Calcium is the most abundant mineral in the body. 99% of the body's calcium is stored in bones and teeth where it performs its most obvious role of supporting their structure. The remaining 1% is required for a variety of important processes. Blood levels of calcium are very tightly regulated and do not tend to fluctuate with changes in dietary intakes; instead the body uses bone as a reservoir for, and source of calcium.
The Recommended Dietary Allowance (RDA) for calcium is 1,000mg/day from the age of 4 for most people, and is higher at 1,300mg/day for adolescents, women who are pregnant or lactating, and people over the age of 50.
Dairy products are the main sources of dietary calcium and it is also found in a number of non-dairy sources including soybeans, some vegetables (e.g. okra, broccoli) and foods that are fortified with calcium e.g. some breakfast cereals. Ideally a dose of 1,000mg/day of calcium should be taken as 500mg at two separate times during the day.
Bone undergoes continuous remodeling and the balance between the breakdown and formation changes with age. Bone formation exceeds breakdown when children are growing, but in older adults (particularly postmenopausal women), breakdown can exceed formation, causing bone loss and an increased risk of osteoporosis.
Many people do not have enough calcium in their diet and over the long-term inadequate calcium intake causes osteopenia (where bone mineral density is lower than normal), which can lead to osteoporosis (weak and brittle bones) and an increased risk of bone fractures, especially in older people.
As well as low long-term calcium intake, several other factors increase the risk of developing osteoporosis, including being female, thin, inactive, older, smoking and a family history of osteoporosis. As well as taking calcium supplements, people at risk should be encouraged to undertake regular exercise, particularly weight-bearing exercises such as walking or running.
Other reasons that people take calcium supplements include:
Antacids - calcium carbonate can help to neutralise stomach acid and is therefore found in some over-the-counter antacid products, such as Tums. Products typically contain 200-400mg of elemental calcium.
Cancer prevention - studies have suggested that calcium may play a role in preventing certain cancers, in particular, colorectal and prostate cancer.
Cardiovascular disease - this is a controversial area. Some studies have found that calcium may help reduce cardiovascular disease risk in a number of ways. However, more recent studies have reached the opposite conclusion and raised concerns about the safety of calcium. At the time of writing, it is not known with certainty whether calcium supplements reduce, increase or have no effect on the risk of cardiovascular disease.
Weight loss - several recent studies have linked higher calcium intakes to lower body weight or reduced weight gain over time, but the evidence is not conclusive and consists of a number of both positive and negative studies.
The most common side effects of calcium supplements are mild gastrointestinal issues including bloating and constipation; these may be reduced or eliminated by spreading the calcium dose throughout the day and/or taking the supplement with meals. A rarer side effect is an increased risk of kidney stones.