Introduction

Calcium is the most common mineral in the human body. About 99% of the calcium in the body is found in bones and teeth while the other 1% is found in blood and soft tissue. Calcium levels in the blood and fluid surrounding the cells must be maintained within a very narrow concentration range for normal physiological functioning. Adequate intakes of calcium are important determinants of bone health and risk of fracture or osteoporosis. The physiological functions of calcium are so vital to survival that the body will demineralise bone to maintain normal blood calcium levels when calcium intake is inadequate. Thus, adequate dietary calcium is a critical factor in maintaining a healthy skeleton.

Food Sources

Minerals from plant sources may vary from place to place because soil mineral content varies geographically.

Dairy products represent rich and absorbable sources of calcium, but certain vegetables and grains also provide calcium. However, the bioavailability of that calcium must be taken into consideration. While the calcium-rich plants in the kale family (broccoli, cabbage, mustard, and turnip greens) contain calcium that is as bioavailable as that in milk, some food components have been found to inhibit the absorption of calcium.

Some important food sources of Calcium (mg/100 g)

Edam cheese (770)	Cheddar cheese (720)	Sesame seeds (670)	Sardines (550)	Tofu (510)
Dried figs (250)	Fruit yoghurt (150)	Whole milk (115)	Muesli (110)	Green Beans (33)

Recommended Dietary Allowance (RDA)

The European Union RDA for the general population is set at 800 mg/day.

In general, the intake of calcium depends on the age of the person. Children require a higher intake of calcium than adults to make their developing bone mass. Pregnant women also need a higher intake of calcium to stabilize their own level of calcium in bones and teeth and also for the developing baby.

The daily intake of calcium should not exceed 1500 mg/day, at 2500 mg/day toxicity symptoms may appear.

Inhibitors/stimulators:

The following food components have been found to stimulate the absorption of calcium.

Vitamin D – Vitamin D is required for optimal calcium absorption.

The following food components have been found to inhibit the absorption of calcium.

Oxalic acid – also known as oxalate, is the most potent inhibitor of calcium absorption, and is found in high concentrations in spinach and rhubarb and in somewhat lower concentrations in sweet potato and dried beans.

Phytic acid – a less potent inhibitor of calcium absorption than oxalate. Yeast possesses an enzyme (phytase), which breaks down phytic acid in grains during fermentation, lowering the phytic acid content of breads and other fermented foods. Only concentrated sources of phytate such as wheat bran or dried beans substantially reduce calcium absorption.

Sodium – Increased sodium intake results in increased loss of calcium in the urine, possibly due to competition between sodium and calcium for re-absorption in the kidney or by an effect of sodium on parathyroid hormone (PTH) secretion. Because urinary losses account for about half of the difference in calcium retention among individuals, dietary sodium has a large potential to influence bone loss.

Protein – As dietary protein intake increases, the urinary excretion of calcium also increases. However, inadequate protein intakes have been associated with poor recovery from osteoporotic fractures. Serum albumin values (an indicator of protein nutritional status) have been found to be inversely related to hip fracture risk.

Phosphorus – Phosphorus is typically found in protein-rich foods, and tends to decrease the excretion of calcium in the urine. However, phosphorus-rich foods also tend to increase the calcium content of digestive secretions, resulting in increased calcium loss in the faeces. Thus, phosphorus does not offset the net loss of calcium associated with increased protein intake.

Caffeine – Caffeine in large amounts increases urinary calcium content for a short time.

Functions in the Body

Bone Structure

Calcium is a major structural element in bones and teeth. The mineral component of bone consists mainly of hydroxyapatite [Ca₁₀(PO₄)₆(OH)₂] crystals, which contain large amounts of calcium and phosphate. Bone is a dynamic tissue that is remodelled throughout life. Bone cells called osteoclasts begin the process of remodelling by dissolving or resorbing bone. Bone-forming cells called osteoblasts then synthesize new bone to replace the bone that was resorbed. During normal growth, bone formation exceeds bone resorption. Osteoporosis may result when bone resorption exceeds formation.

Muscle Function

Calcium helps to mediate the constriction (vasoconstriction) and relaxation (vasodilatation) of blood vessels, nerve impulse transmission, muscle contractions, and the secretion of hormones such as insulin. Excitable cells, such as skeletal muscle and nerve cells, contain calcium channels in their cell membranes that allow for rapid changes in calcium concentrations. For example, when a muscle fibre receives a nerve impulse that stimulates it to contract, calcium channels in the cell membrane open to allow a few calcium ions into the muscle cell. These calcium ions bind to proteins within the cell that release a flood of calcium ions from inside the cell. The binding of calcium to a specific protein called troponin-c initiates a series of steps that lead to muscle contraction. The binding of calcium to a different protein provides energy for muscle contraction.

Clotting factor

Calcium has a very important role in the process to stop bleeding. It is one of the clotting factors that stop bleeding through clot formation. In this process, the calcium ions (Ca²⁺) act as cofactors. They bind to other clotting factors and activate them to form and build the clot and stop the bleeding.

Deficiency

A low blood calcium level usually implies abnormal parathyroid function, and is rarely due to low dietary calcium intake since the skeleton provides a large reserve of calcium for maintaining normal blood levels. Other causes of abnormally low blood calcium levels include chronic kidney failure, vitamin D deficiency, and low blood magnesium levels that occur mainly in cases of severe alcoholism. A chronically low calcium intake in growing individuals may prevent the attainment of optimal peak bone mass. Once peak bone mass is achieved, inadequate calcium intake may contribute to accelerated bone loss and ultimately the development of osteoporosis.

Toxicity

While low intakes of calcium can result in deficiency and undesirable health conditions, excessively high intakes of calcium can also have adverse effects. Adverse conditions associated with high calcium intakes are hypercalcemia (elevated levels of calcium in the blood), impaired kidney function, and decreased absorption of other minerals. Hypercalcemia can also result from excess intake of vitamin D, such as from supplement overuse at levels of 50,000 IU or higher. However, hypercalcemia from diet and supplements is very rare. Most cases of hypercalcemia occur as a result of cancer - especially in the advanced stages.

Another concern with high calcium intakes is the potential for calcium to interfere with the absorption of other minerals, iron, zinc, magnesium, and phosphorus.

Regulation

Calcium concentrations in the blood and fluid that surrounds the cells are tightly controlled in order to preserve normal physiological functioning. When blood calcium levels decrease, calcium-sensing proteins in the parathyroid glands send signals resulting in the secretion of parathyroid hormone (PTH), which, through a series of mechanisms, stimulates the release of calcium from bone by activating osteoclasts, and decreases the urinary excretion of calcium by increasing its reabsorption in the kidneys. When blood calcium rises to normal levels, the parathyroid glands stop secreting PTH, and the kidneys begin to excrete any excess calcium in the urine.