DXA) are all reliable methods to obtain accurate measures of total body fat. However, because of their costs in terms of time andmoney, thesemethods are not practical in large epidemiological studies and for routine clinical use. In these situations, body mass index (BMI) is often used and assumed to represent the degree of body fat. BMI, however, does not distinguish between fat mass and lean (non-fat) mass. For example, well-trained body builders have a very low percentage of body fat, but their BMI may be in the overweight range because of their large lean (muscle) mass. In addition, in the elderly and non-Caucasian populations, the relationship between BMI and body fatness may be different as compared with younger Caucasian populations. 9–14 Another potential limitation of the BMI is that the distribution of fat over the body is not captured. Many studies have shown that an abdominal fat distribution, independent of overall obesity, is associated with metabolic disturbances and increased disease risk. 15–23 An increased abdominal fat accumulation is largely caused by the accumulation of visceral (or intraabdominal) fat (for distinction of these fat depots, see Figure 1). Owing tometabolic differences between different fat depots, they differ in their role of predicting metabolic disturbances and diseases. Table 1 summarizes the capability of the most commonly used methods to assess total adiposity and fat distribution. Abdominal obesity is usually assessed by the easily measured waist circumference, the waist-to-hip circumference ratio (WHR), or the less-commonly used sagittal abdominal diameter (SAD). By the use of sophisticated imaging techniques, such as magnetic resonance imaging (MRI) and computed tomography (CT), different fat depots can be distinguished at the waist level, and it has been shown that in particular the visceral fat depot is associated with metabolic disease risk. 24–30 Because the SAD or waist circumference alone are more strongly correlated with visceral fat than the WHR, 31–35 guidelines tend to focus onwaist circumference to estimate disease risk as suggested by Lean et al. 36 These widely used cut-points (i.e. 102 cm formen and 88 cm for women) were originally based on a replacement of the classification of BMI, 36 but other cut-points have also been suggested on the basis of relationships with visceral fat area. 37