Osteoporosis International

, Volume 16, Issue 11, pp 1330–1338

Body mass index as a predictor of fracture risk: A meta-analysis

Authors

  • C. De Laet
    • Scientific Institute of Public Health
    • WHO Collaborating Centre for Metabolic Bone DiseasesUniversity of Sheffield Medical School
  • A. Odén
    • Consulting Statistician
  • H. Johanson
    • Consulting Statistician
  • O. Johnell
    • Department of OrthopaedicsMalmö General Hospital
  • P. Delmas
    • INSERM Unité 149
  • J. A. Eisman
    • Bone and Mineral Research Program, Garvan Institute of Medical ResearchSt Vincent’s Hospital and University of New South Wales
  • H. Kroger
    • Department of Surgery, Bone and Cartilage Research UnitKuopio University Hospital
  • S. Fujiwara
    • Department of Clinical StudiesRadiation Effects Research Foundation
  • P. Garnero
    • INSERM Unité 403
  • E. V. McCloskey
    • WHO Collaborating Centre for Metabolic Bone DiseasesUniversity of Sheffield Medical School
  • D. Mellstrom
    • Department of Geriatric MedicineGoteborg University
  • L. J. Melton3rd
    • Division of EpidemiologyMayo Clinic
  • P. J. Meunier
    • INSERM Unit 403, Faculty R Laennec
  • H. A. P. Pols
  • J. Reeve
    • Strangeway’s Research Laboratory
  • A. Silman
    • ARC Epidemiology UnitUniversity of Manchester
  • A. Tenenhouse
    • Division of Bone MetabolismThe Montreal General Hospital
Original Article

DOI: 10.1007/s00198-005-1863-y

Cite this article as:
De Laet, C., Kanis, J.A., Odén, A. et al. Osteoporos Int (2005) 16: 1330. doi:10.1007/s00198-005-1863-y

Abstract

Low body mass index (BMI) is a well-documented risk factor for future fracture. The aim of this study was to quantify this effect and to explore the association of BMI with fracture risk in relation to age, gender and bone mineral density (BMD) from an international perspective using worldwide data. We studied individual participant data from almost 60,000 men and women from 12 prospective population-based cohorts comprising Rotterdam, EVOS/EPOS, CaMos, Rochester, Sheffield, Dubbo, EPIDOS, OFELY, Kuopio, Hiroshima, and two cohorts from Gothenburg, with a total follow-up of over 250,000 person years. The effects of BMI, BMD, age and gender on the risk of any fracture, any osteoporotic fracture, and hip fracture alone was examined using a Poisson regression model in each cohort separately. The results of the different studies were then merged. Without information on BMD, the age-adjusted risk for any type of fracture increased significantly with lower BMI. Overall, the risk ratio (RR) per unit higher BMI was 0.98 (95% confidence interval [CI], 0.97–0.99) for any fracture, 0.97 (95% CI, 0.96–0.98) for osteoporotic fracture and 0.93 (95% CI, 0.91–0.94) for hip fracture (all p <0.001). The RR per unit change in BMI was very similar in men and women ( p >0.30). After adjusting for BMD, these RR became 1 for any fracture or osteoporotic fracture and 0.98 for hip fracture (significant in women). The gradient of fracture risk without adjustment for BMD was not linearly distributed across values for BMI. Instead, the contribution to fracture risk was much more marked at low values of BMI than at values above the median. This nonlinear relation of risk with BMI was most evident for hip fracture risk. When compared with a BMI of 25 kg/m2, a BMI of 20 kg/m2 was associated with a nearly twofold increase in risk ratio (RR=1.95; 95% CI, 1.71–2.22) for hip fracture. In contrast, a BMI of 30 kg/m2, when compared with a BMI of 25 kg/m2, was associated with only a 17% reduction in hip fracture risk (RR=0.83; 95% CI, 0.69–0.99). We conclude that low BMI confers a risk of substantial importance for all fractures that is largely independent of age and sex, but dependent on BMD. The significance of BMI as a risk factor varies according to the level of BMI. Its validation on an international basis permits the use of this risk factor in case-finding strategies.

