Vitamin D, cholecalciferol to the scientists, is a hot vitamin that just got a bit cooler. Not hipper but less of a rep. For a while, it seemed that very high daily doses of D might prevent some kinds of cancer and other diseases but most experts see problems with megadoses. The axe fell on D late last November when a panel of experts decided that most people need just 600 international units (IU) a day. Yet, supplement doses can start at 5,000 IUs. That is almost 10 times the recommended amount. Amounts that high raise a red flag to most nutritionists. You can review the complete Institute of Medicine report at http://www.iom.edu/Activities/Nutrition/DRIVitDCalcium/2010-NOV-30.aspx.
What s the short story
Vitamin D, and it cousin, Vitamin A, act like a hormones. They control cell reproduction and too much creates problems. Other vitamins usually assist biochemical reactions along. The way D controls some cellular reproduction activities is the short reason people think that there is a link between D and cancer prevention. D might prevention a normal cell into converting into a cancer cell. There is some epidemiological support for this view. The take home message is that too little and too much of a hormone is like too much of any other steroid. Too little creates conditions for osteoporosis because Vitamin D is part of the bone making process and cell growth regulator. Too much D will create some other complications. One is excess calcium deposition especially if one s calcium intake increases along with D. Instead of going to make hard bones, excess D deposits calcium in tendons make that soft tissue, hard. There's more. Too much D makes bones more likely to crack than harden.
Due to its popularity, D is now in more foods than ever. It used to lurk in milk but now it is in orange juice and many other common foods. In addition, D functions differently in different cultures. In Japan, where calcium consumption is much lower because dairy consumption is lower, higher doses of D are tolerated without problems associated with deposition.
To further complicate the picture, there remains important disagreement as to how D functions among knowledgeable nutrition scientists. The dominant view is that about a 1,000 mg of calcium are needed for bone health. The main idea in this school is that the bones are like banks that need frequent and large deposits of calcium to stay strong. No other mineral is required in such amounts. For example, the most iron anyone needs on a daily basis is 15 mg. Other essential minerals are needed in even smaller and smaller amounts. A minority view holds that less calcium is needed. The reasoning here is that the body does better when calcium circulates instead of being stored in bones. The scientists who support this view explain it this way. When calcium is low, D is activated and does its magic. When calcium levels are high, D remains dormant and can't work its magic. In addition, there are less of certain cancers, such as prostate, in areas where there is lots of year round sunlight. That is because body makes its own D when it is touched by sunlight of warm weather intensity.
For the mainstream view and the most reliable consensus information on supplements and medicines, google MedlinePlus. For more information on the minority view, go to: http://www.oldwayspt.org/mediterraneandiet. For a safe, calcium rich diet, google the DASH diet.
For regular folks considering large doses of supplements, be careful and, if possible, work with a physician experienced in this area. Maybe there are times when huge doses of common vitamins are warranted. Lack of scientific clarity in complex topics creates space for debate and the resolution process can take decades. Another consideration is a known cycle of expectation for any new therapeutic agent. First, hopes soar then crash. The exciting substance now cures or prevents nothing. Finally, a consistent body of research supporting some claims gets produced. Vitamin D s mechanism is very complex, not completely clarified and its action far reaching. That is real reason for disagreement. In the meantime, remember the Goldilocks rule: The middle is just right. . If the nutrition big shots do not agree, caution is warranted.