Obesity, overweight, hypertension, and high blood cholesterol: the importance of age.

Of late, there has been increased interest among epidemiologists in issues of data sharing, partly due to new legal requirements (1,2). Published analyses of data from the National Health and Nutrition Examination Surveys (NHANES) provide examples of possible issues that can arise. These data are made publicly available, there are no restrictions on their use for epidemiologic analyses, and there is no central coordination of analyses. In the current issue of Obesity Research, Brown et al. (3) provide an extensive analysis of NHANES III data on body mass index (BMI), blood pressure, and blood lipids. An article published last year by Must et al. (4) analyzed the same NHANES III data for these and other health conditions. These two articles have several methodological differences. For example, Brown et al. (3) analyzed data for ages 20 years and older, whereas Must et al. (4) restricted their analyses to data for ages 25 years and older. Brown et al. (3) used definitions for hypertension and high blood cholesterol based on physiological or laboratory measurements and on reported pharmaceutical treatment. Must et al. (4) used broader definitions for these conditions and thus got higher prevalence estimates. The BMI categories used are different. Brown et al. (3) used four categories of BMI, with cutoff points at values of 25, 27, and 30; each of the resultant groupings included at least 10% of the population, generally more. Must et al. (4) used six categories, with cutoff points at values of 18.5, 25, 30, 35, and 40. Three of these six categories, the lowest and the two highest, included only a very small proportion of the population (,5% of the population, and sometimes ,1%). In addition, the statistical methods used in the two studies differ slightly. Although the findings in the two studies are broadly similar, there are some notable differences. Brown et al. (3) found statistically significant interactions between race/ethnic group and BMI categories as predictors of hypertension and high blood cholesterol. In contrast, Must et al. (4) found no significant interaction between race/ethnic group and BMI categories. Brown et al. (3) found significant linear associations between high blood cholesterol and BMI category; Must et al. (4) did not. These differences between the two analyses could be due to slight differences in the analytic sample, to differences in the definition of the outcomes, to differences in the construction of BMI categories, or to differences in the statistical methods used. The emphasis in both studies on BMI as a risk factor draws attention away from some aspects of the association of hypertension and high blood cholesterol with age. Both studies found a significant interaction of age with BMI category. Both showed that at older ages, the prevalence or odds ratio for various outcomes associated with higher BMIs is lower than at younger ages. Because of this interaction, Must et al. (4) presented results stratified by age (,55 years; $55 years). Brown et al. (3) also found an interaction of age and BMI but rather than stratifying by age, they included age, BMI category, and the interaction in their models. The more extensive quantitative data on age effects provided by Brown et al. (3) allow the reader a better look at some interesting implications of these effects. The data presented in the article by Brown et al. (3) suggest, somewhat surprisingly, that despite the statistically significant association of BMI with hypertension and high blood cholesterol, reduction of overweight and obesity might not have a large impact on the population prevalence of hypertension and high blood cholesterol. In the multivariate models presented by Brown et al. (3) an age of $60 years is the strongest risk factor for hypertension and high blood cholesterol in both men and women, even after adjusting for BMI category. For hypertension, an age of $60 is associated with a multivariate odds ratio of 25.7 for men and 88.4 for women. According to the analyses by Brown et al., (3) the prevalence of hypertension is low at younger ages (9% among men aged 20 to 39 years) and high at older ages (52% for men aged 60 to 79 years). At older ages, the prevalence is high even in normal-weight individuals (BMI ,25). Among men aged 60 to 79 years with BMI values ,25, the prevalence of hypertension is 50%. This suggests that if the Submitted for publication October 31, 2000. Accepted for publication in final form November 7, 2000. National Center for Health Statistics, Hyattsville, Maryland. Address correspondence to Katherine M. Flegal, Ph.D., National Center for Health Statistics, 6525 Belcrest Road, Room 900, Hyattsville, MD 20782. E-mail: Kflegal@cdc.gov Copyright © 2000 NAASO.