Keywords

BMIFracturesMeta-analysisOsteoporosisProspective studiesRisk

Introduction

Low weight, or low body mass index (BMI), is a well-documented risk factor for future fracture, whereas a high BMI appears to be protective [110]. The increasing prevalence of overweight and obesity in Western societies [11,12] might at first seem a promising development from the point of view of osteoporosis and fracture prevention. From a public health point of view, however, the story is more complicated. Obesity is associated with increased morbidity from diabetes, hypertension, and cardiovascular diseases, and is also associated with increased mortality. The same may also be true for overweight women. Recently, it was estimated that overweight (BMI>25 kg/m2) 41-year-old female nonsmokers lost on average 3.3 years of life, whereas, obese (BMI>30 kg/m2) female nonsmokers lost 7.1 life years [13]. It is important, therefore, to quantify the association between BMI and fracture risk and to explore its relationship to age, gender and bone mineral density (BMD) with the aim of being able to give balanced advice on lifestyle to patients. These relationships are also important when using BMI to assess fracture risk in case finding [2,1416].

The aim of the present study was to explore the relationship of BMI with fracture risk (any fracture, any osteoporotic fracture and hip fracture alone) in men and women using data from 12 prospective population-based cohort studies in an international perspective. BMI was chosen rather than weight to explore this association, because of the wide variation in average weight and height between different countries, which is reduced by adjusting weight for height. Moreover, BMI is as good a predictor of fractures as weight in most studies of hip fracture outcomes [17,18].

Methods

Participants

We used baseline and follow-up data from 12 prospective population-based cohorts comprising the Rotterdam Study, The European Vertebral Osteoporosis Study (later the European Prospective Osteoporosis Study (EVOS/EPOS), The Canadian Multicentre Osteoporosis Study (CaMos), Rochester, Sheffield, Dubbo, Epidémiologie de l’Ostéoporose Study (EPIDOS), a cohort from France (OFELY), Kuopio, Hiroshima and two cohorts from Gothenburg. Details of each of the cohorts are published elsewhere, but are summarized briefly below and in Table 1.
Table 1

Details of cohorts studied (see Methods for cohort abbreviations, BMI body mass index)

Cohort

Gender

Sample

size

Women

(%)

Person-

years

Any

fracture

Hip

fracture

Osteoporotic

fracture

Mean

age (years)

Mean BMI

(kg/m2)

Mean

height (cm)

Mean

weight (kg)

