The increased risk seen only for men in this study may have been because they began taking the supplements at older ages, and women who tend to use calcium for longer periods may "have achieved calcium balance and stable calcium levels long before the study, and the effect of calcium supplement became less profound," the researchers wrote.
A possible reason for why calcium might influence the cardiovascular system is through calcification of the coronary arteries.
"Vascular calcification is an actively regulated process that not only shares key proteins and pathways but is also intricately intertwined with bone mineralization," Xiao and colleagues explained.
However, because of the multiplicity of biological effects of calcium, further research into mechanisms of action are needed, they noted.
Limitations of the study included a lack of information on the duration of calcium use and family history of heart disease, as well as self-report of supplementation.
"Given the extensive use of calcium supplement in the population, it is of great importance to assess the effect of supplemental calcium use beyond bone health," the authors concluded.
In an invited commentary, Susanna C. Larsson of the Karolinska Institute in Stockholm also called for additional studies to more clearly establish the benefits or risks of calcium.
"Meanwhile, a safe alternative to calcium supplements is to consume calcium-rich foods, such as low-fat dairy foods, beans, and green leafy vegetables, which contain not only calcium but also a cocktail of essential minerals and vitamins," Larsson advised.