Calcium plays an important role in the cells as well; it is necessary in cell division. Calcium is needed to activate prothrombin, which helps convert fibrinogen to fibrin and is essential to blood coagulation.

Uses: Calcium is one of the minerals most commonly prescribed by medical doctors (potassium is the other main one) because calcium deficiency is common and causes bone weakness through loss of bone calcium in the disease called osteoporosis. Osteoporosis is more common in the elderly population and occurs four times as often in women as in men. It can also occur at younger ages with chronic dietary insufficiency of calcium or with early menopause. Good evidence shows that there is a relationship between decreased calcium intake and osteoporosis; lack of exercise also increases bone loss. Moderate daily exercise as well as supplementing calcium and vitamin D results in reduced to restore a positive calcium balance. The best way to combat osteoporosis is to prevent it with regular exercise, a calcium-rich diet not too high in phosphorus, calcium supplements, and for menopausal women considering estrogen therapy. Exercise can actually stimulate bone renewal by improving bone uptake of calcium and other minerals.

Osteoporosis (the term literally means "porous bones") is actually a loss of bone mass as the result of the loss of both minerals and protein; this differs slightly from osteomalacia, the bone problem seen in adults with vitamin D deficiency, which involves a softening of bones due to mineral loss alone. Rickets is the childhood equivalent of osteomalacia and is also caused by vitamin D deficiency. Extra calcium can help alleviate these problems somewhat, but the body needs supplemental vitamin D to get appreciable levels of calcium into the blood, tissues, and bones.

Calcium is the primary substance used in the prevention and treatment of osteoporosis, though estrogen used in menopausal hormone replacement therapy can reduce the likelihood of this disease in women. Since osteoporosis is found mainly in menopausal and postmenopausal woman, calcium is commonly seen as a treatment for problems of menopause. It does, in fact, reduce a number of the potential symptoms. Calcium not only helps the bones, especially when supplemented with magnesium and vitamin D, but may also reduce the headaches, irritability, insomnia, and depression sometimes associated with menopause. It is likely that a high percentage-as much as 70-90 percent-of bone fractures in people over 60 years of age are due to osteoporosis. These fractures are often more serious than an average fracture, because demineralized bones shatter when they break and take longer to heal. Actually, because osteoporotic fractures usually occur in the elderly and are so disabling, about one in six people dies within three months after sustaining them. Therefore, by helping to retard osteoporosis, calcium can prevent some fractures.

Osteoporosis is most common in elderly white women with a history of borderline calcium intake. Calcium is often drained from the bones during pregnancy and nursing and becomes hard to replace in later years, especially with reduced consumption of milk products and a lower calcium intake in general. Calcium supplementation can be helpful in reducing the leg cramps of pregnancy and the fatigue and depression after delivery. Children's leg cramps are usually reduced by giving them calcium and magnesium. Calcium supplements tend to stimulate retention of calcium and decrease urinary excretion.