EVOS-EPOS

Men

6,521

52

19,736

213

20

213

64.2

27

171

79.1

Women

6,969

20,945

506

30

506

63.5

27.1

159

68.4

Total

13,490

40,681

719

50

719

CaMos

Men

2,801

69

8,002

124

9

59

59.9

27

174

81.4

Women

6,300

17,832

447

31

248

62.9

26.9

160

68.6

Total

9,101

25,834

571

40

307

Rochester

Men

348

65

1,160

38

0

25

55.4

27.3

175

84.3

Women

653

5,067

251

42

219

57.6

25.5

161

66.3

Total

1,001

6,227

289

42

244

Rotterdam

Men

2,793

59

16,150

185

52

130

68.4

25.7

175

78.3

Women

4,058

23,443

676

168

516

69.9

26.7

161

69.4

Total

6,851

39,593

861

220

646

DOES

Men

819

61

6,365

138

27

110

70.1

26

173

78.2

Women

1,270

9,629

381

76

297

70.7

25.4

160

64.8

Total

2,089

15,994

519

103

407

Gothenburg 1

Men

812

59

6,010

95

73

95

77

25.4

173

75.7

Women

1,158

9,191

255

198

255

78.6

25.3

159

63.9

Total

1,970

15,201

350

271

350

Hiroshima

Men

793

70

3,004

44

7

12

63.2

22.7

163

60.7

Women

1,810

6,821

143

25

78

65.9

23.1

150

52.3

Total

2,603

9,825

187

32

90

OFELY

Women

430

100

2,144

50

64.1

24.2

158

60.5

Sheffield

Women

2,170

100

6,894

292

63

243

80

26.7

156

64.8

Kuopio

Women

11,691

100

56,091

1,043

52.3

26.2

161

68.2

Gothenburg 2

Women

7,065

100

29,603

440

29

312

58.9

24.6

165

67.2

EPIDOS

Women

1,183

100

3,947

291

82.4

25.4

153

59.5

All men

14,887

60,427

837

188

644

66.4

26.2

172.6

77.9

All women

44,757

75

191,607

4,484

953

2674

62.2

25.9

160.4

66.9

Overall

59,644

252,034

5,321

1,141

3318

63.2

26.0

163.3

69.5

The Rotterdam Study, begun in 1990, is an ongoing prospective cohort study that aimed to examine and follow-up all residents aged 55 years and older living in Ommoord, a district of Rotterdam [19]. By 1993, 7,983 residents had been included (response rate 78%). Fracture follow-up was achieved through an automatic link with general practitioner computer systems and hospital admission data [20]. Fracture data were collected and validated by two independent research physicians. For this analysis, validated fracture follow-up was available for 6,851 participants (2,793 men) with an average follow-up time of 6 years. Femoral neck BMD was measured in 5,731 individuals (2,414 men) by dual X-ray absorptometry (DXA) (Lunar DPX-L).

The European Vertebral Osteoporosis Study (EVOS) comprised age- and sex-stratified random samples from 36 centers in 19 European countries [21]. Equal numbers of men and women were drawn in each center within six 5-year age bands (50–74 and 75+ years). BMD was measured in 3,461 men and women from 13 centers by DXA at the femoral neck using pencil beam machines that were cross-calibrated using the European Spine Phantom. This sample provided the framework for the European Prospective Osteoporosis Study (EPOS), where repeated assessment was undertaken in 29 of the centers [22,23]. For this analysis, validated fracture follow-up was available for 13,490 participants (6,521 men) with an average follow-up time of 3 years. Femoral neck BMD was measured in 4,746 individuals (2,141 men).

The Canadian Multicentre Osteoporosis study (CaMos) is an ongoing prospective age stratified cohort. The study is documenting the incidence of fractures and risk factors in a random sample of 9,424 men and women aged 25 years or more selected by telephone listings. The sampling frame is from nine study centers in seven provinces [24]. Characterization of individuals was by interview. BMD was measured by DXA at the femoral neck with Hologic QDR in seven centers and the Lunar DPX Alpha in two centers in 8,297 individuals (2,589 men). Machines were cross-calibrated using the same European Spine Phantom. For this analysis, validated fracture follow-up was available for 9,101 participants (2,801 men) with an average follow-up time of 3 years.

The Rochester cohort was recruited from two random population samples stratified by decade of age, one of women who were subsequently followed for up to 20 years [25], and another sample of women and men followed for 8 years [26]. BMD of the right femoral neck was measured by dual photon absorptiometry in the first cohort (cross-calibrated to DXA) and by DXA (Hologic QDR 2000) in the second group. Fractures were ascertained by periodic interview combined with review of the in-patient and out-patient medical records of all local care providers. For this analysis, validated fracture follow-up was available for 1,001 participants (348 men) with an average follow-up time of 6 years. Femoral neck BMD was measured in 993 individuals (345 men).

The Sheffield cohort comprised women aged 75 years or more selected randomly from the population of Sheffield, UK, and surrounding districts between 1993 and 1999. Approximately 35,000 women, identified from general practitioner listings, were contacted by letter and invited to attend for the assessment of skeletal status; 5,873 women were willing to attend. Of these, 281 women were excluded and the remainder randomly allocated to treatment with placebo or a bisphosphonate, clodronate, to study its effects on fracture risk. The material for this study comprised 2,172 women allocated to treatment with placebo only [27]. All women had baseline assessment of BMD undertaken at the femoral neck using the Hologic QDR 4500. Outcomes were assessed by 6-monthly home visits. For this analysis, validated fracture follow-up was available for 2,170 participants with an average follow-up time of 6 years. Femoral neck BMD was measured in 2,150 individuals.

