Women who take calcium supplements in addition to having a high-calcium diet may be at increased risk of death from all causes, and particularly from cardiovascular disease and ischemic heart disease, according to research recently published in the journal BMJ. Cardiovascular disease is the leading cause of death in people with diabetes.
Calcium intake guidelines have generally focused on preventing low levels of calcium, and indeed, more than 60% of middle-age and older women in the United States now regularly take calcium supplements to preserve bone health (calcium is important for strong bones). But reanalysis of some recent studies has indicated a higher risk of ischemic heart disease (a condition in which there is a reduced supply of blood to the heart) and stroke with calcium supplements.
To evaluate the effect of dietary and supplemental calcium intake on cardiovascular health and mortality, researchers at Uppsala University in Sweden followed 61,433 women born between 1914 and 1948 for nearly two decades. The participants’ diets were determined by questionnaires administered to all the women at the start of the study and to 38,984 of the women approximately 10 years later. The participants were divided into four groups based on their calcium consumption: less than 600 milligrams (mg) a day, from 600 mg to 999 mg a day, from 1,000 mg through 1,399 mg a day, and 1,400 or more mg daily (equivalent to about five 8-ounce glasses of cow’s milk).
Over the course of the 19-year follow-up, 11,944 of the participants died, 3,862 from cardiovascular disease, 1,932 of ischemic heart disease, and 1,100 of stroke. Compared to women with a calcium intake of 600 mg to 999 mg daily, those consuming more than 1,400 mg each day had 1.40 times the risk of death from any cause, 1.49 times the risk of death from cardiovascular disease, and 2.14 times the risk of death from any cause. There was no significant difference in death rate from stroke. Levels of calcium intake below 600 mg a day were associated with an increased risk of death from all causes, plus death from cardiovascular disease, ischemic heart disease, and stroke.
In a separate analysis using the data from the 1997 survey, the researchers found that women with a dietary calcium intake of 1,400 mg or more daily who also took a calcium tablets (500 mg of calcium) had a 2.57-fold risk of death from all causes compared to women with a calcium intake of 600 mg to 999 mg daily. Further analysis revealed that, in women with a high dietary intake of calcium, the addition of a calcium supplement increased the risk of death in a dose-dependent fashion.
“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 study authors noted.
According to the researchers, diets very high or very low in calcium might affect the body’s equilibrium, causing changes in the blood levels of calcium. They emphasized that mortality was not increased with customary levels of calcium intake.
Limitations of the study include that it was observational, meaning it could not establish cause and effect, and that the results may not apply to people of different ethnic origins or to men. (The recent NIH-AARP Diet and Health Study, however, has linked high calcium intake from supplements to heart disease in men. To learn more about that study, click here.)