You are absolutely right, for example there are many studies showing vitamin D may have a protective effect against multiple forms of cancer, here's a nice meta

You are absolutely right, for example there are many studies showing vitamin D may have a protective effect against multiple forms of cancer, here's a nice meta-study summing a bunch of them up: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC1470481/ https://www.ncbi.nlm.nih.gov/pmc/articles/PMC1470481/ On the other hand another study has found that just taking a vitamin D supplement is not enough to statistically improve your odds of not getting cancer: https://www.cancer.gov/news-events/cancer-currents-blog/2018/vitamin-d-supplement-cancer-prevention https://www.cancer.gov/news-events/cancer-currents-blog/2018... Vitamin D is certainly important for a number of biological processes including the regulation of calcium in the body and that is well understood. But for things like cancer prevention and lowering the risk of heart disease it may simply be that naturally occurring vitamin D is just an indicator of other healthy habits like eating a diet high in fish and exercising outdoors etc.