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Calcium has so many structural and metabolic roles in the body that we each harbour around 1.2 kg of the mineral – more than any other – of which most is stored in our skeleton as hydroxyapatite.
While 99% of the dietary calcium you absorb goes straight into your bones and teeth, the other 1% is vital for blood clotting, muscle contraction, nerve conduction and the production of energy.
The best known sources of calcium are cows' milk and dairy products, but it is also found in eggs and tinned salmon (if the tinned salmon includes the soft bones).
For those who follow a plant-based diet, or who have lactose intolerance, calcium can also be obtained from nuts and seeds, pulses, bread made from fortified flour and green leafy vegetables, especially broccoli (but not spinach, whose oxalate content reduces its uptake).
Some types of dietary fibre (phytates from wheat in unleavened breads such as chapati) also bind to calcium in the gut to reduce its absorption.
Did you know? A pint of skimmed or semi-skimmed milk contains around 720mg calcium – almost your daily requirement.
Calcium is only absorbed from your small intestine when sufficient vitamin D is present. Good intakes of both calcium and vitamin D are therefore vital throughout life to build strong bones and to help prevent bone thinning in later life.
Vitamin D regulates calcium metabolism and normal blood levels by:
You need sufficient calcium to build strong bones and to help prevent bone loss in later life.
A large analysis of clinical trials, carried out by the National Osteoporosis Foundation and which involved almost 31,000 adults, suggests that calcium supplements (taken together with vitamin D) can reduce the rate of bone loss and reduce the risk of all types of fracture by 15% and of hip fracture by 30%.1 Calcium supplements are therefore a good idea for middle-aged and older adults, including postmenopausal women.
Calcium also has a key role in ensuring healthy blood clotting, cell division and specialisation, muscle contraction, nerve transmission, the production of energy and for digestive enzymes to work properly.
Did you know? Vitamin K2 helps to ensure calcium is moved away from the arteries and laid down in the bones.
The nutrient reference value (NRV) for calcium is 800mg per day. Some studies have suggested that the most effective doses for preventing bone loss in later life is 1200mg calcium plus 800IU (20mcg) vitamin D.2
The total calcium safe upper limit from both food and supplements (the level that has not been associated with any harm) is 2,500mg per day. The upper safe level for long-term use from supplements alone is suggested as 1,500mg calcium per day.
If you avoid dairy foods due to intolerances or are cutting back to lose weight, you may not get your full calcium requirement.
If you are lacking in vitamin D, you will not absorb calcium well, either. A recent study assessed vitamin D status among 440,581 people and found that deficiency is highest among people of Asian ancestry (of whom 57.2% are affected), followed by those of Black African ancestry (38.5%), mixed (36.5%), Chinese (33.1%) and White European ancestry (17.5%). The researchers concluded that severe vitamin D deficiency remains an issue throughout the UK. It's therefore important to follow government advice to take a vitamin D supplement.3
When intakes of calcium (or vitamin D) are low, the body attempts to replenish blood levels by leaching calcium from your bones, leading to thinning (osteoporosis). Those at greatest risk of calcium deficiency include women (especially those who are pregnant, postmenopausal or who have an eating disorder), individuals with milk allergy or lactose intolerance, adolescents and the elderly.4
Low intakes of calcium are also linked with muscle aches and pains, twitching, spasm and cramps, heart palpitations, high blood pressure, osteoporosis, gum disease and loose teeth.
Supplements that contain calcium carbonate provide the most elemental calcium at 40mg per gram, and this is generally well tolerated and absorbed, especially when taken with food.5 Calcium Citrate provides 21mg elemental calcium per gram, although this lower calcium content is partly compensated by greater absorption. Calcium lactate contains 13mg per gram, and calcium gluconate 9mg per gram.6
Calcium tablets usually include other bone-friendly nutrients such as vitamin D3, vitamin K2, magnesium, boron, copper and zinc.
Select a supplement made to pharmaceutical standards (GMP) to ensure it provides a consistent dose that meets label claims.
Calcium tablets are best taken with meals. Some evidence suggests that they are better taken with an evening meal rather than breakfast, as the movement of calcium in and out of cells is greatest at night, when growth hormone is secreted. If taking a high dose, however, it is usually best to divide it into two or three smaller doses spread throughout the day to maximise the amount you absorb.
Those taking certain medications, such as tetracycline antibiotics, need to ensure they do not eat or drink calcium-containing foods for at least an hour either side of taking their medication. This is because calcium binds with some tetracyclines to reduce their absorption.
Some studies have found a slightly increased risk of heart disease in people taking calcium supplements, but others have not. Those finding a link have tended to take place in countries where vitamin D deficiency is common, such as Sweden, where up to one in two people are lacking in vitamin D during the winter months.7
According to the National Osteoporosis Foundation and the American Society for Preventive Cardiology, calcium supplements (with or without vitamin D) have no harmful effects on the risk for cardiovascular and cerebrovascular disease in generally healthy adults if calcium intake (from food plus supplements) does not exceed 2,000mg per day, and should be considered safe from a cardiovascular point of view.8, 9
People with a known tendency to form kidney stones should seek advice before taking calcium supplements (and drink sufficient fluids).10
Dr Sarah Brewer is Healthspan's Medical Director and holds degrees in Natural Sciences, Surgery and Medicine from the University of Cambridge. Having worked as a GP and hospital doctor, Dr Sarah now holds an MSc in Nutritional Medicine from the University of Surrey and specialises in nutrition. She is also an award-winning writer and author.
Nothing beats a healthy, balanced diet to provide all the nutrients we need. But when this isn't possible, supplements can help. This article isn't intended to replace medical advice. Please consult your healthcare professional before trying supplements or herbal medicines.
1Weaver CM et al (2016). Calcium plus vitamin D supplementation and risk of fractures: an updated meta-analysis from the National Osteoporosis Foundation, Osteoporos Int 27(1):367-76
2Tang BM et al (2007). Use of calcium or calcium in combination with vitamin D supplementation to prevent fractures and bone loss in people aged 50 years and older: a meta-analysis, Lancet 370(9588):657-66
3Sutherland JP et al (2020). Differences and determinants of vitamin D deficiency among UK biobank participants: A cross-ethnic and socioeconomic study, Clin Nutr S0261-5614(20)30639-7
4Beto JA (2015). The Role of Calcium in Human Aging, Clin Nutr Res 4(1): 1–8
5Trailokya A et al (2017). Calcium and Calcium Salts, J Assoc Physicians India 65(2):100-103
6Apgar B (2000). Comparison of Common Calcium Supplements, Am Fam Physician 15;62(8):1895-1896
7Wandell P et al (2018). Vitamin D deficiency was common in all patients at a Swedish primary care centre, but more so in patients born outside of Europe, Z Gesundh Wiss 26(6): 649–652
8Kopeckky SL et al (2016). Lack of Evidence Linking Calcium With or Without Vitamin D Supplementation to Cardiovascular Disease in Generally Healthy Adults: A Clinical Guideline From the National Osteoporosis Foundation and the American Society for Preventive Cardiology, Ann Intern Med 165(12):867-868
9Chung M et al (2016). Calcium Intake and Cardiovascular Disease Risk: An Updated Systematic Review and Meta-analysis, Ann Intern Med 165(12):856-866
10Harris SS, Dawson-Hughes B (2015). Effects of Hydration and Calcium Supplementation on Urine Calcium Concentration in Healthy Postmenopausal Women, J Am Coll Nutr 34(4):340-6