by Leah Frankel, MS, RD | firstname.lastname@example.org
Earlier this year, a study looking at the relationship between calcium supplements and cardiovascular events was published changing the way doctors and other healthcare professionals approach calcium supplementation. This study, which included analysis of 12,000 subjects, found that oral calcium supplements can increase the risk of cardiovascular events, including heart attacks (1). With many people taking calcium supplements to prevent osteoporosis, this study has made many health professional reassess the risks and benefits of taking those tasty calcium supplements for bone health. We’ll discuss the results of this study and what these results mean for you.
Designing a Study
Before we talk about the study results, we should probably go over the way the study was performed. Why, you ask? The way a study is performed can drastically affect the way the results are interpreted by the scientific community. As a member of that community (yes, reading this article gets you into the science club), we want you to be able to critically evaluate the results with us. Now, think back to your high school science classes. You may remember that there are a number of different study designs that researchers can choose from when creating a clinical study. Most research studies are randomized control studies where subjects are assigned to one of several treatment groups and followed up after a particular amount of time to determine if the assigned treatment had an effect on the overall outcome for that group. One example of a randomized control study would be splitting a group of 100 participants into two groups, providing them with different medications and seeing which group had a better outcome after a particular period of time. Another type of study is an observational study where groups of people are studied over time to determine if there are correlations between two factors. An example would be looking at the number of people who are smokers and then following up to find out how many of these people develop lung cancer.
The study examining the effect of calcium supplements on cardiovascular events was a meta-analysis. A meta-analysis combines the subjects and outcomes from a number of other studies in order to have a larger number of study participants from which they can draw a conclusion. The more subjects, the more powerful the study. A study with fewer participants may suggest a particular result (for instance people who were overweight may have an increased chance of developing diabetes), but these results may not be statistically significant since the sample of people they are drawing conclusions from is too small (a study with only two people in it wouldn’t be very meaningful). A meta-analysis allows for the combination of various populations of people and can strengthen a study by “increasing” the number of participants or weaken the results due to improper combination of studies. Also remember that a meta-analysis is not an actual clinic trial, only a statistical analysis of a group of studies. Still with us?
With the calcium study, the researchers looked for research articles that were published between 1990 and 2007. The included “randomized placebo-controlled trails of calcium supplements” looking at the effect of calcium supplementation on “bone density, fracture, colorectal neoplasia, and blood pressure.” In addition, they only included studies that had the following criteria:
- Double blind studies (this means the subjects and the researchers don’t know which subjects get the calcium and which get the placebo)
- Elemental calcium was given at a dose greater than or equal to 500 mg/day
- The average age of the subjects was more than 40 years at the beginning of the study
There were at least 100 participants and the trail lasted more than one year
However, the researchers did not include studies in which:
- Calcium and vitamin D were given together (unless vitamin D was given to the placebo group as well)
- Calcium was supplemented through the diet or multivitamins
Trials where the majority of the “participants had a major disease other than osteoporosis”
After taking into account all of the including and excluding factors, the researchers had 11 studies to use for their analysis.
What Did They Find?
After analyzing the data from these 11 studies, the researchers discovered some interesting outcomes. The subjects who received their calcium from their diet (greater than the average level of 805 mg/day) and who were taking calcium supplements had an increased risk of heart attack. However, there was no increased risk of heart attack in the subjects who had a dietary calcium intake below the average level or who consumed higher amounts of calcium from diet alone. This suggests that it may not be harmful to take calcium supplements if you are getting low levels of calcium through your diet or to consume all of your needed calcium through your diet (think dairy foods!). In addition, participants who took calcium supplements had a 30% higher incidence of heart attacks. While there was an increase in the risk of having a stroke from calcium supplements, the risk of having a heart attack was even more significant. Interestingly, none of the individual trials had significant associations between calcium supplements and risk for cardiovascular events, but once the data was pooled for the meta-analysis the results became significantly.
