Abstract and Introduction

Abstract

Objectives: To systematically review and quantitatively synthesize the effect of vitamin D supplementation on muscle strength, gait, and balance in older adults.
Design: Systematic review and meta-analysis.
Setting: MEDLINE, EMBASE, Cochrane Library, bibliographies of selected articles, and previous systematic reviews were searched between January 1980 and November 2010 for eligible articles.
Participants: Older adults (≥60) participating in randomized controlled trials of the effect of supplemental vitamin D without an exercise intervention on muscle strength, gait, and balance.
Measurements: Data were independently extracted, and study quality was evaluated. Meta-analysis using a fixed-effects model was performed and the I ² statistic was used to assess heterogeneity.
Results: Of 714 potentially relevant articles, 13 met the inclusion criteria. In the pooled analysis, vitamin D supplementation yielded a standardized mean difference of −0.20 (95% confidence interval (CI) = −0.39 to −0.01, P = .04, I ² = 0%) for reduced postural sway, −0.19 (95% CI = −0.35 to −0.02, P = .03, I ² = 0%) for decreased time to complete the Timed Up and Go Test, and 0.05 (95% CI = −0.11 to 0.20, P = .04, I ² = 0%) for lower extremity strength gain. Regarding dosing frequency regimen, only one study demonstrated a beneficial effect on balance with a single large dose. All studies with daily doses of 800 IU or more demonstrated beneficial effects on balance and muscle strength.
Conclusion: Supplemental vitamin D with daily doses of 800 to 1,000 IU consistently demonstrated beneficial effects on strength and balance. An effect on gait was not demonstrated, although further evaluation is recommended.

Introduction

Vitamin D deficiency has recently gained much attention because of its association with cardiovascular disease, cancer, falls, fractures, and mortality.^[1–3] Older adults are especially at risk of developing vitamin D deficiency because of low sunshine exposure, less skin capacity to synthesize vitamin D, poorer absorption of vitamin D with less activation in the kidneys and peripheral tissues, and fewer or lower expression of vitamin D receptors in peripheral tissues.^[4–7]

In older adults, vitamin D deficiency has been associated with important determinants of disability, including poor physical performance, low muscular strength, cognitive impairment, falls, and fractures.^[8–15] Falls and fractures are also strongly associated with muscle weakness and gait and balance deficits.^[16] Because muscle weakness is a feature of the clinical syndrome of vitamin D deficiency, it has been postulated that vitamin D deficiency may precipitate and potentiate muscle weakness and functional decline in older people.^[17]

During the last decade, it has been demonstrated that vitamin D supplementation reduces fall risk in older adults when doses of 700 to 1,000 IU per day are used.^[18,19] This beneficial effect on fall reduction in isolation from exercise prescription seems to occur through action on neuromuscular function.^[17,18] Vitamin D receptors (VDRs) are present in several tissues throughout the body, including bone, muscle, and brain,^[20,21] and their expression and activity decline with aging.^[20] Serum levels of vitamin D decline significantly with aging, and this has been associated with reduced VDR activation and reduced muscle cell function.^[21] This low expression and activity has been well documented in muscle, which affects the response of myocytes to vitamin D.^[21]

Additionally, VDRs have been located in the human cortex and hippocampus and at a cellular level are present in neurons and glial cells.^[20] Vitamin D deficiency in older people has been associated with impairments in the central nervous system, including cognitive decline and balance problems,^[22] and in the peripheral nervous system, including reduction of nerve conduction velocity.^[23] It has been suggested that vitamin D affects neuromuscular control and coordination^[24] and may act as a neurosteroid hormone.^[25]

Based on these age-associated changes, there is a compelling rationale to believe that there is a beneficial effect of vitamin D supplementation on neuromuscular function in older adults, particularly when high doses are used.^[17] A recent review of vitamin D in adult health and disease supported fall risk reduction but did not comment on physical function effects.^[26]

Previous systematic reviews on the effect of vitamin D supplementation on muscle function reported insufficient evidence to support the therapeutic use of vitamin D alone.^[8,27] The use of vitamin D in combination with calcium supplementation was noted to have some supporting evidence, although more-definitive work was recommended.^[27] A potential explanation for these inconclusive findings could be related to variation in important research elements, such as heterogeneity in study quality, and variability in assessment methods of physical performance. Most importantly, and in light of the recent meta-analysis findings on fall risk, an important factor to consider is the dose and treatment regimen of vitamin D evaluated in each study.^[18]

As was hypothesized in a previous review in 2005,^[17] higher doses of vitamin D, at least 800 IU daily, may be necessary to improve muscle strength- and physical performance-related outcomes in older adults. Therefore, the objective of this systematic review was to assess the efficacy of vitamin D supplementation, including variations in dose and treatment regimen, on muscle strength, balance, and gait in older adults.

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