The prestigious Institute of Medicine issued a report recommending that adult North Americans need only 1,000 milligrams of calcium and 600 IU of vitamin D per day, and that most people do not need supplements. Taking too much calcium can cause kidney stones, and taking calcium without also taking vitamin D may increase risk for heart disease. Very large amounts of vitamin D may increase risk for fractures. The authors believe that adolescent girls may be the only group that is getting too little dietary calcium (Report from the Institute of Medicine of the National Academies, November 30, 2010).
Those of you who have read my newsletters for the past few years know that I am very concerned about vitamin D deficiency. However, I do not recommend taking vitamin D pills unless your blood level of vitamin D is less than 75 nmol/L (30 ng/L). How have scientists decided that your blood level of vitamin D should be above 75 nmol/L? A major function of vitamin D is help your body absorb calcium. When you lack vitamin D, ionized blood calcium levels drop. This causes your parathyroid gland to become overactive and produce too much parathyroid hormone. Too much parathyroid hormone forces calcium out of bones to weaken them. The lowest level of vitamin D that keeps parathyroid hormone at normal levels is 75 nmol/L. If your blood level of vitamin D3 is less than 75 nmol/L, you need extra sunlight or to take at least 2000 IU of vitamin D3/day until you reach a normal level.
Hundreds of studies show that people with low blood levels of vitamin D are more likely to suffer many different cancers, heart attacks, strokes, diabetes, osteoporosis, bone fractures, autoimmune diseases, decreased immunity, and so forth. However, except for weakened bones, these are associations, not cause-and-effect. To prove causation, we need studies showing that giving pills to raise blood levels of vitamin D prevents or cures cancers, diabetes, heart disease and other conditions. So far this has been done only for bone diseases, fractures and prevention of influenza. It may be that vitamin D deficiency is only a marker for other factors, such as lack of outdoor exercise, that contribute to all of these diseases. If this is true, then taking vitamin D pills would not correct the underlying problem.
Researchers at Emory University studied vitamin D status in twins living in different North American locations. They concluded that vitamin D deficiency runs in families and is mostly genetic (American Journal of Clinical Nutrition, December 2010).