Note: We are so excited to have the following guest post from colleague and mentor Tanis Fenton, PhD, RD! Dr. Fenton is the Nutrition Research Lead for Alberta Health Services and an Adjunct Assistant Professor in the Department of Community Health Sciences, Faculty of Medicine at University of Calgary.
A recent CBC news article, about calcium supplements and cardiovascular disease (CVD), has had a lot of people talking, and several asked me for a comment. As I summarized my thoughts for them, I wondered if you might like to see my assessment of the topic.
The bottom line – this news article was based on a single study of low level evidence. The study was an observational study, which produces low level evidence that does not prove cause and effect, but produces hypotheses that require testing in randomized trials to be confirmed. (See our previous post on correlation vs. causation)
The article (1) noted an association between calcium supplements and a higher incidence of heart attacks. In contrast, total dietary and dairy calcium were associated with a reduced rates of heart attacks.
What type of study is this? The quoted study’s methods: “Data from 23 980 Heidelberg cohort participants of the European Prospective Investigation into Cancer and Nutrition study, aged 35 to 64 years and free of major CVD events at recruitment, were analysed.” (1).
Thus the Li study is a cohort observational study. As you know, observational studies are low level evidence, only report associations, and cannot prove a cause and effect relationships. The reason observational studies cannot prove cause and effect is because the subjects were not randomly allocated to the different calcium intakes or to a calcium supplement. Some other variable(s) may explain the apparent relationship. Observational studies cannot specifically identify what aspect of the calcium supplementation users lifestyles accounts for the increased risk. The risk may not be due to the supplements, but may be due to another related aspect in these subjects’ lifestyles. This lack of being able to identify the causes are the weakness of observational studies.
You can see from the news report from their web page that much of the CBC article, which was what was on the radio, is carefully written to talk about associations and risk factors (The risk of having a heart attack almost doubled among calcium supplement users compared with non-users). Where the CBC article falls into “cause and effect” statements were the ones made by:
- Dr. Reid about his previous work: “The conclusion we came to was that calcium supplements were probably not a good idea because the cardiovascular problems they cause were greater than the benefits that arose in terms of fracture prevention”
- Dr. Khan: she speculated on a mechanism
- The two members of the fitness class
But higher evidence is available! Last month a careful systematic review of both observational and randomized controlled studies was published (2). Their findings from meta-analyses on both the observational and randomized controlled studies were that there was no clear association between calcium supplements and CVD, from both types of studies. If you read one paper on this topic, I suggest you read this one by Wang et al. (2).
Observational studies do not lose their weakness in terms of evidence when they are combined in a systematic review or meta-analysis. However, it is interesting that the bulk of the previous observational studies do not support this purported association.
Unfortunately, the public may have heard a clear message from the news cast: calcium supplements may cause adverse CVD events, based on the concluding statements made by two people at a fitness class (talk about low level interpretation!!):
- “I’ve been taking it non-stop actually for at least the last 25 years,” on the advice of a doctor, said 67-year-old Ann Kohen. “Now I’m thinking maybe I shouldn’t be.”
- Shoshana Strigberger now sticks with diet and fitness instead of pills. “I used to [take pills] for a short time but it didn’t make me feel good,” Strigberger said. “So I stopped.”
Please do not consider these members of the public to be qualified judges of the evidence. Let’s rely on the strongest evidence available on this topic, which concluded that the evidence does not suggest that calcium supplements cause heart attacks. And none of the studies associated dietary or dairy calcium with cardiovascular disease.
- Li K, 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(12):920-5.
Wang L, Manson JE, Sesso HD. Calcium intake and risk of cardiovascular disease: a review of prospective studies and randomized clinical trials. Am J Cardiovasc Drugs 2012; 12(2):105-16.