The Dubbo Osteoporosis Epidemiology Study (DOES) is a population-based study with multiple assessments of skeletal status in men and women aged 60 years or more from Dubbo, Australia [28]. Participation in the study was 56% of the population. Baseline measurements included BMD at the femoral neck assessed using DXA (GE-Lunar, DPX and Prodigy). Fractures are identified through radiologists’ reports from the two centers serving the region. For this analysis, validated fracture follow-up was available for 2,089 participants (819 men) with an average follow-up time of 8 years. Femoral neck BMD was measured in 2,060 individuals (801 men).

The EPIDOS cohort is a prospective multicenter study of risk factors for hip fractures that included 7,575 French women age 75 years or older [29]. Participants were recruited through mailings using large population-based listings such as voter-registration rolls. Baseline characteristics were obtained through a structured questionnaire as well as through clinical and functional examinations and BMD measurement (Lunar DPX). For this analysis, validated fracture follow-up was available for 1,183 participants comprising 291 hip fracture cases and age-matched controls with an average follow-up time of 3 years [30]. Femoral neck BMD was measured in 1,180 individuals.

The OFELY cohort comprises a cohort of 1,039 women aged 31–89 years stratified by age randomly selected from the regional section of a large health insurance company (Mutuelle Générale d’Education Nationale, Lyon) [31]. Eighteen percent of women contacted participated in the study. Baseline characteristics were obtained using a standardized questionnaire, including the documentation of prior wrist, humeral, vertebral and hip fracture that occurred after the age of 45 years. Only low-trauma fractures (falls from a standing height or less) were recorded, but not the site of fracture. BMD was measured at the spine (L1–L4), at the proximal femur, distal radius and whole body by DXA using a QDR 2000 (Hologic). Women were reviewed annually and incident fractures registered. Peripheral fractures were confirmed by radiography. For this analysis, validated fracture follow-up was available for 430 participants with an average follow-up time of 5 years. Femoral neck BMD was measured in 427 individuals.

The Kuopio osteoporosis risk factor and prevention (OSTPRE) study in Finland comprised a postal enquiry sent to all 14,220 women aged 47–56 who were residents of Kuopio province in 1989 [32]. Of 13,100 women responding to the enquiry, 938 were excluded for incomplete information. For this analysis, validated fracture follow-up was available for 11,691 participants with an average follow-up time of 5 years. The site of fracture was not recorded other than those at the forearm. Femoral neck BMD was measured in 1,743 individuals with the Lunar DPX [32].

The Gothenburg I study comprised four birth cohorts of 2,375 randomly sampled men and women aged 70 years or more followed for up to 20 years after a baseline BMD measurement [33]. The participants were drawn randomly from the population register in Gothenburg by the date of birth to provide cohorts aged 70, 76, 79 and 85 years at the time of investigation. Participation rate was 73%. Bone mineral density was measured at the right heel using dual photon absorptiometry. For this analysis, validated fracture follow-up was available for 1,970 participants (812 men) with an average follow-up time of 8 years. BMD was measured in 1,633 individuals (686 men).

The Gothenburg II study comprised a randomly drawn population cohort of over 7,000 women aged 50–70 years followed for 4 years [34]. The participation rate was 67%. Assessment included a standardized questionnaire that recorded information on risk factors for osteoporosis. Fractures were identified prospectively through the radiology departments serving the region. BMD was assessed at baseline at the distal forearm by using the Osteometer DTX-200. For this analysis, validated fracture follow-up was available for 7,065 participants with an average follow-up time of 4 years. BMD was measured in 7,056 individuals.

The Adult Health Study (AHS) was established in 1958 to document the late health effects of radiation exposure among atomic bomb survivors in Hiroshima and Nagasaki. The original AHS cohort consisted of about 15,000 atomic bomb survivors and 5,000 controls selected from residents in Hiroshima and Nagasaki using the 1950 national census supplementary schedules and the Atomic Bomb Survivors Survey. AHS subjects have been followed through biennial medical examinations since 1958. The participation rate has been around 80% throughout this period. BMD was measured at the proximal femur by DXA in 1994 (Hologic QDR-2000) in 2,588 individuals (791 men). Self-reported fractures were documented at 6-monthly intervals [35,36]. For this analysis, validated fracture follow up was available for 2,603 participants (793 men) with an average follow-up time of 4 years.

