Participants will take 2,000 international units (IU) of vitamin D3 daily for a 6 month period, followed by an increase to 4,000 IU daily for at least 6 months thereafter. The vitamin D supplements will be provided by the study. In keeping with the current standard of care for ALD boys aged 3-12 years, participants will need to visit the study site every six months in order to complete a clinic visit and MRI of the brain with gadolinium. As part of this study, however, participants' will need to submit blood work every 3 months during the first year in order for the study investigators to ensure that the participants' calcium and vitamin D levels are in a safe range and to study the effects of vitamin D on markers in the blood. The MRI protocol during the first year will also include one additional sequence (magnetic resonance spectroscopy) in order to measure brain metabolites.