A Pilot Study to Examine the Effects of Vitamin D Supplementation on Mitochondrial Bioenergetics in Older Adults

Inclusion Criteria: * SPPB score of 10 or less * initial serum 25(OH)D concentration of 12-17.9 ng/mL * not dependent on a walker * willing to provide informed consent and to adhere to the protocol * not involved in another behavioral, exercise, or investigational drug intervention study * self-reported physical performance difficulty * were screened for the EVIDENCE study within the last 2 months Exclusion Criteria: * serious or uncontrolled chronic disease * evidence of impaired cognitive function (MoCA\<18) * taking prescription vitamin D2 or taking \>1000 IU/day of vitamin D3 from all sources; taking an oral corticosteroid; taking hormone replacement therapy * inability or contraindications to consume daily vitamin D supplements * knee or hip surgery within the last 6 months or planned knee or hip surgery within the next year * not willing or eligible to undergo a muscle biopsy (on blood thinners) * weight loss of greater than or equal to 5% in the past 3 months * BMI \> 40kg/m2 * eye surgery within the past month or planned within the next month * if the PI feels the participant is unlikely to follow the protocol