Baseline and outcome variables

Height and weight were measured using standard techniques in all cohorts. BMI was calculated as weight in kilograms divided by height squared in meters. For the purposes of this analysis, we utilized BMD assessed at the femoral neck by DXA, with the exception of the two Gothenburg cohorts where BMD was assessed by DXA at the distal forearm or by DPA at the right heel. We additionally analyzed the BMD data excluding those two cohorts.

Fracture ascertainment was undertaken by self-report (Sheffield, EVOS/EPOS, Hiroshima, Kuopio, EPIDOS, OFELY) and/or verified from hospital or central databases (Gothenburg, CaMos, DOES, Kuopio, Sheffield, EVOS/EPOS, Rochester, Rotterdam). The EPOS and the Rotterdam study also included sequential systematic radiography to define incident vertebral deformities, but these were not used in this analysis. In the analysis, we used information on any clinical fracture and on fractures considered to be osteoporotic. In addition, hip fracture alone was considered separately. An osteoporotic fracture was one considered to be due to osteoporosis by the investigator in the EVOS/EPOS study and in CaMos. For the EVOS/EPOS study, osteoporotic fractures comprised hip, forearm, humeral or spine fractures. For the CaMos Study they comprised fractures of the spine, pelvis, ribs, distal forearm, forearm and hip. In the other cohorts, fractures at sites considered to be characteristic for osteoporosis were extracted [37].

Statistical methods

The association of BMI with the risk of any fracture, osteoporotic fracture and hip fracture was examined using a Poisson regression model in each cohort separately. Covariates included current age and time since start of follow-up, and we performed analyses for both sexes separately, with and without taking BMD information into account. BMD was expressed as sex- and cohort-specific Z -scores. BMI was analyzed either continuously or using specific thresholds. The β-coefficients of each cohort and the two sexes were weighted according to the variance and merged to determine the weighted mean of the coefficient and its standard deviation. The risk ratios (RR) at different BMI levels are then given by e(weighted mean coefficient). We also analyzed the RR per unit difference of BMI for fracture at various levels of BMI, taking a BMI of 25 kg/m2 as the reference, since a BMI of 25 is the internationally accepted threshold between normal weight and overweight [11]. This was slightly below the average in most of the cohorts (see Table 1). To optimally describe the association of RR with level of BMI, we analyzed this relationship using spline functions.

Heterogeneity between cohorts was tested by means of the I2 statistic [38]. Low heterogeneity was noted for hip fracture outcomes ( I 2=8%; p >0.30) and was moderate for osteoporotic fracture outcomes ( I 2=49%). When the interaction between BMI and current age (BMI·current age) was included, there was no significant heterogeneity between cohorts ( p >0.30) for BMI ( I 2=0) nor for the interaction term ( I 2 =0) and a fixed-effects rather than a random-effects model was used.

Results

The total sample included almost 60,000 men and women from the 12 cohorts with a total follow-up of more than 250,000 person years. The distribution of BMI is shown in Table 2. Information on any fracture, on osteoporotic fracture and on hip fracture alone was available for about 58,000, 46,000 and 47,000 participants, respectively. During follow-up, there were 5,321 fractures registered including 3,318 osteoporotic fractures and 1,141 hip fractures. Bone mineral density measurements were available in 65% of individuals. Details by cohort and gender are shown in Table 1.
Table 2

Distribution (%) of men and women categorized by intervals of BMI ( BMI body mass index)

BMI

(kg/m2)

