As human beings, our lifestyles have changed from that of barely-clothed, nearly naked hunter-gatherers spending most of our daylight time outdoors, to fully clothed, smothered in sunscreen, and spending very little time outdoors. This effectively stops our skin from assimilating Vit D from sunlight, which is our primary source of this pro-hormone. No wonder we have so many health epidemics and pandemics, particularly the degenerative diseases and cancers. The immune stimulating effects of D3 - and it MUST be the "3" form, which is cholecalciferol, from animal sources, and not ergocalciferol, which is D2, and from plant sources - are monumental. Be wary of studies that claim vit D supplementation has no effect on bone density or other issues - check that they have used a decent amount (a lot of studies only use 400 or 800IU, and of courser that won't show any beneficial effect because it isn't enough), and that the form was D3 and not D2. Check out potential toxicity issues for yourself. Some people who say they tried vit D and got bone spurs and other calcium deposition problems as a result experienced that because they did not observe some simple facts: Vit D needs Vit K to direct the calcium into the bones - without that process happening, the calcium will OF COURSE get dumped willy-nilly by the body. You can think of Vit K like a mini-bus, picking up the workers and driving then to the job-site. Unless K is there to pick the calcium up, the calcium just wanders around until it lands somewhere and stays there. This is also why some people get kidney stones on high dose D. Other bone factors have to be considered too - estrogen and progesterone are critical to bone health - they produce osteoclasts, which clear away old bone cells, and osteoblasts, which build new bone. You cannot build new bone until the dead stuff has been cleared away, in the same way that you couldn't build a safe new building directly on top of a derelict building with wonky foundations. Other minerals are also important. The calcium:magnesium ratio is especially crucial to get right, and this is going to vary from person to person, with some health conditions benefitting from way more magnesium than calcium. An easy way to check that you are taking enough magnesium is to observe the standard natural medicine protocol of observing any sudden change in stool texture. When you have taken as much mag as your body can use that day, you experience what I can "torpedo poos", which is to say that they pop out quite fast, and are very smooth (not the ideal stool as it indicates food has passed through too fast for optimal absorbtion, but a good way to check you mag requirement). Silica is very important too, because part of bone health is flexibility of the tissue. According to some studies and claims, only 10% of bone structure is actually calcium, the rest is collagen, which is the same springy stuff that gives our skin (and blood vessels and other connective tissues like cartilage, tendon, etc etc) its elasticity. Silica, it is claimed, is as important as Vit K in directing calcium away from soft tissues and into bone. The idea is that the bones are where the body is designed to store calcium so that it can be pulled out of the bones and into the blood as needed - blood levels are kept within very very narrow margins, and any deviation from these margins disrupts health in a major way. So, according to something I read on Eidon's website, only 10% of bone is made of calcium, but bone is the storage depot for calcium. And calcium needs silica as well as Vit D3, Vit K (the MK7 lasts twice as long in the body than the ordinary K2), phosphorous (very plentiful in diet and prob no need to supplement), magnesium, estrogen (definitely in women, I haven't checked to see about men, but presumably they also have the same mechanism of action for osteoblasts and osteoclasts, and contrary to popular belief, men DO produce estrogen, just like women produce testosterone, we each just produce the others major sex hormone in smaller amounts than the opposite sex does), progesterone...........I know some people without osteoporosis and otherwise healthy (I have osteoporosis, breast cancer, fibromyalgia, low adrenal and thyroid function, and other issues) have successfully added high dose D 10k IU/day along with K2, and haven't needed to address other more extensive bone health issues, but please be aware of all these different factors before leaping in without thinking about adding silica and the others. Oh, before I forget, another thing to investigate is Vitamin A, currently a bit out of favour, but think about it - in cod liver oil, which occurs naturally, and indeed used to be prescribed by the government for pregnant women in the UK, and was also prescribed by the government here in war time when we spent a lot of time underground to escape the bombs, levels of Vit A are HIGHER than Vit D. Maybe once D levels have been brought up to optimal status, at which point you can determine your ongoing maintainance dose (which will be lower than you repletion dose), Vit A then needs to be adjusted according to this principle. M Aziz's book is a brilliant source of info, and contains a huge amount of medical references that you can go and check for yourself. It is his personal account of helping himself and his family with high dose D3. He has been taking 10K-12500 IU a day for the past 6-8 years, with nothing but beneficial effects. His bone density has increased. He takes no supplemental calcium or magnesium or progesterone or estrogen, nor silica, just MK7 K2. Also, it is obviously vital to get our GP onside and to have your D levels checked before and during supplementation. Just use you own knowledge of what the test SHOULD say in order for you to be optimal, rather than what the reference ranges on the tests say - they differ wildly from lab to lab, hospital to hospital, so draw your own conclusions. Hope this helps - read M Aziz's book it's a really exciting eye-opener. Currently an e book from the biggest online bookseller.