By Irwin Lim, Rheumatologist
This is worrying. This is controversial. It has caused a stir and led many osteoporosis academics to vigorous argument.
There is mounting evidence that calcium supplements increase the risk of cardiovascular events, particularly heart attacks, in older women.
New research published on bmj.com last week adds to these worrying findings. Calcium supplements are commonly given to older (postmenopausal) women to help maintain bone health and help avoid osteoporosis. Sometimes they are combined with vitamin D, sometimes not.
The Women's Health Initiative (WHI) study was a seven-year trial of over 36,000 women. Overall, this study found no cardiovascular effect of taking combined calcium and vitamin D supplements. However, the majority of participants were already taking personal calcium supplements (separate from the “prescribed” calcium supplements distributed as part of the trial). This added personal use may have obscured any adverse effects.
A team of researchers, led by Professor Ian Reid at the University of Auckland, re-analysed the WHI results to try and provide a better estimate of the effects of calcium supplements, with or without vitamin D, on the risk of cardiovascular events.
They analysed data from 16,718 women who were not taking personal calcium supplements at the start of the trial. Their study (pdf) clearly suggests that those allocated to combined calcium and vitamin D supplements were at an increased risk of cardiovascular events, especially heart attack.
This increase is modest, about 25%–30% for myocardial infarction and 15%–20% for stroke. Percentage increases are not sufficient for you to understand the ‘true’ risk, so the authors provided this:
“treating 1000 patients with calcium or calcium and vitamin D for five years would cause an additional six myocardial infarctions or strokes and prevent only three fractures”
As I said, worrying data. Especially given the very widespread use of calcium supplements, both prescribed by doctors and obtained over-the-counter.
However, the study is hard to interpret. This is because the technique of re-analysing previously attained data in different ways (and ways not previously defined when the study was originally designed) can have many potential hazards. In short, the validity of the data will have some question marks attached to it.
In the accompanying editorial, Professors Bo Abrahamsen and Opinder Sahota argue that there is insufficient evidence available to support or refute the association. They write:
"it is not possible to provide reassurance that calcium supplements given with vitamin D do not cause adverse cardiovascular events or to link them with certainty to increased cardiovascular risk. Clearly further studies are needed and the debate remains ongoing"
What do I tell patients? Well, it’s far better to get your calcium from your diet. If you can’t tolerate or don’t like dairy products, a dietitian will be able to help identify other food sources you should consume.
If you’re already on calcium supplements, it’s really hard to tell if you need to stop. The data just isn’t quite good enough for that yet.
Dr Irwin Lim is a rheumatologist and a director of BJC Health.
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