Friday, January 22, 2021

Calcium absorption

A constant supply of calcium is necessary throughout our lifetime, but is especially important during phases of growth, pregnancy, and lactation (breast feeding). Ninety-nine percent of body calcium is stored in bone in the form of hydroxyapatite crystal [Ca10(PO4)6(OH)2], while the remaining 1% is distributed in the plasma, interstitium, intracellular fluid, and within the cells in mitochondria and endoplasmic reticulum.

The intestinal calcium absorption is a crucial physiological process to maintain bone mineralization and calcium homeostasis. It occurs through transcellular and paracellular pathways: a transcellular active transport process, located largely in the duodenum and upper jejunum; and a paracellular, passive process that functions throughout the length of the intestine. Both pathways are regulated by hormones, nutrients and other factors.

The small intestine represents the major site of active calcium absorption in most species; it is responsible for approximately 90% of the total calcium absorption, whereas the rate of absorption in the colon appears to be less than 10%.

Dietary calcium is absorbed in the small intestine with the help of vitamin D. The level of calcium absorption from dietary sources drops to 7 in post-menopausal women. The body will absorb more calcium if there is a deficiency.

The largest quantity, 65% of the absorbed calcium occurs primarily by passive transport in the ileum. In the jejunum the intestinal calcium absorption is about 17%, and in duodenum 8%. In the duodenum and jejunum there is active calcium transport in addition to passive transport.

Factors that improve calcium absorption include adequate amounts of protein, magnesium, phosphorous, and vitamin D.

Conditions that reduce calcium absorption include high or excessive intakes of oxalates and phytates, found in foods such as spinach and unleavened whole wheat products.

Calcium deficiency can increase risk of bone disorders such as osteoporosis. It is well known that calcium is involved in the nerve impulse transmission, muscle contraction, blood coagulation, secretory activity, cell death, cell differentiation, immune response and enzyme activation. The dysregulation of calcium homeostasis is associated with bone disorders, metabolic diseases, and increment in the risk of epithelial cancers.
Calcium absorption

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