The Romans used lime (calcium oxide), clacked lime (calcium hydroxide), and hydraulic cement in construction works. Calcium (Latin calx, meaning "lime") was first isolated in its metallic form by Sir Humphrey Davy in 1808 through the electrolysis of a mixture of calcium oxide and mercury oxide.
Chelated calcium refers to the way in which calcium is chemically combined with another substance. Calcium citrate is an example of such a chelated preparation. Calcium may also be combined with other substances to form preparations such as calcium lactate or calcium gluconate. Calcium carbonate can be refined from limestone, natural elements of the earth, or from shell sources, such as oyster. Shell sources are often described on the label as a "natural" source. Calcium carbonate from oyster shells is not "refined" and can contain variable amounts of lead.
Calcium is the most abundant mineral in the human body and has several important functions. More than 99% of total body calcium is stored in the bones and teeth where it supports the structure. The remaining 1% is found throughout the body in blood, muscle, and the intracellular fluid. Calcium is needed for muscle contraction, blood vessel constriction and relaxation, the secretion of hormones and enzymes, and nervous system signaling. A constant level of calcium is maintained in body fluid and tissues so that these vital body processes function efficiently.
The body gets the calcium it needs in two ways. One method is dietary intake of calcium-rich foods including dairy products, which have the highest concentration per serving of highly absorbable calcium, and dark, leafy greens or dried beans, which have varying amounts of absorbable calcium. Calcium is an essential nutrient required in substantial amounts, but many diets are deficient in calcium.
The other way the body obtains calcium is by extracting it from bones. This happens when blood levels of calcium drop too low and dietary calcium is not sufficient. Ideally, the calcium that is taken from the bones will be replaced when calcium levels are replenished. However, simply eating more calcium-rich foods does not necessarily replace lost bone calcium, which leads to weakened bone structure.
Hypocalcaemia is defined as a low level of calcium in the blood. Symptoms of this condition include sensations of tingling, numbness, and muscle twitches. In severe cases, tetany (muscle spasms) may occur. Hypocalcaemia is more likely to be due to a hormonal imbalance, which regulates calcium levels, rather than a dietary deficiency. Excess calcium in the blood can cause nausea, vomiting, and calcium deposition in the heart and kidneys. This usually results from excessive doses of vitamin D and can be fatal in infants.
The Surgeon General's 2004 report "Bone Health and Osteoporosis" stated that calcium has been singled out as a major public health concern today because it is critically important to bone health and the average American consumes levels of calcium that are far below the amount recommended. Vitamin D is important for good bone health because it aids in the absorption and utilization of calcium. There is a high prevalence of vitamin D insufficiency in nursing home residents, hospitalized patients, and adults with hip fractures.
Calcium supplements are widely used to reduce bone resorption in osteoporosis, and many studies support this use. Calcium supplementation is also used for colorectal neoplasia and in pregnancy.