Calcium deficiency has been implicated as a possible cause of GH.^1,2 In two preliminary studies, women who developed GH were found to have significantly lower dietary calcium intake than did pregnant women with normal blood pressure.^3,4 Calcium supplementation has significantly reduced the incidence of GH in preliminary studies⁵ and in many,^{5,7,8,9,10,11} though not all,¹² double-blind trials. Calcium supplements may be most effective in preventing GH in women who have low dietary intake of calcium. The National Institutes of Health (NIH) recommends an intake of 1,200 to 1,500 mg of calcium daily during normal pregnancy.¹³ In women at risk of GH, studies showing reduced incidence have typically used 2,000 mg of supplemental calcium per day,^{14,15,5,7,9,11} without any reported maternal or fetal side effects.^14,15 Nonetheless, many doctors continue to suggest amounts no higher than 1,500 mg per day.

Calcium deficiency has been associated with preeclampsia.¹⁵ In numerous controlled trials, oral calcium supplementation has been studied as a possible preventive measure.^16 17 1819 While most trials have found a significant reduction in the incidence of preeclampsia with calcium supplementation,^161722231925 One study reported that calcium supplementation reduced both the severity of preeclampsia and the mortality rate in the infants.¹⁸

An analysis of double-blind trials46 found calcium supplementation to be highly effective in preventing preeclampsia. However, a large and well-designed double-blind trial and a critical analysis of six double-blind trials concluded that calcium supplementation did not reduce the risk of preeclampsia in healthy women at low risk for preeclampsia.²⁷ For healthy, high-risk (in other words, calcium deficient) women, however, the data show a clear and statistically significant beneficial effect of calcium supplementation in reducing the risk of preeclampsia.^{2723183118333435191938 39 4041}

The National Institutes of Health recommends an intake of 1,200 to 1,500 mg of elemental calcium daily during normal pregnancy.⁴² In women at risk of preeclampsia, most trials showing reduced incidence have used 2,000 mg of supplemental calcium per day.¹⁷ Nonetheless, many doctors continue to suggest amounts no higher than 1,500 mg per day.

Women who consume more calcium from their diets are less likely to suffer severe PMS.³³ A large double-blind trial found that women who took 1,200 mg per day of calcium for three menstrual cycles had a 48% reduction in PMS symptoms, compared to a 30% reduction in the placebo group.³⁴ Other double-blind trials have shown that supplementing 1,000 mg of calcium per day relieves premenstrual symptoms.^35,36

Caution: Calcium supplements should be avoided by prostate cancer patients.

Researchers have yet to clearly determine whether lactose-intolerant people absorb less calcium.³⁷ As lactose-containing foods are among the best dietary sources of calcium, alternative sources of calcium (from beverages, foods, or supplements) are important for lactose-intolerant people. A typical amount of supplemental calcium is 1,000 mg per day.

Women who consume more calcium from their diets are less likely to suffer severe PMS.³⁸ A large double-blind trial found that women who took 1,200 mg per day of calcium for three menstrual cycles had a 48% reduction in PMS symptoms, compared to a 30% reduction in the placebo group.³⁹ Other double-blind trials have shown that supplementing 1,000 mg of calcium per day relieves premenstrual symptoms.^40,41

Caution: Calcium supplements should be avoided by prostate cancer patients.

Although insufficient when used as the only intervention, calcium supplements help prevent osteoporosis.⁴² Though some of the research remains controversial, the protective effect of calcium on bone mass is one of very few health claims permitted on supplement labels by the U.S. Food and Drug Administration.

In some studies, higher calcium intake has not correlated with a reduced risk of osteoporosis—for example, in women shortly after becoming menopausal⁴³ or in men.⁴⁴ However, after about three years of menopause, calcium supplementation does appear to take on a protective effect for women.⁴⁵ Even the most positive trials using isolated calcium supplementation show only minor effects on bone mass. Nonetheless, a review of the research shows that calcium supplementation plus hormone replacement therapy is much more effective than hormone replacement therapy without calcium.⁴⁶ Double-blind research has found that increasing calcium intake results in greater bone mass in girls.⁴⁷ An analysis of many trials investigating the effects of calcium supplementation in premenopausal women has also shown a significant positive effect.⁴⁸ Most doctors recommend calcium supplementation as a way to partially reduce the risk of osteoporosis and to help people already diagnosed with the condition. In order to achieve the 1,500 mg per day calcium intake many researchers deem optimal, 800 to 1,000 mg of supplemental calcium are generally added to the 500 to 700 mg readily obtainable from the diet.

While phosphorus is essential for bone formation, most people do not require phosphorus supplementation, because the typical western diet provides ample or even excessive amounts of phosphorus. One study, however, has shown that taking calcium can interfere with the absorption of phosphorus, potentially leading to phosphorus deficiency in elderly people, whose diets may contain less phosphorus.⁴⁹. The authors of this study recommend that, for elderly people, at least some of the supplemental calcium be taken in the form of tricalcium phosphate or some other phosphorus-containing preparation.

One trial studying postmenopausal women combined hormone replacement therapy with magnesium (600 mg per day), calcium (500 mg per day), vitamin C, B vitamins, vitamin D, zinc, copper, manganese, boron, and other nutrients for an eight- to nine-month period.⁵⁰ In addition, participants were told to avoid processed foods, limit protein intake, emphasize vegetable over animal protein, and limit consumption of salt, sugar, alcohol, coffee, tea, chocolate, and tobacco. Bone density increased a remarkable 11%, compared to only 0.7% in women receiving hormone replacement alone.