Calcium carbonate is a naturally occurring inorganic salt found in geological sources like chalk and limestone, and biological sources such as marine shells. This compound is widely utilized in human applications due to its high calcium concentration and basic, acid-neutralizing properties. It serves multiple roles, including acting as an active ingredient in supplements and medicinal products, and as a food additive. Its primary function is to provide calcium, a mineral fundamental for maintaining bone density, supporting muscle contraction, and ensuring proper nerve function.
Over-the-Counter Supplement Forms
Calcium carbonate is a highly concentrated source of calcium selected for over-the-counter dietary supplements aimed at bone health maintenance. It contains approximately 40% elemental calcium by weight, making it one of the most compact calcium salts available. For example, a 1,250 mg tablet supplies 500 mg of elemental calcium, the amount the body actually absorbs.
The raw material for these supplements is frequently purified limestone or biogenic sources like ground oyster shells. For human consumption, the material must meet strict purity guidelines, such as those set by the United States Pharmacopeia (USP), which ensures low levels of heavy metals.
Supplements are best absorbed when taken with food because the compound requires stomach acid to dissolve into absorbable calcium ions. Food stimulates the production of hydrochloric acid, which converts the insoluble calcium carbonate into a soluble form the body can use. Taking the supplement without a meal can significantly reduce its absorption, especially in older adults. To maximize absorption, it is advised to consume calcium in smaller doses, typically 500 mg of elemental calcium or less, spread throughout the day.
Medicinal Uses and Antacid Products
Calcium carbonate is a common active pharmaceutical ingredient (API) used for acute medicinal purposes, primarily as an antacid. This use is based on its ability to quickly neutralize excess hydrochloric acid in the stomach. The reaction results in the formation of calcium chloride, water, and carbon dioxide, providing rapid relief from symptoms of heartburn, acid indigestion, and sour stomach.
Chewable antacid tablets offer immediate relief because the calcium carbonate is delivered directly to the stomach in a form that reacts quickly with the acid. The medicinal application focuses on the acute treatment of gastrointestinal ailments, distinct from the chronic management of calcium levels.
Calcium carbonate also has a specialized therapeutic application as a phosphate binder in patients with chronic kidney disease (CKD). When the kidneys fail to properly excrete phosphate, the compound is taken with meals to bind to dietary phosphate in the gastrointestinal tract. This action helps to manage hyperphosphatemia, a condition that can lead to bone disorders in CKD patients.
Food Sources and Fortified Items
Calcium carbonate enters the diet through naturally occurring sources and as a widely used food additive, often designated as E170 in Europe. Natural sources include certain shellfish and marine corals, whose shells are primarily composed of this compound and are sometimes processed for use in health foods. While certain dark leafy greens also contain calcium, the form found there is generally not pure calcium carbonate.
Calcium carbonate is intentionally added to processed foods for fortification and as a processing aid. It is a preferred fortifying agent in products like plant-based milk alternatives, breakfast cereals, and orange juice. This practice helps individuals who may not consume traditional dairy products meet their daily calcium requirements.
Other Additive Functions
As a food additive, calcium carbonate serves several functions beyond fortification, including acting as an anti-caking agent to prevent clumping in powdered foods. It is also used as an acidity regulator and a colorant, contributing a white color to items like confectionery and chewing gum. The purity standards for food-grade calcium carbonate are regulated to ensure safety for consumption.