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Prospective cohort study of women in the Iowa Women's Health Study with 8 years of follow up.
34 486 women who were 55-69 years of age. Women were excluded if they had IHD at baseline, were premenopausal, did not complete ≥30 items on the food frequency questionnaire, or had implausibly high or low daily energy intake scores.
Assessment of risk factors
Women completed postal questionnaires on known and suspected risk factors for cardiovascular disease in 1986. A 127 item semiquantitative food frequency questionnaire measured usual food intake and vitamin and mineral supplement use. For analysis, women were divided into quartiles based on intake of food, nutrients, and other characteristics.
Main outcome measure
Death from IHD (1986–94) obtained from questionnaires and state and national databases.
During follow up, 387 deaths from IHD occurred (57% from acute myocardial infarction, 30% from chronic IHD, and 13% from arteriosclerotic cardiovascular disease). Multivariate analysis showed that total calcium intake was associated with IHD mortality. Compared with women in the lowest quartile of total calcium intake, those in the second and fourth quartiles had reduced risks of IHD mortality, but those in the third quartile did not (table⇓). A daily intake of 1–500 mg of supplemental calcium was also associated with a reduced risk of IHD (relative risk 0.76, 95% CI 0.58 to 1.00). Dietary calcium; total, dietary, and supplemental vitamin D; and total and fat containing dairy intake were not associated with IHD mortality.
A higher intake of total calcium, but not vitamin D or dairy products, was associated with a decreased risk of ischaemic heart disease mortality in postmenopausal women.
The results of Bostick et al suggest that among postmenopausal women, the risk of dying of IHD may be reduced by consuming relatively high concentrations of calcium. This association seems plausible given the findings of previous research showing that higher consumption of calcium lowers blood cholesterol concentrations,1 may reduce blood pressure,2 and may prevent hypertension.
The strengths of this study are that it was prospective and included a large, well defined cohort derived from a general population. In addition, the validity of the nutritional questionnaire is impressive and well supported by published data. The limitations of the study are the many confounders related to the evaluation of the effects of vitamins and minerals on health or disease states. Data on possible confounders, such as sunlight exposure as a source of vitamin D, were not provided.
The findings suggest that there is no increase in IHD mortality as a result of increased calcium intake, provided the calcium does not come from fat containing milk products. Furthermore, the findings are consistent with a 30-35% reduction in IHD mortality among postmenopausal women with a high intake of calcium. More studies are needed to substantiate these findings and to clarify practice recommendations, particularly in relation to the non-significant results for the third quartile of calcium intake. Diet and vitamin supplementation should be one of a broader spectrum of lifestyle factors considered for health promotion, disease prevention, and treatment. Studies continue to report that people who have healthy eating habits, exercise more frequently, and do not smoke have better health outcomes.3, 4
Source of funding: National Cancer Institute.
For correspondence: Dr R M Bostick, Division of Population Sciences, South Carolina Cancer Center, University of South Carolina, 15 Richland Medical Park, Suite 301, Columbia, SC 29203, USA. Fax +1 803 434 7529.