There have been several articles recently published in medical journals (JAMA, New England Journal of Medicine, British Medical Journal) about the possible risks associated with taking calcium supplements. The study groups are respected researchers, and include our own National Institute of Health. The studies addressed only the question of calcium supplements, and not the impact on health of a diet that was high in calcium.
I must state in advance that one problem with the results of the studies, besides their contradictory results, is that we have no good explanation for their results. While this does not mean the results are not valid and true, this lack of reasoning as to cause and effect makes it more difficult to believe in the result. It is also puzzling why the two main studies found different effects in men and women, and the results of one contradicted the results of the other.
The amount of calcium in your blood is very closely controlled by the body, and it is the ionized calcium, Ca++, a divalent cation that is the active form. Calcium exists in your blood both free as a cation and bound to serum proteins, and the aforementioned studies did not measure calcium levels. Because magnesium also exists as a divalent cation it often precipitates out in laboratory processes along with calcium, which led to the belief that the use of a supplement that contained magnesium as well as calcium was beneficial; no such result has ever been shown to be valid. However, since calcium is mildly constipating and magnesium is a mild laxative, combining the two elements into one pill makes theoretical clinical sense. Strontium is also a divalent cation, and unfortunately can be taken up by your bones along with calcium, which explains some of the bone marrow cancers induced by the radioactive strontium fallout from an atomic explosion.
The calcium levels are controlled by parathyroid hormone, which also activates Vitamin D into its useful form and thereby promotes the uptake of calcium by your bones both directly and indirectly. As a rule, the amount of calcium in your diet does not affect the level of calcium in your bloodstream. It is generally agreed that the calcium in your diet should contain between 600mg to 1200mg of calcium daily, along with 400 Units of Vitamin D, in order to maintain bone strength and prevent osteoporosis because activated Vitamin D promotes the absorption of calcium from your gut. We also know that ingesting too much calcium if you have chronic kidney disease can promote calcification of the arteries in your body, probably because kidney failure induces a state of secondary hyperparathyroidism, but a further discussion of this condition is beyond the scope of this article. Parathyroid hormone also increases the absorption of phosphate from your intestine which makes clinical sense since the largest mineral constituent of your bones is hydroxylapatite, which contains both calcium and phosphorous.
Now as to the studies. The question asked was whether or not taking calcium supplements (1000 mg or more in men and 1400 mg or more in women) had a benefit on the patient's health. One study showed an increase in overall mortality, and cardiovascular mortality as well as in heart attacks, but not in stroke. This increase in mortality was shown to exist in men, but not in women. Another study, published at about the same time, showed a similar increase in overall and cardiovascular mortality in women, but not in men(!).In each study the control group was same-sex patients who did not take calcium supplements.
So what are we to believe? There even are no clear-cut studies showing that calcium supplements prevent osteoporotic fracture; in fact some studies show an opposite result. (But taking extra Vitamin D at a dose of 800 mg/day does seem to prevent such fractures.) It is difficult to recommend calcium supplements in view of the above quoted data. It would be useful to have a study that measured calcium levels in people having heart attacks were it not for the fact that no one knows whether or not a MI acutely affects calcium levels. And it would seem heartless to tell the patients to read all three studies and to make up their own minds. I think that because no study showed by clear-cut evidence that taking calcium supplements was beneficial for one's health, then the recommendation of the doctor should come down on not recommending calcium supplements, thereby combining two tenets of medicine: "first, do no harm" and "less is more".