Too Much Calcium May Be Harmful for Women

By Michael Smith, North American Correspondent, MedPage Today

Published: February 13, 2013

Reviewed by Robert Jasmer, MD; Associate Clinical Professor of Medicine, University of California, San Francisco and Dorothy Caputo, MA, BSN, RN, Nurse Planner

But in a large cohort of Swedish women followed for nearly 2 decades, there was no increase in the risk of stroke in women who took more than 1,400 mg of dietary calcium per day, according to Karl Michaëlsson, MD, of Uppsala University in Uppsala, Sweden, and colleagues.

The findings are seemingly counter-intuitive, Michaëlsson and colleagues wrote online in BMJ, because guidelines have focused on avoiding low levels of calcium.

Indeed, they noted, more than 60% of middle-age and older women in the U.S. regularly take calcium supplements.

But re-analysis of some recent trials observed a higher risk of ischemic heart disease and stroke with calcium supplements, they added, although that was not seen in another study.

To help clarify the issue, they turned to the Swedish Mammography Cohort, a population-based cohort that includes 61,433 women born between 1914 and 1948 with a median follow-up of 19 years.

The primary outcome measures, based on registry data, were time to death from all causes, as well as from cardiovascular disease, ischemic heart disease, and stroke.

The researchers assessed diet by food frequency questionnaires, allowing an estimate of calcium intake, both from diet and supplements. Participants were divided into groups based on calcium intake – less than 600 mg a day, between 600 and 999 mg daily, from 1,000 through 1,399 mg a day, and 1,400 or more mg daily (about five 8-ounce glasses of cow's milk).

All told, 11,944 women died, including 3,862 from cardiovascular disease, 1,932 from ischemic heart disease, and 1,100 from stroke.

Compared with women whose intake was between 600 and 999 mg day, they found dietary intakes of more than 1,400 mg a day were associated with higher death rates:

All causes: hazard ratio 1.40 (95% CI 1.17 to 1.67)
Cardiovascular disease: HR 1.49 (95% CI 1.09 to 2.02)
Ischemic heart disease: HR 2.14 (95% CI 1.48 to 3.09)

However, higher dietary intakes of more than 1,400 mg per day was not associated with a significantly increased risk for stroke (HR 0.73).

"The increase was moderate with a high dietary calcium intake without supplement use, but the combination of a high dietary calcium intake and calcium tablet use resulted in a more pronounced increase in mortality," the authors stated.

At the same time, levels of calcium below 600 mg a day were associated with an increased risk of all-cause mortality, as well as cardiovascular disease, ischemic heart disease, and stroke.

Diets very low or very high in calcium, might override the normal tight homeostatic control, causing changes in blood levels of calcium, said Michaëlsson and colleagues.

They cautioned that the dietary assessments were prone to limitations that affect both accuracy and precision, and that an observational study cannot show causality. Also, portion sizes were not assessed on an individual basis.

Finally, the results might not apply to people of different ethnic origins or to men.

The study had support from the Swedish Research Council. The journal said the authors declared no conflicts of interest.

From the American Heart Association: