I found out that I had a strong/moderate vitamin D deficient two years ago after I broke my hand (bike accident) and the bones wouldn't heal. I started on a D3 supplement but my levels didn't go up (and the bone healing also didn't proceed), and so I had injections of megadoses, followed by regular supplementation, which I still take. (2000 IU/day). Eventually, the bones healed. Here in Minnesota and in similar places, there isn't enough sunlight for most of the year to maintain vitamin D at healthy levels, and deficiencies are a widespread problem.