Age and Sex Differences in Body Mass Index as a Predictor of Hip Fracture

A NOREPOS Study

Anne Johanne Søgaard; Kristin Holvik; Tone K. Omsland; Grethe S. Tell; Cecilie Dahl; Berit Schei; Haakon E. Meyer

Disclosures

Am J Epidemiol. 2016;184(7):510-519. 

In This Article

Abstract and Introduction

Abstract

It is unclear whether very high body mass index (BMI; weight (kg)/height (m)2) lowers risk of hip fracture. Our objectives in this study were 1) to examine the association between BMI and subsequent hip fracture according to sex and age and 2) to explore whether the importance of known risk factors varied across BMI. We followed 61,787 participants (29,511 female and 32,276 male) in the Cohort of Norway (ages 50–79 years at baseline in 1994–2003) with regard to hip fracture. BMI was calculated from measured height and weight. During a median follow-up period of 8.4 years, 1,603 women and 951 men suffered a hip fracture. Hazard ratios for hip fracture and associated 95% confidence intervals were estimated. After adjustment for potential confounders, women with BMI <22 had a hazard ratio of 1.38 (95% confidence interval (CI): 1.18, 1.60) for hip fracture, as compared with women with BMI 22–24.9; and women with BMI ≥30 had a hazard ratio of 0.57 (95% CI: 0.49, 0.66). Corresponding results in men were hazard ratio = 1.66 (95% CI: 1.35, 2.05) and hazard ratio = 0.77 (95% CI: 0.62, 0.96), respectively. Below age 70 years, there was no further decrease in fracture risk at BMIs of 25 or more, while in women aged 70–79 years, the risk continued to decrease with increasing BMI. The associations between risk factors and hip fracture were similar in strength across BMI strata.

Introduction

Hip fractures and obesity are prevalent in Western societies, and both are associated with increased morbidity and mortality.[1–8] The societal burden of hip fractures is predicted to increase because of the increasing number of elderly in the population.[9–11] Because the prevalence of obesity has been increasing over the last several decades, more knowledge about fracture risk in the upper range of body mass index (BMI) is needed. The authors of several reviews and meta-analyses have concluded that there is a reduced risk of hip fracture with increasing BMI.[12–19] However, some authors have challenged this view and have suggested that being overweight or obese does not reduce the risk of hip fracture compared with being normal weight.[20–23] There could be several reasons for these controversies. The information about possible confounders in reviews and meta-analyses is limited,[3,12,16,24] and most studies have not been designed to detect possible age and sex differences in the BMI-fracture relationship across the BMI range.

A possible obscuring element in the BMI–hip fracture association could be the interplay between BMI and other risk factors. Since an increasing proportion of the population burden of fractures in the Western world is due to fractures in the overweight and obese, it is important to determine whether the risk factors for hip fracture are the same regardless of BMI.[22] Our aims in this study were to 1) evaluate whether the association between BMI and hip fracture differed according to sex and age and 2) explore whether the importance of risk factors for hip fracture differed in persons with high BMI as compared with persons with normal or low BMI.

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