Higher Vitamin D Levels Associated With Improved Lower Extremity Function

Yael Waknine

August 31, 2004

Aug. 31, 2004 — Serum 25-hydroxyvitamin D (25[OH]D) concentrations higher than 40 nmol/L are associated with improved lower extremity function in ambulatory patients aged 60 years and older, regardless of activity level, sex, age, race or ethnicity, and calcium intake, according to the results of a population-based survey published in the September issue of the American Journal of Clinical Nutrition.

"The protective effect of vitamin D on fractures has been attributed to the established moderate benefit of vitamin D for calcium homeostasis and bone mineral density," writes Heike A. Bischoff-Ferrari, MD, from the Robert B. Brigham Arthritis and Musculoskeletal Diseases Clinical Research Center at Brigham and Women's Hospital in Boston, Massachusetts, and colleagues.

"However, an alternative explanation might be that vitamin D affects factors directly related to muscle strength and function, thus reducing fracture risk through fall prevention, in addition to its benefits on calcium homeostasis."

The investigators explored the relationship between vitamin D levels and muscle strength and function in 4,100 subjects aged 60 to more than 90 years (mean age, 71.4 ± 7.9 years; 49% women). Serum concentrations of 25(OH)D were measured in all subjects by radioimmunoassay.

Lower extremity function was assessed using the timed eight-foot walk test and the repeated sit-to-stand test. Decreased performance time was associated with increased lower extremity functionality.

Subjects classified as active (75%) had walked one mile without stopping, swam, jogged, bicycled, danced, exercised, or gardened in the previous month. Inactive subjects (25%) had not.

After controlling for activity level (active or inactive), sex, age, race or ethnicity, and calcium intake, regression analysis showed that subjects in the highest 25(OH)D quintile had a mean 5% decrease in time of 0.27 seconds in the eight-foot walk test compared with those in the lowest quintile of 25(OH)D concentration (95% confidence interval [CI], -0.44 s to -0.09 s; P < .001).

In addition, subjects in the highest 25(OH)D quintile had a mean 3.9% decrease of 0.67 seconds in the sit-to-stand test compared with those in the lowest quintile (95% CI, -1.11s to -0.23 s; P = .017).

25(OH)D concentration was positively associated with improved lower extremity function throughout the reference range (22.5 - 94.0 nmol/L), with most of the improvement occurring at levels between 22.5 and ~ 40.0 nmol/L. Further improvement of less magnitude continued in the 40.0 to 94.0 nmol/L range.

"[I]n both active and inactive ambulatory U.S. women and children aged ≥ 60 y, higher concentrations of 25(OH)D are associated with better musculoskeletal function in the lower extremities," the authors write, noting that while concentrations of 40 nmol/L or greater are desirable for optimal function, concentrations as high as 100 nmol/L appear advantageous.

"Given the high prevalence of low 25(OH)D concentrations and inactivity in this national survey and the positive association between 25(OH)D and lower-extremity function, vitamin D supplementation may offer a way to improve lower-extremity function in both active and inactive subjects aged ≥ 60 y," the authors conclude.

The study was supported by the Harvard Hartford Foundation, the Kirkland Scholar Award, the Irene and Fredrick State Nutrition Education Fund, the Swiss Foundation for Nutrition Research, and the International Foundation for the Promotion of Nutrition Research and Nutrition Education.

Am J Clin Nutr. 2004;80:752-758

Reviewed by Gary D. Vogin, MD

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