Body Mass Index Is Inversely Related to Mortality in Older People After Adjustment for Waist Circumference

Ian Janssen, PhD; Peter T. Katzmarzyk, PhD; Robert Ross, PhD

Disclosures

J Am Geriatr Soc. 2005;53(12):2112-2118. 

In This Article

Abstract and Introduction

Objectives: To examine the individual and combined influence of body mass index (BMI) and waist circumference (WC) on mortality risk in older people.
Design: Longitudinal cohort study.
Setting: Cardiovascular Health Study, a longitudinal study of cardiovascular disease and its risk factors in older people.
Participants: Five thousand two hundred men and women aged 65 and older.
Measurements: BMI and WC were measured at baseline. The risks of all-cause mortality associated with BMI and WC were examined using Cox proportional hazards models over 9 years of follow-up.
Results: When examined individually, BMI and WC were both negative predictors of mortality, but when BMI and WC were examined simultaneously, BMI was a negative predictor of mortality, whereas WC was a positive predictor of mortality. After controlling for WC, mortality risk decreased 21% for every standard deviation increase in BMI. After controlling for BMI, mortality risk increased 13% for every standard deviation increase in WC. The patterns of associations were consistent by sex, age, and disease status.
Conclusion: Higher BMI values indicated a lower mortality risk once the risk attributable to WC was accounted for, whereas higher WC values indicate a higher mortality risk once the risk attributable to BMI was accounted for. Both BMI and WC should be measured in the clinical setting, but in older adults higher BMI is associated with lower mortality rates.

Obesity has reached epidemic proportions in the United States and in many other countries around the globe.[1] Overweight and obesity prevalences are particularly high in elderly adults. Recent estimates in the United States indicate that 74% of men and 66% of women aged 60 and older are overweight or obese according to their body mass index (BMI, weight in kg/height in m2).[2] Moreover, more than half of American adults aged 60 and older are abdominally obese as determined by waist circumference (WC).[3] These statistics are troubling given that the proportion of older adults in the population will increase considerably over the next few decades and that overweight, obesity, and abdominal obesity are risk factors for numerous diseases and premature mortality.[1,4]

Current national guidelines are that overweight be defined as a BMI of 25kg/m2 to 29.9kg/m2 and obesity as a BMI of 30kg/m2 or more.[4] These guidelines indicate that health risk increases in a graded fashion when moving from normal weight to overweight to obese BMI categories and that, within each of these BMI categories, individuals with high WC values (≥102 cm in men, ≥88 cm in women) have a greater relative health risk than individuals with normal WC values.[4] This classification system has been recommended for all adults, regardless of age, but the influence of overweight and obesity on health risk in those aged 65 and older is controversial. In fact, a recent review of studies examining the relationship between BMI and mortality in older people reported that few studies found a positive association between BMI and all-cause mortality.[5]

A number of hypotheses have been put forth to explain the lack of association between BMI and mortality in older people,[5,6] among them being that the positive effects of greater lean mass in overweight and obese older people counterbalance the negative effects of greater fat mass on mortality. Thus, the higher lean mass in overweight and obese elderly persons may act as a nutritional reserve during traumatic events and in so doing protect against death. Consistent with this line of thought, one study recently reported that BMI was negatively associated with mortality in people aged 50 to 64 after adjustment for WC, whereas WC showed a positive dose-response relationship with mortality after adjustment for BMI.[7] These authors speculated that BMI is a reflection of lean mass for individuals with the same WC, whereas WC is a reflection of total and abdominal fat content for individuals with the same BMI. This is an important observation, which suggests that, after consideration of WC, greater BMI may represent a unique aspect of body composition—one that decreases health risk.

Additional studies, particularly in elderly populations, are required to confirm these findings.[7] If further evidence verifies that higher BMI indicates lower mortality risk once the risk attributable to WC is accounted for and that higher WC indicates a greater mortality risk once the risk attributable to BMI is accounted for, it may indicate that distinct obesity guidelines are required for identification of mortality risk in older people. In particular, after consideration of the health risk determined by WC, overweight and obesity according to BMI would be considered favorable rather than adverse.

The purpose of this study was to examine the effects of BMI and WC on mortality risk in elderly men and women with special emphasis on the independent predictive value of these two anthropometric variables. It was hypothesized that higher BMI would be a negative predictor of all-cause mortality after adjustment for the risk of greater WC.

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