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Yes, it sure is possible, and there are many cases on record of vitamin D overdose. Most of these cases were recorded in the first half of the 20th century, in the heyday of massive vitamin D supplementation. (Or should I call it the FIRST heyday of same, being as how we seem to be in a second one right now?) Back then, they were using high vitamin D and cod liver oil to treat several different diseases, including arthritis and lupus. And they got some pretty good results, too. The only problem was that the doses were SO intemperate that there were a fair number of overdose cases along with. I mean doses like 50,000 or 100,000 or even more I.U.s daily, for months! Or gigantic doses, like a half-million I.U.s daily for shorter periods. (A great deal of interesting detail on this whole era is in the vitamin D chapter of Bicknell & Prescott’s “Vitamins In Medicine”, circa 1960.)
I think that this period of overly enthusiastic vitamin D therapy gives us useful information regarding vitamin D safety. The margin of safety appears to be large. If vitamin D can be given in such huge amounts — hundreds of times the RDI, or more — with only a modest fraction of recipients developing symptoms of excess, then that says something. It says: be cautious, yes, but you need not be timid, ratcheting up from the current very low RDIs (400 I.U.s, 600 I.U.s, etc.). And this view is consistent with most scholarly opinion of today.
Doses of 2-4000 I.U.s daily are almost certainly free from harm for adults, though it would be wise to adjust downward depending on sun exposure: More sun exposure, less vitamin D. It would also be wise to attend to magnesium nutrition; see notes below. Many vitamin D enthusiasts pop 6,000 or 8,000 I.U.s daily; these should be considered experimental doses, probably best with physician supervision. Cannell, of the vitamin D council — vitamindcouncil.org — recommends supplementation sufficient to keep your blood level of 25(OH)D in the range of 50-80 nmol/L (that’s from memory but I think it is correct or close). If you have good insurance and/or can afford doctor visits and expensive lab work, you should read Cannell’s stuff on that site and follow his very scholarly advice. If you don’t, then you can wing it with a few thousand I.U.s per day, less in summer or when exposed to sun. Your ultimate perfect/optimal dose might be higher, but better to be safe.
One caveat: Mildred Seelig, a great researcher and clnician who spent most of her life studying magnesium metabolism and deficiency (and its role in disease), was adamant that vitamin D (and calcium, and phosphorus, or especially all three) could be harmful in the face of magnesium deficiency or insufficiency. These agents (D, Ca, P), which she called “calcemic” agents (raising the calcium level in the blood) can cause metastatic calcification, i.e. widespread soft tissue calcium deposition, with harmful or even disastrous results. Vitamin D overdose is indeed characterized by symptoms of such calcification, in the kidneys and elsewhere. It is important to note that the majority of the U.S. population gets insufficient magnesium; typically not even the RDI of 300 mgs — and that RDI is quite low. Seelig and others have argued — convincingly I think — that the RDI should be much higher, perhaps even twice that. So, bottom line: if you are embarking on a program of vitamin D supplementation, you might be wise to boost your magnesium intake if it has been low, or if you are in the habit of eating things that tend to deplete magnesium (which includes most of America’s favorite calorie sources: sugar, fat, alcohol).
The full text of Seelig’s *magnum opus* (which deals extensively with this vitamin D issue, among much else) is available on the web; google for her name plus “Magnesium Deficiency in the Pathogenesis of Disease”. It is a bit dated now, but still brimming with fascinating detail, and superbly documented. I would say that in light of more current research, she was half-wrong about vitamin D. She was a tad too fearful of it, though her concerns about magnesium nutrition were spot-on and have never been competently refuted.
Added: I forgot to mention skin color. Darker skin slows down the rate of vitamin D production in response to sunshine. Blacks have lower vitamin D levels than whites, especially if they live far from the equator (e.g. North America); they are often seriously deficient. Even near the equator, vitamin D insufficiency is widespread. I happen to know this because I’ve been following the literature on international vitamin D nutrition for several years. It is surprising how residents of VERY sunny lands have vitamin D deficit almost as often as us northerners. It is a matter of skin pigmentation, as well as clothing and cultural habits. If you cover your skin from head to foot, and/or spend all your time indoors, especially if you have darker pigmented skin, then you will come up with low vitamin D levels even if you live on the equator. Bottom line: darker skin? Take more vitamin D. Like toward the 4,000 I.U. end in that 2-4,000 range.
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