As we discussed earlier on, there are some disadvantages to meta-analyses; the researchers discussed some of the limitations of this study in their research paper. One of the limitations is that this investigation excluded studies where vitamin D was given in combination with the calcium supplement (they only included studies if vitamin D was given with calcium and given to the placebo group). This means that the results from this study may not apply to people taking calcium supplements with vitamin D. In addition, since the studies included in the meta-analysis were not initially looking at cardiovascular effects, the data on heart attacks, strokes, and mortality were not gathered in a typical manner. Additionally, seven of the smaller trials that were included in the meta-analysis either had incomplete or no data on cardiovascular events; this is equivalent to 15% of the total number of subjects in the analysis. The researchers do not think this was significant since these were all rather small trials and the larger trials, which had complete data sets, were consistent and therefore the additional data wouldn’t have changed the outcome of the study.
Comparing These Results to Other Studies
One study that agreed with these findings looked at Finnish postmenopausal women, and showed that there was a 24% increase in coronary heart disease when the women were taking calcium supplements, regardless of whether they were taking vitamin D (2). On the other hand, a US observational study found that women with the highest intake of calcium had the lowest cardiovascular mortality (3). These results suggest that the cardiovascular risks from high calcium intake might be restricted to use of oral calcium supplements (rather than dietary calcium).
The researchers also suggested some possible ways in which calcium supplements can increase the risk of heart attacks. Calcium supplements have been shown to increase the level of serum calcium (calcium in your bloodstream) rather quickly after taking the supplement (4). We also know that higher levels of serum calcium are associated with an increase in the frequency of heart attacks secondary to atherosclerotic plaques (see picture above) (5). However, eating calcium from dairy sources does not have as much of an effect on serum calcium levels (6). Taking into account the modest benefits of calcium supplements on preventing bone fractures and the risk of heart attacks, strokes and death, the researchers determined that “treatment of 1000 people with calcium supplements for five years would cause an additional 14 myocardial infarctions (heart attacks), 10 strokes, and 13 deaths, and prevent 26 fractures.” You can do the math.
What Does This Mean For You?
This study has shown that calcium supplementation increases the risk for developing a heart attack by 30%. Before we jump to conclusions too quickly, it is important that we remember the limitations of this study. As we mentioned earlier, this study did not include vitamin D supplementation. In addition, it specifically looked at oral calcium supplementation. In general, it is more beneficial to get nutrients, including calcium, from food sources as opposed to supplements. Many nutrients are better absorbed when consumed from foods; calcium is absorbed best when eating dairy foods. Rather than taking a calcium supplements, think about ways you can increase your calcium intake from foods!
Your Take Home Message: Calcium supplementation leads to a 30% increased risk of having a heart attack. However, this study did not look at the effect of vitamin D,and the results do not apply the calcium consumed in foods. To prevent fractures and maintain bone health, consume all of your required daily calcium from dietary sources and put the supplements away until we know more about their risks. Focus on getting enough calcium by consuming several low or nonfat dairy foods a day.
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1. Bolland MJ, Avenell A, and JA Baron, et al. Effect of calcium supplements on risk of myocardial infarction and cardiovascular events: meta-analysis. British Medical Journal 2010; 241:c3691.
2. Pentti K, Tuppurainen MT, and R Honkanen et al. Use of calcium supplements and the risk of coronary heart disease in 52-62 year old women: the Kuopio Osteoporosis Risk Factor and Prevention Study. Maturitas 2009; 63:73-78.
3. Bostick RM, Kushi LH, and Y Wu et al. Relation of calcium, vitamin D, and dairy food intake to ischemic heart disease mortality among postmenopausal women. American Journal of Epidemiology 1999; 149:151-161.
4. Reid IR, Schooler BA, and SF Hannan et al. The acute biochemical effects of four proprietary calcium preparations. Australia and New Zealand Journal of Medicine 1986; 16:193-197.
5. Lind L, Skarfors E, and L Berglund et al. Serum calcium: a new, independent, prospective risk factor for myocardial infarction in middle-aged men followed for 18 years. Journal of Clinical Epidemiology 1997; 50:967-973.
6. Green JH, Booth C, and R Bunning. Postprandial metabolic responses to milk enriched with milk calcium are different from responses to milk enriched with calcium carbonate. Asia Pacific Journal of Clinical Nutrition 2003; 12:109-119.