Calcium and fracture risk
Multiples studies have addressed the effect of calcium supplements with or without Vitamin D on fracture risk. While several studies shown a benefit on bone mineral density, the results on fracture reduction are conflicting (1,2,3). A meta-analysis of 17 trials with fracture as the primary outcome showed a modest relative risk reduction of borderline statistical significance (4). In a systematic review of 26 trials (5), there were again small and inconsistent beneficial reductions in fracture risk. Because of the minimal, if any, reduction in fracture risk, it means large numbers of patients would need to be treated to prevent a single fracture.
The conflicting results have been attributed to differences in their definitions of fracture and study designs, study bias and the populations studied (6). The beneficial effects of calcium and Vitamin D appear to be greater in individuals who are hospitalized or institutionalized compared to those in the community (7).
Which supplements are available?
Calcium carbonate (Caltrate®) and calcium citrate (Citracal ®) are the most widely available supplements. Calcium carbonate is better absorbed when taken with meals. It is not well absorbed however in patients with achlorhydria (low acid environment); calcium citrate is therefore preferred as a first line option for patients taking proton pump inhibitors or H2 blockers.
High doses of calcium supplements may result in the formation of kidney stones. There is no evidence of an increased risk of nephrolithiasis from high dietary calcium intake. In the Women’s Health Initiative, women on calcium and Vitamin D supplements reported a higher rate of urinary tract stone formation compared to placebo (8). Gastrointestinal side effects such as nausea, vomiting and constipation have also been reported.
Calcium supplements and cardiovascular disease
In 2008, the Auckland calcium study reported an increased risk of myocardial infarction in post-menopausal women taking calcium supplements for 5 years compared to placebo (9). Of note, women were taking 1000mg of elemental calcium, 10% of the study population were > 80 years old at baseline and cardiovascular disease was not the primary endpoint. Vascular calcification is one postulated mechanism behind the increased cardiovascular risk.
The Women’s Health Initiative which randomized 36,282 women to calcium supplements (1000mg) and Vitamin D (400 IU) daily or placebo showed no increase in myocardial infarction in the calcium supplement group (8). However, 54% of the study population were already taking personal calcium supplements which were not part of the study protocol (10).
Subsequently, several meta-analyses including re-analysis of the WHI (including only women who were not taking calcium supplements at baseline) have been published with conflicting data (10,11,12). At present this topic remains a source of debate and there is insufficient evidence to declare one way or the other whether calcium supplements increase myocardial infarction or not. In light of the modest potential fracture benefit and possible cardiovascular risk, widespread use of supplements is not recommended.
What is clear is that calcium from dietary sources do not lead to an increased cardiovascular risk. Healthy Bones Australia recommends the optimal calcium intake to be 1000mg in adults > 19 years and 1300mg daily in post-menopausal women and men over 70. Most of this should be obtained through dietary sources. If dietary sources are inadequate, then supplements in the order of 500 – 600mg daily can be used. It is also recommended that calcium intake should not exceed more than 2000mg daily.