A number of minerals are essential for body functions. They are classified as macrominerals or microminerals (trace elements) depending on their dietary requirement. The macrominerals include calcium, phosphorus, potassium, sodium, chloride, sulfur and magnesium. The trace elements include copper, cobalt, chromium, fluorine, iodine, iron, manganese, molybdenum, nickel, selenium, silicon, tin, vanadium and zinc. Other minerals such as lead, arsenic and cadmium are toxic. The essentiality of several other elements in human nutrition is still under investigation.

Minerals may function as cofactors of enzymes, as components of organic compounds and as structural components of bones and teeth and are catalysts for many biological processes. Minerals may also participate in contraction and conduction of nerve impulses.

Calcium is the most abundant cation in the human body. The body of an adult usually contains about 1200 g of calcium. More than ninety-nine percent of the calcium is present in bones and teeth as a component of hydroxyapatite crystals-Ca10(PO4)6(OH)2 and amorphous calcium phosphate. The remaining 1% of total body calcium is necessary for a variety of metabolic processes including enzyme activation, hormone function, nerve transmission, muscle contraction, blood clotting and membrane transport.

Sources of Calcium

Milk and milk products are the richest dietary sources of calcium. Lesser amounts of calcium are found in shellfish, egg yolk, canned sardines and salmon (with bones), soybeans and certain green leafy vegetables such as turnip and mustard greens, broccoli and kale. Calcium is also found in such foods as spinach and rhubarb; however, the presence of oxalic acid in these foods may bind with calcium rendering it unavailable for absorption.

Table: Calcium Content of Some Common Foods

Dairy Products: (Calcium /100g)

  • cheese, parmesan, grated : 1376 mg
  • cheese, mozzarella, partly skimmed : 645 mg
  • cheese, cheddar : 721 mg
  • cheese, swiss, process : 961 mg
  • cheese, blue : 528 mg
  • cheese, ricotta, partly skimmed : 272 mg
  • cheese, cottage, creamed : 60 mg
  • ice cream, 50% fat : 157 mg
  • milk, skim : 124 mg
  • milk, lowfat, 2% fat : 122 mg
  • milk, whole, 3.3% fat : 119 mg
  • yogurt, plain, 1.5% fat : 182 mg
  • buttermilk : 116 mg

Fruits: ( Calcium /100g)

  • rhubarb, frozen, cooked, added sugar : 145* mg

Vegetables: (Calcium /100g)

  • beet greens, boiled : 114 mg
  • spinach, boiled : 136* mg
  • broccoli, boiled : 46 mg

Meat/Fish/Poultry and Alternates: (Calcium /100g)

  • sardines, canned in oil : 382 mg
  • salmon, canned, solids and liquid : 213 mg
  • nuts, Brazil : 176 mg
  • seeds, sunflower, dry : 116 mg
  • almonds, whole : 266
  • beans, red, kidney, cooked : 28 mg

* Ca bound by oxalic acid greatly reduces the amount absorbed
Source: U.S. Department of Agriculture, USDA Nutrient Database for Standard References, Release 13,

As a result of fortification, infant cereals are excellent sources of calcium. More recently, the fortification of foods such as orange juice and flour, have become additional sources of calcium in the diet. In hard water areas, drinking water may also be a significant source.

Absorption and Metabolism

Absorption of calcium from the gut is influenced by a number of factors. One of the most important factors is the individual’s need for calcium. The body can adapt to a wide range of intakes by altering the efficiency of absorption and excretion. The efficiency of calcium absorption in the gut decreases with age.

Calcium absorption is enhanced by an acid pH which keeps calcium in solution. Vitamin D, lactose and certain amino acids such as lysine and arginine, also enhance calcium absorption.

Calcium absorption is decreased by foods containing large amounts of phytates or oxalates both of which will form an insoluble calcium salt which is unavailable for absorption. However, this reaction is considered to be of little biological significance if calcium intake is liberal. Excess fat in the diet or impaired digestion of fat may reduce calcium absorption through the formation of insoluble calcium soaps.

Animal studies have linked both a low calcium-to-phosphorus ratio and a high phosphorus intake to bone demineralization and soft tissue calcification. However, results from studies on humans regarding this relationship are not conclusive. A dietary calcium to – phosphorus ratio of 1:1 is generally recommended.

There is also an interrelationship among calcium, phosphorus and protein. Increasing dietary protein will increase urinary calcium excretion if phosphorus intake is held constant. Serum calcium levels are maintained between 9-11 mg/dl.

Control of serum calcium is dependent upon the hormones, 1,25-dihydroxy D3 calcitonin and parathyroid hormone (PTH). PTH, in conjunction with 1,25 dihydroxy D3, raises serum calcium levels while calcitonin lowers them to keep them within the desirable physiologically range.

Calcium is present in the blood in several forms. Ionized calcium accounts for 50% of the total plasma calcium and is the physio logically active form of calcium. The remaining calcium is bound to serum proteins (40-45 percent of total plasma levels) or complexed with citrate, bicarbonate or phosphate.

Deficiency of Calcium

Low dietary calcium intake may produce rickets in children. Severe calcium deficiency in children may also result in stunted growth, muscle weakness, parathyroid hyperplasia, hyperirritability, tetany and death. In adults a calcium deficiency will exacerbate osteomalacia caused by vitamin D deficiency.

Insufficient dietary calcium is thought to be a factor contributing to osteoporosis. Osteoporosis is characterized by a significant reduction in bone density causing bone fragility and susceptibility to fracture. Osteoporosis is a prevalent disease among older women and, to a lesser extent, older men. To reduce the risk of osteoporosis individuals are advised to ascertain the recommended calcium intake from adolescence and to increase intake after menopause. As well, physical activity tends to reduce the risk of osteoporosis.

Skeletal demineralization occurs with prolonged immobilization and bed rest, due to large losses of calcium and phosphorus in the urine.

Among various macrominerals, calcium has attracted significant interest as a potential chemopreventive agent. It has been postulated that low intake of calcium and fiber along with high fat intake predisposes humans to a heightened state of colon cell proliferation leading to the formation of adenomatous polyps with subsequent degeneration to malignant transformation and development of colon cancer. Nutritional data also indicate that increased consumption of calcium-containing foods is associated with a reduced incidence of colon cancer.

It appears that there may be a subpopulation of hypertensives who are sensitive to calcium deficiency. Increasing the dietary calcium intake of these individuals to between 1200 and 1500 mg per day tends to lower their blood pressure.

Toxicity of Calcium

Hypercalcemia is usually due to hyperparathyroidism or excess vitamin D. It is also associated with patients having neoplasms with metastases leading to deposition of calcium in the kidneys.

Hypercalcemia may result in muscle rigidity, dehydration, lethargy, nausea, vomiting, anorexia and ultimately coma and death.