Original ReportBody mass index and mortality among US male physicians☆
Introduction
Obesity is a burgeoning health problem in both developed and developing countries. In the United States, national surveys have documented an increase in obesity (defined as body mass index ⩾ 30 kg/m2) despite the dissemination and adoption of a wide variety of fat-free and low-fat foods. The 2000 Behavioral Risk Factor Surveillance System estimated an increase in prevalence of obesity among adults age 18 or older from 12.0% in 1991 to 19.8% in 2000 (1). Another large survey, the National Health and Nutrition Examination Survey (NHANES) reported an increase in prevalence of obesity among adult men and women aged 20 to 74 years (2) from 14.5% between 1971 and 1974 (NHANES II) to 30.9% in most recent data from NHANES 1999–2000. Recent surveys from countries in Asia and South America show that these populations are also becoming more obese (3). Such trends are alarming, given the clear association between obesity and morbidity (4) and mortality 5., 6..
While there is minimal controversy concerning the mortality hazards for individuals at the upper end of the weight distribution curve, there is still some dispute concerning hazards for those at the lower end of the curve. Previous studies have not demonstrated a clear relationship between body weight and mortality—some studies have shown no association 7., 8., 9., 10., 11. and others have shown a direct linear association 12., 13., 14., 15., 16., 18., 19., 20.. Still others have found J-shaped 7., 8., 9., 17., 21., 22., 23. or U-shaped curves 24., 25., 26., 27., 28., 29., 30., 31., 32. which suggest that both lean and obese individuals are at increased risk of all-cause mortality.
The relationship between body weight and mortality can be obscured by several potential biases and confounders. These include failure or inability to control for cigarette smoking (smokers tend to be lean and also have elevated mortality rates), failure or inability to control for subclinical disease (underlying disease may cause weight loss and also increase mortality), and over-adjustment for intermediate risk factors such as hypertension, diabetes, and high cholesterol (biological effects of obesity that mediate its influence on mortality, rather than confounding variables) (33). Moreover, in most studies demonstrating a direct association between body weight and mortality, the relationship was observed predominantly among younger and middle-aged participants, but not among the elderly 11., 17., 18., 34..
The Physicians' Health Study enrollment cohort afforded a unique opportunity to assess the relationship between body mass index and all-cause mortality, as well as mortality from specific causes, among more than 85,000 US male physicians aged 40 to 84 years. In this cohort, confounding by educational attainment, socioeconomic status, and access to health care was minimized and we were able to examine factors such as age, cigarette smoking, and physical activity that may influence the association between body mass index and mortality.
Section snippets
Study population
The Physicians' Health Study was a randomized, double-blind, placebo-controlled trial testing the role aspirin and β-carotene in primary prevention of cardiovascular diseases and cancer. A detailed description of the study design has been published elsewhere 35., 36.. Briefly, in 1982 and 1983, letters of invitation, informed consent forms, and baseline questionnaires were mailed to 261,248 eligible US male physicians aged 40 to 84 years. By December 31, 1983, 112,528 men had responded to the
Results
During 5 years of follow-up, we identified 2856 deaths. Of these, 1212 were due to cardiovascular diseases, 891 were due to cancer, and 753 were due to other causes. In Table 1, we present the baseline characteristics of the 85,078 participants according to BMI categories at baseline. As expected, a greater proportion of participants in higher BMI categories reported a positive history of hypertension, high cholesterol, and diabetes, while fewer subjects reported engaging in vigorous physical
Discussion
In this cohort of apparently healthy male physicians aged 40 to 84 years at baseline who were followed for 5 years, we observed a linear trend between body mass index and all-cause mortality after accounting for pre-existing disease. This linear trend was observed particularly among those who never smoked, an alternate method of accounting for effects of smoking. Smoking status was a powerful confounder of the association between body mass index and mortality and therefore, simply controlling
Acknowledgements
The authors acknowledge the contributions of the entire staff of the Physicians' Health Study, including Charlene Belanger, Mary Breen, Vadim Bubes, Jean MacFayden, Geneva McNair, David Potter, Leslie Power, Harriet Samuelson, Miriam Schvartz, Mickie Sheehey, Joanne Smith, Phyllis Johnson Wojciechowski, P.J. Skerrett, and Philomena Quinn for their expert help. We are also indebted to all dedicated and committed participants of this study.
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Supported by a grant (HL-42441) from the National Institutes of Health, Bethesda, MD, USA. Dr. Lotufo is the recipient of a fellowship (97-02424-8) from FAPESP (Fundação de Amparo a Pesquisa do Estado de São Paulo), São Paulo, Brazil.