Calcium citrate is the calcium salt of citric acid. It is commonly used as a food additive (E333), usually as a preservative, but sometimes for flavor. In this sense, it is similar to sodium citrate. Calcium citrate is also found in some dietary calcium supplements (e.g. Citracal or Caltrate). Calcium makes up 24.1% of calcium citrate (anhydrous) and 21.1% of calcium citrate (tetrahydrate) by mass. The tetrahydrate occurs in nature as the mineral Earlandite.
Calcium citrate is sparingly soluble in water. Needle-shaped crystals of tricalcium dicitrate tetrahydrate [Ca3(C6H5O7)2(H2O)2]2H2O were obtained by hydrothermal synthesis. The crystal structure comprises a three-dimensional network in which eightfold coordinated Ca2+ cations are linked by citrate anions and hydrogen bonds between two non-coordinating crystal water molecules and two coordinating water molecules.
Calcium citrate is an intermediate in the isolation of citric acid from the fungal fermentation process by which citric acid is produced industrially. The citric acid in the broth solution is neutralized by limewater, precipitating insoluble calcium citrate. This is then filtered off from the rest of the broth and washed to give clean calcium citrate.
In many individuals, bioavailability of calcium citrate is found to be equal to that of the cheaper calcium carbonate. However, alterations to the digestive tract may change how calcium is digested and absorbed. Unlike calcium carbonate, which is basic and neutralizes stomach acid, calcium citrate has no effect on stomach acid. Calcium carbonate is harder to digest than calcium citrate, and calcium carbonate carries a risk of \"acid rebound\" (the stomach overcompensates by producing more acid), so individuals who are sensitive to antacids or who have difficulty producing adequate stomach acid may choose calcium citrate over calcium carbonate for supplementation. According to recent research into calcium absorption after gastric bypass surgery, calcium citrate may have improved bioavailability over calcium carbonate in Roux-en-Y gastric bypass patients who are taking calcium citrate as a dietary supplement after surgery. This is mainly due to the changes related to where calcium absorption occurs in the digestive tract of these individuals.
Calcium citrate increases plasma calcium levels. This reduces calcium flux from osteocyte activity by reducing the secretion of parathyroid hormone (PTH) 3. Calcium does this by stimulating a G-protein coupled calcium receptor on the surface of parathyroid cells. The reduction in calcium flux increases the amount of calcium deposited in bone resulting in an increase in bone mineral density. The reduction in PTH secretion also reduces the amount of vitamin D metabolized to its active form, calcidiol. Since calcidiol increases the expression of calcium dependent ATPases and transient receptor potential cation channel subfamily V member 6 (TRPV6) both of which are involved in calcium uptake from the gut, a reduction in calcidiol results in less calcium absorption. Additionally, TRPV5, the channel responsible for calcium reabsorption in the kidney, is downregulated when PTH secretion is reduced thus increasing calcium excretion via the kidneys. Another hormone, calitonin, is likely involved in the reduction of bone resorption during periods of high plasma calcium.
Patients taking more than 4g of calcium a day are at risk of hypercalcemia and metabolic alkalosis 4. Chronic intake of calcium supplements is associated with adverse gastrointestinal symptoms such as constipation and flatulence 4 1. 781b155fdc