Published: May 17, 2013 | Updated: May 18, 2013

By Kristina Fiore , Staff Writer, MedPage Today

Action Points

• In this prospective case-cohort study of hip fractures, there was an increased risk of hip fracture in subjects in the lowest compared to the highest quartile of serum 25-hydroxyvitamin D level.
• The association was stronger in men than in women.

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Having low serum levels of vitamin D is tied to risk of hip fracture in older patients, Norwegian researchers found.

In a cohort of several population studies in Norway, patients in the lowest quartile of 25(OH)D levels had a 38% increased risk of hip fracture compared with those in the highest quartile of 25(OH)D levels, Kristin Holvik, PhD, of the University of Bergen in Norway, and colleagues reported online in the Journal of Clinical Endocrinology & Metabolism.

“In accordance with findings of previous community-based studies, low vitamin D status was a modest risk factor for hip fracture in our population,” the researchers wrote.

Despite “considerable interest,” they wrote, the relationship between circulating levels of25-hydroxyvitamin D, or 25(OH)D, and hip fracture isn’t fully characterized — particularly in Norway, which has among the highest rate of hip fractures worldwide.

To study the relationship more closely, Holvik and colleagues conducted the Norwegian Epidemiologic Osteoporosis Studies (NOREPOS), which collected data from four community health studies between 1994 and 2001 — totaling 21,774 patients ages 65 to 79.

Information on subsequent hip fractures came from electronic hospital discharge registers.

The overall median 25(OH)D level was 53.5 nmol/L, and vitamin D levels were slightly higher in men than in women.

During a median observation time of 8.2 years, a total of 1,232 patients had a hip fracture — 340 men and 892 women. Complete data were available for 1,175 of the hip fracture patients: 307 men and 868 women.

Overall, Holvik and colleagues saw an inverse association between 25(OH)D levels and hip fracture (P=0.008).

In models adjusting for several potential confounders, including age, gender, study center, and body mass index (BMI), the researchers found that patients with vitamin D levels in the lowest quartile (below 42.2 nmol/L) had a 38% increased risk of hip fracture compared with those in the highest quartile (at least 67.9 nmol/L) (P=0.009).

The association was stronger in men (HR 1.65, 95% CI 1.04-2.61), and it was not significant in women (HR 1.25, 95% CI 0.95 to 1.65), they noted.

The results also suggest a preventive effect with levels above 75 nmol/L compared with levels below 50 nmol/L, the researchers added.

Indeed, hip fracture risk fell continuously with increasing serum levels of the vitamin, “with the steepest and most consistent decrease at levels in the magnitude of 40 to 60 nmol/L,” they wrote.

The study was limited by variations in participation rates among the four centers, and because the categorization of exposure was based on a single measurement.

Still, the researchers concluded that theirs is the largest prospective case-cohort study to find an increased risk of hip fracture in patients with the lowest levels of serum vitamin D.