"These data support the continued use of calcium supplements by women who are able to remain compliant with their use… Calcium supplementation is safe and effective for reducing the risk of osteoporotic fracture," wrote lead author Richard Prince from the University of Western Australia.
Osteoporosis is characterized by low bone mass, which leads to an increase risk of fractures, especially the hips, spine and wrists, and is estimated to affect about 75m people in Europe, the USA and Japan. Women are four times more likely to develop osteoporosis than men.
The new randomised, double-blind, placebo-controlled study, published in the Archives of Internal Medicine (Vol. 166, pp. 869-875), reports the effects of a 600 milligram supplement of calcium in carbonate form twice a day with meals, or an identically looking placebo (Wyeth Consumer Healthcare, Baulkham Hills, Australia) on the occurrence of osteoporotic fractures for 1460 women with an average age of 75.
Participants had their bones measured using a combination of techniques, including quantitative ultrasonography (QUS) and dual x-ray absorptiometry (DXA). Measures of serum vitamin D levels were also taken, as vitamin D deficiency has been linked to increased risk of osteoporosis by reducing calcium absorption into the bone.
After five years of study, the researchers found that there was no significant difference between the placebo group and the calcium supplemented group.
"The lack of significance of calcium is likely due to a lack of compliance with the medication regime… [This] reflects the difficulties of implementation of preventive health practice in all such studies," wrote lead author The women (57 per cent) who took more than 80 per cent of the supplements did benefit, with an absolute risk of 10 per cent, compared to 15.4 per cent for the placebo group.
"Thus, the effect of 1.2 grams of calcium appears to be sufficiently large to be applicable to compliant individuals, irrespective of dietary calcium intake," said Prince.
The current recommended daily intake of calcium is 800 mg, with an upper safe limit of 2500 mg.
The recently reported Women's Health Initiative (WHI) trial also reported similar findings. The joint vitamin D3 (400 IU) - calcium carbonate (1000 mg) supplementation programme, over a period of about seven years, concluded that women who were compliant with the trial had a 29 per cent lower incidence of fractures.
The Western Australian researchers found that compliance could be linked to specific characteristics of the participants. Non-compliant women were found to be older, weaker, and slower.
Despite the positive results for the benefits of calcium supplementation, the implementation of supplements as a public health approach would not work, argue the researchers, due to this lack of long-term compliance.
In terms of adverse effects, the WHI study reported that the compliant subjects did suffer from an increased risk of kidney stones. In the new Australian study, which used a higher dose of calcium, no increase in such a side effect. Indeed, the most common adverse effect was reported to be a slightly increased risk of constipation.
This use of the supplements is widely accepted by the general public, with calcium supplements reported to be the biggest seller in the US supplements industry, with annual sales of about $993 (€836) million in 2004, according to the Nutrition Business Journal.
According to the International Osteoporosis Foundation, the total direct cost of osteoporotic fractures is €31.7 billion ($37.6) in Europe, and $17.5 (€14.7) billion in the US (2002 figure).