Cause-Specific Excess Deaths

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Cause-Specific Excess Deaths Associated With Underweight, Overweight, and Obesity

Katherine M. Flegal, PhD; Barry I. Graubard, PhD; David F. Williamson, PhD; Mitchell H. Gail, MD, PhD

JAMA. 2007;298(17):2028-2037.

ABSTRAC

Context The association of body mass index (BMI) with cause-specific mortality has not been reported for the US population.

Objective To estimate cause-specific excess deaths associated with underweight (BMI <18.5), overweight (BMI 25-<30), and obesity (BMI 30).

Design, Setting, and Participants Cause-specific relative risks of mortality from the National Health and Nutrition Examination Survey (NHANES) I, 1971-1975; II, 1976-1980; and III, 1988-1994, with mortality follow-up through 2000 (571 042 person-years of follow-up) were combined with data on BMI and other covariates from NHANES 1999-2002 with underlying cause of death information for 2.3 million adults 25 years and older from 2004 vital statistics data for the United States.

Main Outcome Measures Cause-specific excess deaths in 2004 by BMI levels for

categories of cardiovascular disease (CVD), cancer, and all other causes (noncancer, non-CVD causes).

Results Based on total follow-up, underweight was associated with significantly increased mortality from noncancer, non-CVD causes (23 455 excess deaths; 95% confidence interval [CI], 11 848 to 35 061) but not associated with cancer or CVD mortality. Overweight was associated with significantly decreased mortality from noncancer, non-CVD causes (–69 299 excess deaths; 95% CI, –100 702 to –37 897) but not associated with cancer or CVD mortality. Obesity was associated with significantly increased CVD mortality (112 159 excess deaths; 95% CI, 87 842 to 136 476) but not associated with

cancer mortality or with noncancer, non-CVD mortality. In further analyses, overweight and obesity combined were associated with increased mortality from diabetes and kidney disease (61 248 excess deaths; 95% CI, 49 685 to 72 811) and decreased mortality from other noncancer, non-CVD causes (–105 572 excess deaths; 95% CI, –161 816 to –49 328). Obesity was associated with increased mortality from cancers considered obesity-related (13 839 excess deaths; 95% CI, 1920 to 25 758) but not associated with mortality from other cancers. Comparisons across surveys suggested a decrease in the association of obesity with CVD mortality over time.

Conclusions The BMI-mortality association varies by cause of death. These results help to clarify the associations of BMI with all-cause mortality.

INTRODUCTION

In a previous study,¹ we estimated excess all-cause mortality associated with underweight, overweight, and obesity in the United States in 2000 using data from national surveys. We found significantly increased all-cause mortality in the underweight and obese categories and significantly decreased all-cause mortality in the overweight category compared with normal weight. To gain further insight into these findings, we now extend that work, using additional mortality data with longer follow-up, to examine the association of cause-specific mortality with different weight categories among US adults in 2004.

METHODS

The general approach used to estimate excess deaths builds on that of Allison et al.² Numbers of deaths in 2004 for people 25 years and older came from US vital statistics.³ We used underlying cause-of-death information that had been recoded using a standard list of 113 causes of death; details of the corresponding International Classification of Diseases, Ninth Revision (ICD-9) and ICD-10 codes are shown elsewhere.⁴ We grouped deaths into 3 major categories: cardiovascular disease (CVD), cancer, and all other causes (ie, noncancer, non-CVD deaths). For some analyses, these categories were divided further. Table 1 shows the groupings and the total number of deaths from each cause in the United States, based on 2004 mortality data³ for people 25 years and older. Following the National Cancer Institute Obesity and Cancer Fact Sheet,⁵ we considered deaths from colon cancer, breast cancer, esophageal cancer, uterine cancer, ovarian cancer, kidney cancer, and pancreatic cancer to be deaths from obesity-related cancers.

Table 1. Cause of Death and Number of Deaths Among US Adults 25 Years and Older in 2004 by Specified Cause

All other data came from the National Health and Nutrition Examination Survey (NHANES) program of the National Center for Health Statistics. In each survey a different nationally representative cross-sectional sample of the US population was examined. To estimate relative risks, we used baseline data from NHANES I, 1971-1975; NHANES II, 1976-1980; NHANES III, 1988-1994, and underlying cause-of-death mortality data through 2000 for these surveys (Table 2).^6-12 Height and weight were measured using standardized procedures. Body mass index (BMI) was calculated as weight in kilograms divided by height in meters squared. Baseline age was age at the time of the examination.

