The purpose of this study is to identify the magnitude of muscle lipid redistribution and muscle oxygen use in individuals 60 years old or older following combined treatment of vitamin D repletion and aerobic training compared to vitamin D repletion alone, aerobic training alone, or no treatment.

The investigators plan to recruit approximately 56 aged individuals with vitamin D insufficiency for a 13 week trial. All subjects will be 60 years of age or older. Participants will be randomized to one of four groups: vitamin D (50,000IU/wk) + aerobic training (7 days during the 13th week); vitamin D (50,000IU/wk) + normal daily activity during the 13th week; placebo + aerobic training (7 days during the 13th week); or placebo + normal daily activity during the 13th week.

Following screening and informed consent process a physical activity questionnaire will be administered. A blood draw will then be conducted at baseline, midpoint and endpoint. Collected blood for the purposes of this study will assess vitamin D status (25(OH)D), parathyroid hormone, ionized calcium, insulin, and a comprehensive metabolic panel. At baseline and endpoint muscle lipid will be assessed using Magnetic Resonance Spectroscopy and muscle oxygen consumption will be determined using a novel hybrid diffuse optical instrument consisting of a near-infrared spectroscopy and a diffuse correlation spectroscopy. In addition, at each study time point 3 dietary recalls will be completed and physical activity will be measured using an accelerometer.

Inclusion Criteria:

• Age: 60 and older
• BMI: 18.5-29
• Communicates fluently in English

Exclusion Criteria:

• Metal implant not suitable for MRI
• Any form of hormone replacement therapy
• Engage in more than 1 hr/week of vigorous activity
• Participated in a high-intensity resistance or aerobic training program in the last 3 months
• Lower extremity injury or surgery in the past 3 months
• Foot sores or bone conditions severely limiting ability to move or perform exercise or joint pain made worse by mild exercise
• Diabetes, inflammatory bowel disease, renal disease or uncontrolled hypertension.
• Currently have or had a history of cardiovascular or pulmonary disease that would preclude the involvement in the performance of exercise
• History of myopathy (including congenital myopathies)
• History of neurological conditions related to spinal derangement, disk disease, peripheral neuropathies, tremor and rigidity
• Past medical history of hyperparathyroidism, kidney stones or rhabdomyolysis
• Currently receiving treatment for vitamin D deficiency
• Vitamin D level >30ng/mL
• Currently smoking