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Institute of Biomedical Sciences
Academia Sinica
No. 128, Section 2, Academia Sinica Road
Taipei, Taiwan
pan{at}ibms.sinica.edu.tw
Centers for Disease Control and Prevention
Hyattsville, MD 20782
Dear Sir:
We appreciate Dr Cheng's letter. He echoes our point (1) that there may be a need to set lower body mass index (BMI; in kg/m2) cutoffs for Chinese adults, and he raises the possibility that a decision to lower the cutoff could affect the well-being of Chinese, who make up a large proportion of the world's population. A World Health Organization (WHO) expert consultation also concluded recently that a substantial proportion of Asians with BMIs lower than the existing WHO cutoffs for overweight (BMI >25) are at high risk of type 2 diabetes and cardiovascular disease (2). We would like to go beyond this point to emphasize that screening for obesity is a complex issue. The most appropriate and useful cutoffs may vary with time and circumstances in any given population or race. Additional complexities are related to considerations such as the availability of resources and the negative effect of misclassification.
Our article (1) provided evidence that, relative to American whites and blacks surveyed during a similar period (1988–1994), Taiwanese had a higher excess risk associated with several obesity-related metabolic disorders at each fixed BMI level than did other populations. In addition, for each increment of BMI, the relative risk for several metabolic disorders was higher in Taiwanese than in other populations. Although evidence (3) shows that this phenomenon can be explained in part by the higher percentage body fat at a specific BMI in Chinese, the reasons for a higher percentage body fat and for a higher risk of metabolic diseases in Taiwanese are not altogether clear. It is possible that both genetic background and environmental factors such as lack of physical activity (4), intrauterine malnutrition (5), and poor magnesium status (6) could affect a person's susceptibility to metabolic disorders. If, for example, improving living standard and public health efforts were to increase physical activity or reduce exposure to intrauterine malnutrition and mineral deficiency, the susceptibility of Chinese to metabolic diseases at a given BMI might change even without changes in genetic make-up.
It is crucial to understand the meaning of BMI cutoffs for different age, sex, and ethnic groups so that one can apply BMI cutoffs effectively for screening. Lowering the BMI cutoff for obesity screening in Chinese may well be appropriate at the present time, but the susceptibility to metabolic disorders may change in either direction for Chinese in the future. This complex situation may be addressed more comprehensively when technological developments lead to more convenient and accurate ways to measure obesity.
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