Calcium is a key structural component of bone and is built into bone as a mineral complex that includes calcium and phosphate. Our skeleton houses 99% of our body’s calcium stores. Calcium built into bone also serves as a calcium reservoir for maintaining calcium levels in the blood. 

Calcium is absorbed in the small intestine both by passive diffusion and by active absorption regulated by vitamin D. Individuals who have more vitamin D are able to absorb more calcium [2]Heaney, R.P., et al., Calcium absorption varies within the reference range for serum 25-hydroxyvitamin D. J Am Coll Nutr, 2003. 22(2): p. 142-6.

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In addition to mineralising the skeleton, calcium plays a critical role in nerve and muscle function.

An inadequate intake of calcium results in lower levels of calcium in the blood. When this occurs a highly effective compensation mechanism is initiated. Calcium sensing receptors (CaSR) located on the parathyroid glands act as a highly accurate sensor of blood calcium levels [3]Centeno, V., et al., Molecular mechanisms triggered by low-calcium diets. Nutr Res Rev, 2009. 22(2): p. 163-74.

. When blood calcium decreases, PTH is released, which leads to:

• Stimulating osteoclast cells to resorb bone and release calcium by acting on the osteoblast
• Increasing gastrointestinal calcium absorption by activating vitamin D
• Reabsorbing calcium by the kidneys

For healthy individuals, the recommended daily calcium intake can be achieved through a healthy diet which contains enough calcium-rich foods. 

Dairy is the main source of calcium in the diet, with calcium- and vitamin D-fortified dairy products (such as yogurt and milk) providing around 40% of the RNI of calcium (400 mg) and 200 IU of vitamin D per portion [4]Kanis, J.A., et al., European guidance for the diagnosis and management of osteoporosis in postmenopausal women. Osteoporos Int, 2019. 30(1): p. 3-44.

[5]Rizzoli, R., Dairy products, yogurts, and bone health. Am J Clin Nutr, 2014. 99(5 Suppl): p. 1256S-62S.

.  As a source of both protein and calcium, dairy products are associated with higher bone strength in bone men and women [6]Durosier-Izart, C., et al., Peripheral skeleton bone strength is positively correlated with total and dairy protein intakes in healthy postmenopausal women. Am J Clin Nutr, 2017. 105(2): p. 513-525.

[7]Langsetmo, L., et al., High dairy protein intake is associated with greater bone strength parameters at the distal radius and tibia in older men: a cross-sectional study. Osteoporos Int, 2018. 29(1): p. 69-77.

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Other food sources of calcium include certain green vegetables (e.g., broccoli, curly kale, bok choy); whole canned fish with soft, edible bones such as sardines or pilchards; nuts (almonds in particular); and tofu set with calcium. 

Some calcium-fortified breads, cereals, fruit juices, soy beverages and several brands of commercial mineral water also contain significant amounts of calcium. These foods provide a suitable alternative for people who are lactose-intolerant or vegan.

• List of calcium-content in foods 
• Resources for patients and public: IOF Calcium Calculator and Calcium fact sheet

Some leafy produce, like spinach and rhubarb, contains oxalates, a naturally occurring compound in certain vegetables. Oxalates reduce the absorption of calcium contained in these vegetables. Whether or not they also interfere with calcium absorption from other calcium-containing foods eaten at the same time seems to be dependent on additional factors. The same is true of 'phytates' in dried beans, cereal husks, and seeds, but to a lesser extent [8]Shkembi B, Huppertz T. Calcium Absorption from Food Products: Food Matrix Effects. Nutrients. 2022; 14: 180.

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For individuals who cannot get enough calcium through their diets, supplements may be beneficial. These should be limited to 500-600 mg per day and it is generally recommended that they be taken combined with vitamin D.

Supplementation is recommended for patients at high risk of calcium and vitamin D insufficiency, and in those who are receiving treatment for osteoporosis [9]Harvey, N.C., et al., The role of calcium supplementation in healthy musculoskeletal ageing : An expert consensus meeting of the European Society for Clinical and Economic Aspects of Osteoporosis, Osteoarthritis and Musculoskeletal Diseases (ESCEO) and the International Foundation for Osteoporosis (IOF). Osteoporos Int, 2017. 28(2): p. 447-462.

. Note that supplementation with calcium alone does not reduce fracture risk, however calcium together with vitamin D supplementation may lead to a modest reduction in fracture risk (although population-level intervention has not been shown to be an effective public health strategy). 

The side effects of calcium supplementation include renal stones and gastrointestinal symptoms, however increased cardiovascular risk consequent to calcium supplementation is not convincingly supported by current evidence [9]Harvey, N.C., et al., The role of calcium supplementation in healthy musculoskeletal ageing : An expert consensus meeting of the European Society for Clinical and Economic Aspects of Osteoporosis, Osteoarthritis and Musculoskeletal Diseases (ESCEO) and the International Foundation for Osteoporosis (IOF). Osteoporos Int, 2017. 28(2): p. 447-462.

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When pharmacological calcium supplements are needed, they should be taken with a meal to improve tolerance and increase calcium absorption.

The two main forms of calcium in supplements are carbonate and citrate. Calcium carbonate is more commonly available and is absorbed most efficiently when taken with food, whereas calcium citrate is absorbed equally well when taken with or without food.