Calcium is the fifth most required element by the human body after oxygen, carbon, hydrogen, and nitrogen. It contributes 1.9% to the body weight. Ninety-nine percent of the calcium in the body is stored in the bones, remaining 1% is in teeth and other soft tissues and only 0.1% is in the extracellular fluid (ECF). (1) The ECF contains ionized calcium (Normal value - 4.8 mg/dl) and albumin bound calcium (Normal value - 3.2 mg/dl) with complexed calcium with phosphate and citrate at about 1.6mg/dl to maintain a normal serum calcium level between 8.8 to 10.4 mg/dL. (1,2) The ionized calcium is the physiologically active form of calcium.
Functions of Calcium (1-3)
- Calcium salts provide rigidity to the skeleton. Calcium phosphate is a major component of the mineral complex (called hydroxyapatite) that gives structure and strength to bones.
- Many neuromuscular and other cellular functions depend on the maintenance of the ionized calcium concentration in the ECF.
- Blood clotting
- Deficiency of calcium can lead to weaker bones, tetany and seizures. Deficiency can occur due to poor intake, malabsorption, vitamin D deficiency, hypoparathyroidism and renal disease. Long term deficiency can lead to osteomalacia and osteoporosis.
Phosphorus is a component of DNA, RNA, ATP, and also the phospholipids that form all cell membranes. Nearly every cellular process that uses energy obtains it in the form of ATP. Thus, it is an essential element for all living cells and important for energy utilization in the body. (4) It is present in the body in the form of phosphates (PO4). It forms calcium phosphate with calcium and forms the skeleton of the bones. Bone contains about 85% of the body's phosphate. The rest is located primarily inside cells and only 1% is present in the ECF (4,5).
Metabolism of Calcium and Phosphorus
The metabolism of calcium and phosphorus is closely related and influenced by parathyroid hormone (PTH), vitamin D, and, to a lesser extent, calcitonin.
Calcium is absorbed in the proximal small intestine by active (transcellular) transport when intake is low and by diffusion when intake is high. Many factors influence absorption of calcium such as substances, which form insoluble complexes with calcium like phosphate. Phytates present in the husks of many cereals as well as in nuts, seeds, and legumes, can form insoluble calcium phytate salts in the gastrointestinal tract. Excess oxalates can precipitate calcium in the bowel. Human milk has a high calcium-phosphate ratio (2.2) as compared to cow's milk (0.77). Thus higher absorption of calcium is seen with human milk. Intestinal absorption of calcium is influences by Vitamin D. (1)
Calcium enters the ECF from the gut by absorption and from bone by resorption. Calcium leaves the ECF via the gastrointestinal tract, kidneys, and skin and enters into bone via bone formation. (1) PTH increases the renal tubular reabsorption of calcium, promotes intestinal calcium absorption by stimulating the renal production of 1,25-dihyroxycholecalciferol [1,25(OH)2D], and, if necessary, resorbs bone. However, if there is a deficiency of vitamin D, serum calcium decreases and secondary hyperparathyroidism occurs leading to hypophosphataemia. (1)
The normal serum inorganic PO4 concentration in adults ranges from 2.5 to 4.5 mg/dL. It is 50% higher in infants and 30% higher in children, possibly because of the important roles these PO4-dependent processes play in growth. (6). Phosphate absorption from the intestines is enhanced by vitamin D. Low serum phosphorus can occur with hyperparathyroidism, burns and long term diuretic use. Patients present with seizures, muscle weakness and hemolytic anemia. Long standing hypophosphatemia can occur with malabsorption, poor intake, and use of aluminum containing antacids. Patients present with osteomalacia and muscle weakness.
The normal product of calcium and phosphorus (in mEq/L) is 60. When the product exceeds 70, precipitation of CaPO4 crystals in soft tissue is much more likely. (2)
Foods Rich in Calcium and Phosphorus (3)
Excellent sources of calcium include dairy products, spinach, turnip greens, mustard greens, and collard greens. Milk and juices fortified with calcium are also good sources of calcium. Foods rich in phosphates are all-bran cereal, almonds, cashew nuts, cheese, dried fruit, egg, garlic, legumes, lentils, meat, milk, peanuts, sesame seeds, sunflower seeds, whole wheat bread and yogurt . Patients with chronic renal failure should avoid foods rich in phosphates as serum phosphorus levels in these patients are high.
Recommended daily allowance (RDA) of calcium
9-18 years= 1300mg/d
RDA of phosphorus
Calcium and phosphorus are two of the most important macrominerals required for the body's growth, bones and function. Dietary requirements are greatest during periods of growth and pregnancy. Many people do not get sufficient amounts of calcium in their diet (7) and thus fortification of food with calcium may be required for healthy bones.
1. Calcium. Available at URL: http://www.fao.org/docrep/004/y2809e/y2809e0h.htm. Accessed on 15th November 2010
2. Disorders of calcium concentration: fluid and electrolyte metabolism: Merck Manual Professional. Available at URL: http://www.merckmanuals.com/professional/sec12/ch156/ch156g.html. Accessed on 15th November 2010
3. WHFoods. Calcium. Available at URL: http://www.whfoods.com/genpage.php?tname=nutrient&dbid=45. Accessed on 15th November 2010
4. Phosphorus - Wikipedia, the free encyclopedia. Available at URL: http://en.wikipedia.org/wiki/Phosphorus. Accessed on 15th November 2010
5. Phosphates: Minerals and Electrolytes: Merck Manual Home Edition. Available at URL: http://merckmanuals.com/home/sec12/ch155/ch155j.html. Accessed on 15th November 2010
6. Disorders of phosphate concentration: fluid and electrolyte metabolism: Merck Manual Professional. Available at URL: http://www.merckmanuals.com/professional/sec12/ch156/ch156h.html. Accessed on 15th November 2010
7. Calcium - effects, food, nutrition, deficiency, needs, body, diet, absorption, protein, fat, vitamin, weight, vitamins, Deficiency and Toxicity, Requirements and Supplementation. Available at URL: http://www.faqs.org/nutrition/Ca-De/Calcium.html#ixzz166Gu3Osp.