Until 1997, the recommended dietary allowance (RDA) for vitamin D was 200 IU for all adults. Faced with growing evidence of vitamin D deficiencies in Americans, the RDA for 51- to 70-year-olds was increased to 400 IU, and to 600 IU for people older than 70.
Is more better? New research suggests that it is, and many authorities are recommending 800 or even 1,000 IU a day. Remember, though, that you can get too much of a good thing. Like the other fat-soluble vitamins, vitamin D is stored in the body's adipose (fat) tissue. That means your body can mobilize its own reserves if your daily intake falters temporarily — but it also means that excessive doses of vitamin D can build up to toxic levels. At those extremes, vitamin D can raise blood calcium to levels that can cause grogginess, constipation, and even death. But it takes massive overdosing to produce toxicity, and doses up to 2,000 IU a day are considered safe.