For an average of seven years the researchers studied 800 50-79 year-olds selected from nearly 40,000 candidates who were not using estrogens or other bone-active therapies. The mean age of the participants was 70 years.
Of the 800 selected from 40 clinical centres across the US, 400 patients of the same sex and race had suffered hip fractures while 400 control had not.
The University of Pennsylvania School of Medicine researchers concluded those women who had hip fractures had lower blood levels of vitamin D.
They measured serum 25-hydroxyvitamin D 25(OH) levels – the form in which vitamin D is stored in the body.
“In our prospective, nested case–control study, we found that women with the lowest 25(OH) vitamin D concentrations (47.6 nmol/L) at study entry had a significantly greater increased risk for subsequent hip fracture during the next seven years than did women with the highest concentrations (70.7 nmol/L),” the researchers wrote.
They added that the correlation between vitamin D concentration and hip fracture was linear and unaffected by age. The number of physical falls did not differ between the control and active groups.
The results are consistent with the Third National Health and Nutrition Examination Survey as well as a 2005 cohort study conducted among Swedish women which found those with 25(OH) levels below 52.5 nmol/L had twice the risk of hip fracture.
However, two other cohort studies failed to find a significant relationship between vitamin D levels and hip fracture.
Studies with younger populations (average age of 53 years) had not produced a strong correlation causing the researchers to reason: “Vitamin D concentration may be more strongly linked to frailty-related fractures, such as hip fractures, which tend to occur in much older women.”
The researchers said there existed no clear molecular explanation for the vitamin D-hip fracture link, but noted: “C-terminal telopeptide of type I collagen, a marker of bone resorption, tended to be higher among case-patients with the lowest 25(OH) vitamin D concentrations, an association that may be driven by higher parathyroid hormone levels in this group.”
Limitations of the study, published in the August 19 issue of the Annals of Internal Medicine, included the fact almost all of the candidates were white and bone density was not measured despite the fact it is a common contributing factor hip and other bone factures.
The researchers also tested whether the vitamin D-hip fracture relationship was independent of poor physical functioning, frailty, falls, sex-steroid hormones, renal function, or bone turnover.
While more fractures were recorded among those with poor physical function and the frail, they found the relationship between 25(OH) levels and the likelihood of hip fracture was statistically significant.
An optimum 25(OH) level was not established.
Source: Annals of Internal Medicine
August 19, 2008, Volume 149, Number 4, Pages 242-250
“Serum 25-Hydroxyvitamin D Concentrations and Risk for Hip Fractures”
Authors: Jane A. Cauley, DrPH; Andrea Z. LaCroix, PhD; LieLing Wu, MS; Mara Horwitz, MD; Michelle E. Danielson, PhD; Doug C. Bauer, MD; Jennifer S. Lee, MD; Rebecca D. Jackson, MD; John A. Robbins, MD; Chunyuan Wu, MS; Frank Z. Stanczyk, PhD; Meryl S. LeBoff, MD; Jean Wactawski-Wende, PhD; Gloria Sarto, MD; Judith Ockene, PhD; and Steven R. Cummings, MD.