Re: "body mass index categories in observational studies of weight and risk of death" and "editorial: body mass index and risk of death".

The article by Flegal et al. (1) on body mass index (BMI; weight (kg)/height (m)) categories in studies of weight and death and the related editorial by Friedman (2) discussed the use of BMI categories that differ from those published by the World Health Organization (WHO) (3) and the National Heart, Lung, and Blood Institute (4) in 1998 (i.e., <18.5, 18.5–24.9, 25.0–29.9, 30–34.9, 35–39.9, and ≥40). Friedman noted the potential usefulness of more fine-grained BMI categories with standard subdivisions established by an authoritative group (2). Neither author mentioned the narrow BMI ranges recommended by a WHO expert consultation held in 2002 that involved 26 international experts and was chaired by Dr. Shiriki Kumanyika (5). The summary report, published in The Lancet in 2004 (5), showed additional BMI ranges to be used for determining public health and clinical action (Table 1). Countries were urged to use as many of these categories as feasible for reporting purposes in order to facilitate international comparisons. Among the members of the expert consultation (in which I participated), there was strong sentiment that the cutpoints from the 1998 WHO report should not be changed but augmented. Above a BMI of 25, uniform intervals that spanned 2.5 kg/m were favored. In the normal BMI category (18.5– <25.0), unequal ranges were used (18.5–<23 and 23–<25), partly to avoid additional decimal places (the midpoint of the normal-weight category would be 21.75). A BMI of 23.0 was chosen to subdivide the normal-weight range and was recommended as the cutpoint for triggering public health action for many Asian populations. This advice was influenced by studies of Asians that had shown increases in risk starting at BMI levels between 22 and 25 (5). The cutpoint of 23.0 has since been used by several authors in studies examining the implications of overweight in Asian populations. Incidentally, a cutpoint of 23.0 divides the normal-weight, nonpregnant, US adult population represented by the 2005–2010 National Health and Nutrition Examination Surveys into groups more equal in size than might be expected given the unequal BMI intervals (57% vs. 43%; author’s unpublished data, 2014). An additional BMI cutpoint between 18.5 and 23, as suggested by Dr. Freidman, may help to separate persons who are in the normal-weight range due to healthy diet and exercise behaviors from those who have anorexia associated with disease, but accurate separation of those groups is likely to require other variables in addition to BMI. The lower bound of 18.5 for normal weight was presented in the 1995 report of the WHO (6) following endorsement of this cutoff by the International Dietary Energy Consultative Group (7, 8). In both reports, the emphasis was on the need to identify chronic energy deficiency due to food shortages, and weight loss associated with chronic disease was not the focus. Nevertheless, the WHO report (6) mentioned the importance of using relatively young, physically active, healthy adults to establish a satisfactory reference standard for defining the lower limit of desirable body weight in affluent societies with a plentiful food supply. The 2004 summary of the WHO consultation (5) and the publications by Flegal (1) and Friedman (2) are all in agreement that standardized BMI cutpoints can aid comparisons among studies examining different populations. When small subdivisions of BMI are merited, the augmented cutpoints from the WHO (5) can provide vetted values.