Harvard Medical Center randomized more than 25,000 men and women to take either Vitamin D supplementation or placebo for more than 5 years. There were no differences in the chance of strokes, heart attacks, or cancer and this finding was true for both those who started with normal Vitamin D levels, or with low levels. This was a high quality, low risk of bias study, so the conclusions are valid. New England Journal of Medicine, Jan 2019.
