Calcium
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
Functions
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
0-8 years=210-800mg/d
9-18 years= 1300mg/d
RDA of phosphorus
0-8 years=100-500mg/d
9-18 years=1,250mg/d
Conclusion
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.
References
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. |
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