Ninety-nine percent of the human body is made up of only six elements. Calcium comes in fifth, at 1.5 percent of the total by mass. The vast majority of calcium in the human body is bound into a calcium phosphate salt (hydroxylapatite) in teeth and bones, though it also plays a role in many bodily functions. By far the most common isotope of calcium, both in the human body and in the earth generally, is calcium-40, making up 97% of the total.
Despite its major skeletal role, calcium is not a permanent bodily deposit. Rather, there is a constant exchange of calcium between bone and extracellular fluid, regulated by the amount of human serum albumin, which serves as a carrier. Calcitonin reduces blood calcium, while parathyroid hormone releases calcium from the bones back into the blood. These hormones are produced by the thyroid and parathyroid glands respectively.
Along with sodium and potassium, calcium also serves as a cellular ionic messenger across cell membranes and nerve dendrite-axon connections, evoking specific cell actions. These messages control everything from heart rhythm to muscle contraction.
We take in the calcium we need to survive and remain healthy entirely through our diet. Exact daily intake recommendations vary, but human adults generally need between 1000 and 1500 mg of calcium each day, rising with age to preserve bone density. Lactating mothers especially need higher intakes of calcium. Shortages in daily intake of calcium will be drawn from the bones and teeth. Calcium-rich foods include milk and dairy products, and to a lesser extent broccoli, okra, oranges, and dandelion greens. As well, many foods such as soy milk and orange juice are calcium-fortified. Calcium supplements are generally much more poorly absorbed than dietary calcium. If calcium supplements are taken, no more than 500 mg of calcium should be taken at a time, as higher doses won't be absorbed as well and thus most of the supplement calcium will simply be egested as feces. Constipation, a common side-effect of calcium supplements, can be avoided by taking magnesium at the same time. Vitamin D is also essential to successful absorption of calcium.
Besides loss of bone density leading to osteoporosis, lack of calcium can also influence brain function and cause teeth to fall out. However, too much calcium in the diet can bring about decreased kidney function and kidney stones; and can also block other essential minerals from being absorbed into the body, which could cause other deficiency diseases. To date, study results of the effects of calcium on colorectal cancer have had conflicting results.