Men

Women

Total

<20

7.5

8.9

8.5

20–24

30.9

38.5

36.5

25–29

47.2

35.8

38.8

30–34

12.4

12.9

12.8

35–39

1.7

3.1

2.7

40+

0.2

0.8

0.7

Without information on BMD, low BMI in men and women combined was associated with a significantly increased age-specific risk of fracture, while at higher BMI values the risk of fracture was decreased. The risk ratio per unit increase in BMI (gradient of risk; GR) was for any fracture 0.98 (95% confidence interval [CI], 0.97–0.99), for osteoporotic fracture 0.97 (95% CI, 0.96–0.98) and for hip fracture 0.93 (95% CI, 0.91–0.94) (Fig. 1A). As the figure shows, the RRs per unit change of BMI in men and women were very similar and not significantly different ( p >0.30). Since all osteoporotic fractures also included hip fractures, we analyzed osteoporotic fractures excluding hip fractures. As expected, the GR increased slightly (i.e., predictive value decreased) but remained significantly lower than 1 in men and women combined (data not shown).
Fig. 1

Risk Ratio ( RR) for fracture per unit increase in BMI; ( A)adjusted for current age and time since start of follow-up, and ( B) additionally adjusted for BMD ( BMI body mass index, BMD bone mineral density) [04Ca001]

When information on BMD was included (Fig. 1B), the GRs changed markedly and remained significantly different from unity only for hip fractures in women. When the two studies from Gothenburg were excluded (because BMD was not measured at the femoral neck), the GR for hip fracture in women was not significantly different from unity.

Since the GRs were similar in men and women both before and after correction for BMD and not significantly different, further results are presented for men and women combined, but all analyses were also carried out for men and women separately.

The effect of age on gradient of risk is shown in Fig. 2. For any fracture and for any osteoporotic fracture, the GR per unit of BMI increased with advancing age (without adjustment for BMD). In contrast, for hip fractures the gradient of risk decreased with age, although this trend was not significant. When hip fractures were excluded from the osteoporotic fractures, a similar trend with age was observed as seen for all osteoporotic fractures. After correction for BMD, the risk gradients showed nonsignificant trends with age, and at most ages were not significantly different from 1.
Fig. 2

Relative fracture risk per unit increase in BMI by age for men and women combined; ( A) adjusted for time since start of follow-up, and ( B), additionally adjusted for BMD ( BMI body mass index, BMD bone mineral density) [04Ca002]

We also analyzed the risk ratio (RR) for fracture at various levels of BMI, taking a BMI of 25 kg/m2 as the reference. As expected, the RR increased with decreasing BMI. The magnitude of the effect was greater for hip fracture than for any osteoporotic fracture or any fracture. The RR of fracture risk with BMI was, however, nonlinear (Fig. 3 and Table 3). RR was markedly higher at the lower values of BMI, particularly with a BMI of 20 kg/m2 or less. By contrast, between a BMI of 25 kg/m2 and 35 kg/m2 the differences in RR were small. For example, given the overall risk gradient for hip fracture (0.93/unit BMI) an increase of 5 BMI units from 25 kg/m2 to 30 kg/m2 would be expected to correspond to a 32% reduction in hip fracture risk. The observed difference was, however, only 17% (1.00 vs 0.83, see Table 3). With a 10-unit BMI difference the expected risk reduction would be more than 50%, whereas, the difference between a BMI of 25 kg/m2 and one of 35 kg/m2 was only 25% (1.00 vs 0.75, see Table 3). At the low end of the BMI spectrum, on the other hand, a change of 5 units from a BMI of 25 kg/m2 to a BMI of 20 kg/m2, corresponded to a doubling of the hip fracture risk (1.95 vs 1.00, see Table 2). For any osteoporotic fracture there was a 27% difference in fracture risk comparing a BMI of 20 kg/m2 vs 25 kg/m2 and an 11% difference comparing a BMI of 30 kg/m2 v 25 kg/m2.
Fig. 3

Relative fracture risk at various levels of BMI (kg/m2) for men and women combined. The reference is a BMI=25, ( A) adjusted for current age and time since start of follow-up, and ( B) additionally adjusted for BMD. The bold solid line describes hip fracture, the solid line any osteoporotic fracture, and the dotted line any fracture ( BMI body mass index, BMD bone mineral density) [04Ca003]

