Calcium is traditionally considered to play a pivotal role in bone physiology, but has otherwise been neglected by mainstream medicine. However, a growing body of evidence supports a role for calcium in a number of important physiological processes. The weight loss effects of calcium are well reported and may stem from the ability of calcium to improve insulin sensitivity and bind fatty acids in the gut. In addition, calcium has been shown to be protective of cardiovascular disease through multiple mechanisms that include its weight loss effects as well as its ability to lower plasma cholesterol, blood pressure, inflammation, insulin insensitivity and thrombosis. The data from observational studies on calcium’s effects on the cardiovascular system are not as strong as the clinical trials data. This in part relates to the fact that doses in observational studies are generally lower than those in clinical trials, largely due to the fact that supplements are often administered in clinical trials and this increases calcium intake above that which can be obtained from food.
Because of the sometimes small effects seen in observational studies it is common to perform a meta-analysis on such data to find statistically significant trends. Pooling the data in this way is useful as it can increase the subjects number and is therefore suited to nutritional studies where small effects over long periods might be seen with certain nutrients. In one such meta-analysis, researchers analysed prospective studies investigating the association between calcium and the risk of stroke1. Of the eleven studies identified as fulfilling their initial criteria, the researchers reported a non-linear association between calcium intake and the risk of stroke. Therefore higher intakes appear to be beneficial, but an upper limit of intake produces little further benefit when this ceiling is reached. In a stratified analysis, the researchers noted that a low to moderate intake of 700 mg per day was strongly associated with reductions in the the risk of stroke, but at high intakes above 700 mg per day the association was weaker.
These results are interesting in that they suggest that calcium is protective of stroke. In particular, the dose response information presented here suggests that those with low intakes of calcium below 700 mg per day are at increased risk of stroke. While this may sound pedantic as it essentially means the same thing, it is important to understand that in many cases nutrients are protective of disease in studies investigating Western populations because the subjects are often deficient in so many micronutrients. For example, 700 mg of calcium is a low intake below the recommended amount and as such it is no surprise that individuals with poor micronutrient status are affected by disease. In addition, low intakes of one micronutrient are often considered a marker of a low quality diet, and frequently those with insufficient intakes of one micronutrient have insufficient intakes of other nutrients. However, while this is true, there are multiple mechanisms known by which low calcium intakes could increase the risk of stroke, so the effect may be causal.
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