Calcium: Friend or Foe?

What a hit calcium has taken in the news lately. Another recent study suggests that women with calcium intakes of greater than 1,400 mg per day were more than twice as likely to die compared with women taking between 600 and 999 mg (1). The study was published in the British Medical Journal and looks at women in the Swedish mammography cohort. Drawbacks of the study are that it is a population study, looking at what the women were doing rather than carefully controlling the calcium levels to determine a cause and effect. Additionally, the study did not control for hormone replacement therapy use.

That being said, the research against calcium seems to be mounting. In an analysis of a double-blind trial intended to assess the effect of calcium on fracture incidence, the incidence of cardiovascular events (including myocardial infarction, stroke or sudden death) was 47% higher in women who received 1,000 mg per day of calcium for 5 years.(2) It is important to keep in mind that the study was not designed to study cardiovascular disease, and confounding factors were not equally distributed across both groups. For example, the supplemented group had a greater number of individuals with high cholesterol and smokers. Both of these factors increase the risk of a cardiovascular event. Another analysis of the National Institutes of Health (NIH)-AARP (formerly known as the American Association of Retired Persons) Diet and Health Study found that supplemental calcium was associated with heart disease death in men.(3) However, this study was again not designed to look at cardiovascular disease and suffers from the same confounding concerns of the first study. Another study, the EPIC-Heidelberg study, suggests that dietary calcium may be more better than supplemental calcium.(4) In this study, participants taking 820 mg of calcium per day from diet and supplements had a significantly reduced risk of myocardial infarction compared to individuals in the lowest level of intake (less than 513 mg per day), however users of calcium supplements had an increased risk compared to non-supplement users. Clinicians have theorized that the risk may be due to large boluses of calcium, unlike the smaller intakes that occur with dietary calcium, or that starting large supplemental doses of calcium may cause abrupt changes in serum calcium concentration leading to adverse effects.(5) The increased risk could also be due to calcium induced magnesium or other trace element depletions. Other analyses, using data from the Women’s Health Initiative and the Framingham Offspring Study do not show increased risk of heart disease or coronary calcification from calcium supplementation. (6,7)

Weighing both the importance of calcium for bone and the potential concerns, it seems wise to ensure that intake of calcium from the diet and supplements should be between 750 and 1200 mg per day, and limited to less than 1,500 mg per day. Limiting calcium to 1,000 mg per day may be desired, as well as choosing highly absorbable forms of calcium, such as calcium citrate or calcium citrate/malate, and ideally supplementing with 300 mg of calcium at a time. It seems wise to increase the amount of calcium in the diet, limiting supplemental calcium correspondingly. Additionally serum vitamin D levels should be checked and maintained in the optimal range.

Sources of calcium include dairy products like milk, yogurt and cheese (which contain approximately 300 mg per serving), sardines or salmon with bones (which contain approximately 250-300 mg calcium per 3 oz. serving), nuts or seeds (which contain approximately 40-70 mg calcium per 1 oz serving), dark green leafy vegetables (which contain approximately 50-110 mg per 1/2 cup cooked serving), and calcium supplements.

References

1. Michaelsson K, Melhus H, Warensjo E, et al. Long term calcium intake and rates of all cause and cardiovascular mortality: community based prospective longitudinal cohort study.BMJ. 2013; 346: 228.
2. Bolland MJ, Barber PA, Doughty RN, et al. Vascular events in healthy older women receiving calcium supplementation: randomised controlled trial. BMJ. 2008 Feb 2;336(7638):262-6.
3. Xiao Q, Murphy RA, Houston DK, et al. Dietary and Supplemental Calcium Intake and Cardiovascular Disease Mortality: The National Institutes of Health-AARP Diet and Health Study.JAMA Intern Med. 2013 Feb 4:1-8.
4. Kuanrong L, Kaaks R, Linseisen J, Rohrmann S. Associations of dietary calcium intake and calcium supplementation with myocardial infarction and stroke risk and overall cardiovascular mortality in the Heidelberg cohort of the European Prospective Investigation into Cancer and Nutrition study (EPIC-Heidelberg). Heart 2012;98:920-925.
5. Bolland MJ, Grey A, Avenell A, Gamble GD, Reid IR. Calcium supplements with or without vitamin D and risk of cardiovascular events: reanalysis of the Women's Health Initiative limited access dataset and meta-analysis. BMJ. 2011 Apr 19;342:d2040. 
6. Samelson EJ, Booth SL, Fox CS, et al. Calcium intake is not associated with increased coronary artery calcification: the Framingham Study. Am J Clin Nutr. 2012 Dec;96(6):1274-80.
7. Prentice RL, Pettinger MB, Jackson RD, et al. Health risks and benefits from calcium and vitamin D supplementation: Women's Health Initiative clinical trial and cohort study. Osteoporos Int. 2013 Feb;24(2):567-80.