Table 3

Risk Ratio ( RR) for fracture at various levels of BMI (kg/m2) for men and women combined, adjusted for current age and time in study, without and with adjustment for BMD. The reference is a BMI of 25 kg/m2 ( BMD bone mineral density, BMI body mass index, CI confidence interval)

Any fracture

Osteoporotic fracture

Hip fracture

BMI

RR

95% CI

RR

95% CI

RR

95% CI

Not adjusted for BMD

15

1.66

1.31–2.09

1.79

1.35−2.37

4.48

3.11–6.45

20

1.21

1.12–1.30

1.27

1.16–1.38

1.95

1.71–2.22

25

1.00

reference

1.00

reference

1.00

reference

30

0.92

0.85–1.00

0.89

0.81–0.98

0.83

0.69–0.99

35

0.85

0.74–0.98

0.74

0.62–0.90

0.75

0.50–1.11

Adjusted for BMD

15

1.00

0.75–1.33

1.07

0.78–1.48

2.16

1.42–3.28

20

0.98

0.9–1.08

1.02

0.92–1.13

1.42

1.23–1.65

25

1.00

reference

1.00

reference

1.00

reference

30

1.01

0.91–1.11

0.96

0.86–1.08

1.00

0.82–1.21

35

0.99

0.82–1.19

0.91

0.73–1.13

1.18

0.78–1.80

Re-analyzing the data using only those cohorts with a uniform acquisition of data on fractures (Rochester, Rotterdam, DOES, Hiroshima and Sheffield) did not alter the relationship of BMI and hip fracture risk. For osteoporotic fracture, a high BMI had a greater and significant protective effect in the absence of BMD (data not shown).

The data for Table 3 (unadjusted for BMD) were recalculated in the 65% of individuals who had also a BMD test. The findings did not differ from those utilizing the entire cohort.

After adjustment for BMD, BMI was not predictive of fracture risk except for hip fracture at a BMI of 20 kg/m2 or less (Table 3). There were no significant differences in these relationships between men and women. The results were not changed when cohorts with uniform characterization of fractures were analyzed. After adjustment for BMD, the risk of hip fracture was 33% higher comparing individuals with a BMI of 20 kg/m2 with 25 kg/m2 (42% higher in all cohorts; see Table 3).

For osteoporotic fractures, the I2 statistic was 49% (95% CI, 8–71), indicating that about half of the observed differences between studies was due to heterogeneity. This heterogeneity disappeared when account was taken of age ( I 2 =0). For hip fractures there was also low heterogeneity ( I 2 =8% (0–44)) indicating that the data about the relationship between BMI and fracture risk were homogenous between cohorts.

Discussion

The principal finding of the present study, undertaken in large and internationally drawn cohorts, is the confirmation that low BMI is associated with a substantial increase in fracture risk of similar magnitude in men and women, whereas, a high BMI is protective. This risk associated with a low BMI was present at most ages and for all types of fracture studied, but was strongest for hip fracture. Overall, the RR for hip fracture decreased 0.93 per unit increase in BMI.

This risk gradient was, however, not constant across the range of BMI. In the most common range of normal weight and overweight (the average in our populations was 26.0 kg/m2), the gradient of risk per unit BMI was relatively low (see Fig. 3A). In contrast, the gradients were steeper at low values of BMI, below 22 kg/m2. This threshold of 22 kg/m2 and lower corresponded overall in our cohorts to about 10% of men and 17% of women. A nonlinear relationship of BMI with fracture risk, with thresholds of BMI of approximately 22 kg/m2 and 26 kg/m2, has been described previously in case-control studies of men and women [17,18]. In the present study, where the individual participant data were pooled from several prospective studies, we could derive the form of this relationship. The present study additionally demonstrates that this gradient of risk is markedly reduced when adjusted for BMD, suggesting that BMD is an important intermediary or confounder.

These findings have important consequences for case-finding strategies based on clinical risk factors. Firstly, obesity should not be regarded as an important protective factor for hip fracture risk [39,40]. Rather, leanness should be regarded as a significant risk factor. Secondly, the use of low BMI as a risk factor will identify populations with a low BMD and hence a high risk of fracture. The finding that leanness is much more important as a risk factor for hip fracture—than obesity is a protective factor—means that advice concerning body weight and osteoporosis need not be inconsistent with the weight control advocated for the prevention of cardiovascular disease or diabetes.

