Researchers in New Zealand have discovered that calcium supplements may increase the risk of heart attack in healthy postmenopausal women.

The study is published online today, Wednesday 16th January, in the British Medical Journal (BMJ) and is the work of researchers at the University of Auckland.

Calcium supplements are often prescribed to women past their menopause as a way to keep bones healthy, and some doctors believe it can also prevent arteries clogging up by lowering circulating levels of bad cholesterol.

However, the evidence is scanty, so the researchers at the University of Auckland’s Faculty of Medical and Health Sciences decided to investigate the effect of calcium supplementation on myocardial infarction, stroke, and sudden death in healthy postmenopausal women.

They did this by carrying out a secondary analysis of an existing randomized placebo controlled trial in an urban-situated academic medical centre in New Zealand, involving 1,471 postmenopausal women over 55 and of average age 74 who were randomly assigned to receive a calcium supplement or a placebo.

The original study was looking at the effect of calcium on bone fracture. The women were also asked questions about calcium in their diet, and they were assessed every six months for five years.

The researchers worked out that including the daily supplement, women in the calcium group were getting on average about 1,861 milligrams (mg) of calcium a day, while those in the placebo group, who got their daily calcium only from their diet, averaged about 853 mg a day.

The main outcome measures noted in the study were the following cardiovascular-related events: death, sudden death, angina, myocardial infarction (heart attack), chest pains, stroke, transient ischaemic attack (mini stroke).

The results showed that:

  • Reports of myocardial infarction (heart attack) were significantly higher in the calcium group than in the placebo group (45 events in 31 women versus 19 events in 14 women).
  • The occurrence of any three vascular events, myocardial infarction, stroke, or sudden death was also significantly more common in the calcium group (101 events in 69 women versus 54 events in 42 women).
  • Because the results were so important, the researchers went back and checked hospital records and death certificates to look for any unreported events.
  • After adjusting the figures for the newly found and previously unreported events, they discovered that myocardial infarction (heart attack) was still more common in the calcium group (36 events in 31 women versus 22 events in 21 women on placebo).
  • The figures for heart attack, stroke or sudden death also went up in the calcium group (76 events in 60 women versus 54 events in 50 women on placebo) but were shown to have borderline significance.

The researchers concluded that:

“Calcium supplementation in healthy postmenopausal women is associated with upward trends in cardiovascular event rates.”

However they cautioned against rash treatment decisions by saying that:

“This potentially detrimental effect should be balanced against the likely benefits of calcium on bone.” This was particularly important in the case of elderly women they said.

According to an article in WebMD today, a US expert said the findings could be a “fluke” and were not weighty enough to warrant changing any current treatments involving calcium supplements.

“Vascular events in healthy older women receiving calcium supplementation: randomised controlled trial.”
Mark J Bolland, P Alan Barber, Robert N Doughty, Barbara Mason, Anne Horne, Ruth Ames, Gregory D Gamble, Andrew Grey, Ian R Reid.
BMJ 2008:394405257
published online, 15 January 2008.

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Sources: BMJ article, Newswise press release, WebMD.

Written by: Catharine Paddock