Table 2. Descriptive Information About the Surveys Providing Mortality Data

We calculated relative risks for each category of causes of death using Cox proportional hazard models with age as the time scale.¹³ Because the proportional hazards assumption was not met across age, we divided the data into 3 age strata, 25 to younger than 60 years, 60 to younger than 70 years, and 70 years and older and fit models separately within each age stratum. Because age is the time scale, these age strata refer to attained age, rather than to age at baseline. For all analyses, we used BMI categories of 18.5 or less (underweight), 18.5 to less than 25 (normal weight, reference category), 25 to less than 30 (overweight), 30 to less than 35 (obesity grade 1), and 35 or greater (obesity grades 2 and 3).^14-15 The model included BMI categories, sex, smoking status (never, former, current), race (white, black, other), and alcohol consumption categories in ounces per day (none, <0.07, 0.07-<0.35, and 0.35).

To calculate the proportion of deaths in 2004 associated with each BMI level, we first calculated the relative risks for specific causes of death for all covariates from a data set that combined data from all 3 surveys (Table 2). We then applied each set of cause-specific relative risks from a given age group to the current distribution of the covariates (BMI group and all other covariates) in that age group in the general population, as estimated from the NHANES 1999-2002 cross-sectional survey data; for these purposes, non-Hispanic whites and blacks were categorized as white and black. Race was assessed by observation in NHANES I and II and by participant self-report in NHANES III and NHANES 1999-2002.

As described previously,¹ we calculated cause-specific attributable fractions for each BMI category by comparing the estimated risks from the current distribution of covariates with the estimated risks with BMI category set to normal weight, holding all else constant. The estimated number of excess deaths for each cause was then calculated by multiplying the total number of deaths in that age group in 2004 by the corresponding attributable fraction and summing over age. Standard errors were calculated by applying a delta method for complex sample designs that takes into account uncertainties in the relative risks for BMI categories, the distribution of BMI, the distribution of covariates, and the estimated effects of covariates, as well as the added variability due to the complex sample designs of the NHANES surveys.^16-18

Secondary analyses examining subgroups of mortality categories were performed by dividing the major mortality categories into finer groupings. The sum of the estimated excess deaths from these secondary analyses differs slightly from the estimated excess deaths in the whole category because they are derived from separately fitting different proportional hazard regressions, but the differences are small.

Estimates were made from the combined data to obtain more precision and to represent the US population over the more than 2 decades (1971-1994) covered by the baseline years of these surveys. Several issues arise in combining the surveys. For the present analyses, follow-up through 2000 was available for all surveys. As a result, NHANES I had more than 29 years of follow-up, NHANES II had more than 24 years of follow-up, and NHANES III had slightly more than 12 years of follow-up. Accordingly, the number of deaths contributed by the earlier surveys is larger than the number contributed by NHANES III.

One approach is to combine all data for all surveys, which gives the most weight to NHANES I, with its higher number of deaths. Another is to hold follow-up roughly constant across surveys. Willett et al¹⁹ have recommended that for studies of weight and mortality follow-up should not continue more than 10 or 15 years without remeasurement to limit weight misclassification. We report results from both approaches, using roughly balanced follow-up by limiting follow-up to no more than 15 years for each survey, balanced follow-up, or using all follow-up data for all surveys, total follow-up. Both approaches give less emphasis to the most recent data, since NHANES III has less than 15 years of follow-up and the fewest number of deaths.

Data were analyzed using the SAS System for Windows (Release 9.1) (SAS Institute Inc, Cary, North Carolina) and Research Triangle Institute’s SUDAAN (Release 9.0) software programs (RTI International, Research Triangle Park, North Carolina). All analyses included sample weights that account for the varying probabilities of selection due to sampling and nonresponse. NHANES III and NHANES 1999-2002 underwent institutional review board approval and included written informed consent. Institutional review board approval using current standards was not obtained for NHANES I or NHANES II, but internal human subject review was conducted. Estimates were considered significantly different from 0 if the 95% confidence interval (CI) did not include 0.

RESULTS

Cause-specific relative risks and 95% CIs for balanced and total follow-up are shown in Figure 1. The estimated numbers of excess deaths and 95% CIs by BMI category for CVD, cancer, and noncancer, non-CVD causes are shown in Table 3. The excess deaths from Table 3 expressed as a percentage of total deaths from the corresponding cause are shown in Table 4. For example, the 8674 excess CVD deaths associated with underweight shown in Table 3 represent 1% of the total 858 723 deaths from CVD in the United States that is shown in Table 1.

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