Its a good idea to linger on Vitamin D a bit since it brings up the broader issue that there are some nutrients that are easily available as supplements which people are assuming that more is always better. Along with vitamin D, potassium, magnesium and b-complex come to mind. High potassium is a danger to elderly folks with heart issues, high magnesium acts like a calcium channel blocker, some b vitamins in high doses are linked to cancer progression. etc. All of these nutrients have a sweet spot in their serum and cellular levels within which they can be a huge help. On either end of the curve however, they start turning toxic. Its a boring idea borrowed from toxicology, "Hormesis", but it applies. Unless a danger of deficiency exists its probably wise to assume that less-is-more, especially as you get older.