After adjustment of fracture risk by BMD, a low BMI was still a significant risk factor for hip fracture. Thus, low BMI can be used to enhance the predictive value of BMD in case finding. The mechanisms for this are conjectural but might include muscle weakness [41], perhaps associated with nutritional deficiencies of protein or vitamin D [41,42], decreased padding over the greater trochanter [43], or a greater liability to fall [10]. This BMD-independent risk was not observed, however, when only cohorts using femoral neck BMD were analysed.

BMI and other risk factors have been studied mainly in relation to hip fracture risk. As expected, the risk gradient we found was higher for hip fractures than for all osteoporotic fractures. However, for any fracture or for all osteoporotic fractures, the impact of BMI was still significant, even when hip fractures were excluded from the analysis. For BMD, it is customary to express the relation with fracture risk as a gradient of risk per SD change. For hip fracture, for example, the most commonly cited number is that of an RR of 2.6 per SD decrease in BMD at the femoral neck [44]. In the populations that we studied, the average SD for BMI was around 4 kg/m2, corresponding to a RR for hip fracture of 1.4 per SD decrease in BMI, much lower than the estimate for BMD. The analogy is, however, not wholly appropriate in view of the nonlinearity of risk with BMI.

We found no significant differences in the gradients of fracture risk between men and women; but, we found a significant increase of risk associated with low BMI with age, for any fracture and for osteoporotic fractures, but an opposite, though not significant, trend for hip fractures. In the case of any fracture or osteoporotic fracture, an increased exposure with age to relative gonadal deficiency might provide an explanation, at least in women [45]. A further possible explanation is that in young individuals, low BMI may be associated with physical fitness and a lower risk of fracture. In contrast, in the elderly, where hip fractures are more common, low BMI may more likely be related to frailty.

The strength of the present study is that the estimate of risk is derived from several studies in an international setting from population-based cohorts, using the original individual participant data. The large sample size permitted us to examine the general relationship of BMI with fracture risk and also the detailed relationship with age and level of BMI. The study also has some limitations. The definition of what was considered an osteoporotic fracture was not the same in all cohorts. For hip fractures, the definition is similar in all cohorts and there is substantial homogeneity between studies; here we found higher gradients of risk across the range of BMI. For osteoporotic fractures our results were essentially similar when we analyzed only those cohorts with uniform documentation of fracture.

The use of BMI, rather than weight, as a measure for body composition has the great advantage that there is less variability across countries and between sexes as can be seen from Table 1. A potential drawback is that BMI can be influenced by the height loss associated with vertebral deformities. Therefore, in individuals with important height loss, the risk conferred through BMI on fracture risk could be underestimated [46]. The use of maximal attained height rather than current height might be a solution in clinical practice, if it could be shown that risk prediction could thereby be improved.

We conclude that low BMI confers a risk of fracture of substantial importance that is largely independent of sex. The significance of BMI as a risk factor varies according to the level of BMI and to a lesser extent on age. Its validation on an international basis permits the use of this risk factor, at least in the absence of a BMD measurement, in case-finding strategies. Even with BMD, a low BMI may remain an independent risk factor for hip fracture in those with a BMI of less than 20 kg/m2. These data also show that there should be no conflict between advice for weight control with reasonable target values, such as for the prevention of diabetes or cardiovascular disease, and the prevention of osteoporotic and hip fractures.

Acknowledgements

We are grateful to Drs. T.V. Nguyen and J.R. Center for their help with the DOES Study. We would like to thank the Alliance for Better Bone Health, Hologic, IGEA, Lilly, Lunar, Novartis, Pfizer, Roche and Wyeth for their unrestricted support of this work. We are also grateful to the EU (FP3/5), the International Osteoporosis Foundation, the International Society for Clinical Densitometry and the National Osteoporosis Foundation for supporting this study

Copyright information

© International Osteoporosis Foundation and National Osteoporosis Foundation 2005