Calcium is traditionally considered the bone-health nutrient. This is because 99% of calcium is contained within the bone, in which calcium creates a mineral complex with phosphate (hydroxyapatite), giving bone its characteristic strength and function. However, besides forming part of bone, calcium is also involved in other functions, such as muscle function, nerve transmission, intracellular signaling, and others.
Bone is a very dynamic organ in which the rate of formation and resorption (or destruction) is different throughout the lifespan. During childhood and adolescence, the rate of formation is higher than the rate of resorption, leading to maximum bone gain. In older adults these rates are switched, causing loss of bone. This bone loss may lead to osteopenia, osteoporosis, and an increased risk of bone fractures.
Is an increased intake of calcium associated with reduced risk of fractures?
As a way to prevent bone loss and fractures in middle-aged and older adults, there is usually a push from public health initiatives and healthcare professionals to increase the intake of calcium. However, an increased intake of calcium (dietary + supplements) may not necessarily lead to fewer fractures. Two of the most recent systematic reviews and meta-analyses showed that increasing dietary calcium and the supplementation of calcium (and vitamin D) did not reduce the risk of fractures. However, this remains controversial as a previous meta-analysis showed that the supplementation of calcium and vitamin D was associated with a 15% risk reduction in middle-aged and older adults (community-dwelling and institutionalized).
The use of calcium supplements may lead to a higher risk of cardiovascular disease
The hypothesis is that an increased intake of calcium may lead to a positive calcium balance (intake lower than output in urine + feces), in the absence of increased bone formation. This positive calcium balance may lead to the calcification of tissues other than the bone, such as the vasculature, increasing the risk of cardiovascular disease. However, this association remains controversial. A sub-analysis of the Multi‐Ethnic Study of Atherosclerosis (MESA), showed that those that had higher dietary calcium intake (without supplements) had a lower risk of coronary artery calcification, while those that were taking supplements had a higher risk. However, the National Osteoporosis Foundation and the American Society for Preventive Cardiology suggested that calcium intake (diet + supplements) that does not exceed the tolerable upper level of intake of 2000 to 2500mg/d is not associated with cardiovascular outcomes, and should be considered safe.
Special caution of high intake of calcium in subjects with reduced kidney function
A high intake of calcium, particularly those that use calcium supplements, may represent a problem for those with reduced kidney function, as they may not be able to excrete the extra calcium. In a study by Hill and collaborators, a calcium intake of 2500mg (1000 mg through diet + 1500mg from calcium carbonate), compared with a dietary calcium intake of 1000mg, led to 500mg of calcium retention in patients with reduced kidney function. Similarly, Spiegel and collaborators compared a diet with 800mg vs. 2000mg of calcium in subjects with normal and reduced kidney function. In those on the 2000mg calcium diet, there was a positive calcium balance, which was more marked in those with reduced kidney function. With these results, a calcium intake higher than 800-1000mg (diet + supplements) should be avoided in those patients with a reduced kidney function to prevent calcifications of tissues other than the bone.
Should health care professionals recommend a high intake of calcium?
The current evidence, in middle-aged and older adults, does not seem to support high calcium intakes (above the tolerable upper level) for the prevention of fractures. Additionally, high calcium intakes achieved with the use of supplements may be associated with increased risk of cardiovascular disease. However, the use of supplements may be useful for patients that have low calcium intake. Finally, special attention should be taken in patients with reduced kidney function, as a high intake (diet + supplements) may lead to an increased risk of cardiovascular disease.