A Randomized Controlled Trial of the Effects of Vitamin D on Muscle Strength and Mobility in Older Women with Vitamin D Insufficiency

Kun Zhu, PhD; Nicole Austin, PhD; Amanda Devine, PhD; David Bruce, MD; Richard L. Prince, MD

Disclosures

J Am Geriatr Soc. 2010;58(11):2063-2068. 

In This Article

Discussion

The present study found that, in older women with low vitamin D status receiving calcium, ergocalciferol supplementation improved muscle strength in subjects with baseline muscle strength and mobility in the lowest tertile. Mobility as reflected by the TUAG improved significantly in those in the slowest tertile at baseline with supplementation.

Previous intervention studies on the effects of vitamin D on lower extremity muscle strength and function have yielded inconsistent results. Whereas some trials showed an effect of vitamin D on improving lower extremity muscle strength and function in older people,[2,15,16] others did not.[17–19] Most of these studies included male and female participants who were not selected for low vitamin D status. In one recent study of 226 men and women aged 70 and older with vitamin D insufficiency (serum 25(OH)D 6–20 ng/mL), 8,400 IU vitamin D weekly supplementation for 16 weeks increased serum 25(OH)D from 14 to 26 ng/mL. The intervention reduced mediolateral sway in participants who had large sway but not those who had normal sway at baseline.[20] Consistent with this, the present study showed that, in older women with baseline serum 25(OH)D levels less than 24 ng/mL, vitamin D supplementation significantly improved muscle strength in participants with low baseline muscle strength.

The TUAG includes basic mobility skills and is an effective method of assessing functional mobility in older adults.[21] It has also been shown to be a sensitive and specific measure for identifying community-dwelling older adults aged 65 and older who are prone to fall.[25] The results of the current study showed that supplementation of vitamin D2 1,000 IU/d improved TUAG time 17.5% in participants with baseline values longer than 12 seconds. This is of clinical significance, because 12 seconds has been suggested to be the cutoff point for normal mobility.[26] A previous study showed that vitamin D supplementation could improve neuromuscular coordination in older people with serum 25(OH)D concentrations less than 30 nmol/L (12 ng/mL) and a history of falling.[27] Therefore, the improvement on the TUAG could be related to improved muscle strength and neuromuscular function.

The current study used vitamin D2 as the supplement, and although some studies have reported that vitamin D2 is not as effective as vitamin D3 in maintaining serum 25(OH)D concentrations,[28,29] a recent study showed that vitamin D2 was as effective as vitamin D3 in maintaining serum 25(OH)D concentrations.[30] Epidemiological studies have suggested that, for optimal lower extremity strength, serum 25(OH)D concentrations greater than 20 ng/mL are desirable.[31,32] In the present study, 1,000 IU of ergocalciferol resulted in 80% of subjects in the vitamin D group achieving a serum 25(OH)D concentration greater than 20 ng/mL. Despite this, it could be argued that the dose of vitamin D used was suboptimal, and whether higher doses of vitamin D may provide further functional benefit is being actively pursued.

For muscle strength measurements based on voluntary contractions, subjects may learn how to perform a "good" test. In the present study, participants were given three attempts for each muscle group to reduce potential learning effect bias. In addition, the double-blind randomized placebo-controlled design of the present study ensured that any "learning effect" would occur in both groups and would not introduce bias to the study. A post hoc hypothesis that treatment was most effective in the weakest and slowest replaced the primary hypothesis that vitamin D reduced falling by improving mean muscle strength and mobility in all subjects. Multiple testing was not adjusted for in the subgroup analysis, so the results should be accepted with some caution. Nevertheless, similar effects were observed in two muscle groups and the TUAG in the present study and in a previous study.[20] Finally, the subjects were Caucasian community-dwelling elderly women with low vitamin D status, so the interpretations of these findings are limited to this population.

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