Calcium is the most abundant mineral in the human body, which contains approximately 1.2 Kg, 99% of which is stored in bones and teeth, with the remaining 1% present in the blood and other bodily tissues. The skeleton acts as a reservoir for calcium; if the intake of calcium is insufficient, bones become depleted in calcium. Dietary sources of calcium include dairy products, fish and some green vegetables. Oxalic acid, which is found in some green vegetables like spinach, is a potent inhibitor of calcium absorption. The body is best equipped to absorb calcium and build up bone tissues up to the age of 35, after which bone density is lost at a rate of approximately 0.3% per year (1% in post-menopausal women). Calcium cannot be absorbed from the digestive system without vitamin D, which is made in the skin in response to sunlight (vitamin D levels decline with age, and a daily intake of 200-400 IU/day is required). Vitamin K promotes the uptake of calcium into bone tissue. Many adults receive just half the amount of calcium they require from the diet each day. Calcium supplements are available in a variety of chemical forms, most commonly calcium carbonate, calcium citrate, calcium gluconate and calcium phosphate. The elemental calcium provided varies with chemical form, e.g. calcium carbonate provides 40% elemental calcium, whereas calcium gluconate provides only 9% elemental calcium. The lower the elemental calcium content, the more tablets that are required to meet recommended intakes.