A growing body of evidence suggests that vitamin D status is important in biologic processes involved in the maintenance of physical function. To advance the investigators understanding of the role of vitamin D in physical function, the investigators will conduct a feasibility pilot study to collect key information to help design a full-scale randomized trial to determine whether vitamin D supplementation will delay declines in physical function. The primary goals of the pilot study are to determine cost-effective strategies for identifying persons at high risk for functional decline with insufficient vitamin D levels, determine the serum vitamin D response to a vitamin D supplementation regimen designed to attain sufficient vitamin D levels, and provide preliminary data of key functional measures (balance, physical performance and muscle power) for the future larger study design. A secondary goal is to begin to examine potential mechanisms by which vitamin D supplementation may enhance physical performance by exploring the effects of vitamin D supplementation on changes in skeletal muscle gene expression.

A growing body of evidence suggests that vitamin D status is important in biologic processes involved in the maintenance of physical function. However, whether remediation of vitamin D insufficiency will improve physical function and the potential mechanisms involved are unclear. Previous vitamin D supplementation trials have produced mixed results with respect to physical function; however, most trials did not specifically recruit individuals who were vitamin D insufficient nor is the potential mechanism of action understood well enough to appropriately select those individuals most likely to benefit. To advance our understanding of the role of vitamin D in physical function, the investigators will conduct a feasibility pilot study to collect key information to help design a full-scale randomized trial to determine whether vitamin D supplementation will delay declines in physical function. The primary goals of the pilot study are to determine cost-effective strategies for identifying persons at high risk for functional decline with insufficient vitamin D levels, determine the serum vitamin D response to a vitamin D supplementation regimen designed to attain sufficient vitamin D levels, and provide preliminary data of key functional measures (balance, short physical performance battery (SBBP) and muscle power) for the future larger study design. A secondary goal is to begin to examine potential mechanisms by which vitamin D supplementation may enhance physical performance and muscle contractility by exploring the effects of vitamin D supplementation on changes in skeletal muscle gene expression using microarrays.

Inclusion Criteria:

• Short Physical Performance Battery (SPPB) score of ≥ 4 to < 10
• Vitamin D insufficient (serum 25(OH)D ≥ 10 to < 25 ng/mL)

Exclusion Criteria:

• Serious or uncontrolled chronic disease including: insulin-dependent or uncontrolled diabetes; cancer requiring treatment in past year, except non-melanoma skin cancers; past or current ischemic heart disease, uncontrolled angina, heart failure, peripheral artery disease (PAD), or stroke; chronic respiratory disease; uncontrolled endocrine/metabolic disease; neurological or hematological disease; liver or renal dysfunction; and severe musculoskeletal impairment
• Impaired cognitive function (MMSE ≤ 24)
• Dependent on a cane or walker
• Use of anti-coagulants
• Unwillingness to undergo a muscle biopsy
• Taking prescription vitamin D2 or OCT vitamin D3 supplements of > 1000 IU/d
• Inability or contraindications to consume daily vitamin D and calcium supplements
• Weight loss of ≥ 5% or more in the past 6 months
• Involved in any other intervention