Calcium absorption and metabolism

On average, adults absorb around 25 to 50 per cent of dietary calcium. Some is
absorbed passively while some is transported via a vitamin D-mediated process.
Most absorption occurs in the small intestine. The calcium then passes into the
exchangeable calcium pool that is in the body fluids. This pool turns over 20 to
30 times a day whereas the calcium in bone turns over every five to six years.

Blood levels of calcium are tightly regulated by the hormones calcitonin,
parathyroid hormone and vitamin D. These hormones act together to regulate
calcium levels as dietary intake and requirements vary. They control absorption
from the gut, excretion in the kidney and the rate of bone formation and
breakdown. In the absence of vitamin D, less than 10 per cent of dietary calcium
may be absorbed. When intake is inadequate, calcium is removed from storage
sites in bone and used to keep blood levels constant. Other hormones which
affect calcium levels include estrogens, glucocorticoids, thyroid hormone, insulin
and growth hormones.

Calcium absorptionCalcium absorption is enhanced when calcium intake is low and also by moderate
exercise. Lactose, vitamin D and adequate (but not excessive) protein improve
calcium absorption. High levels of fat reduce absorption. Compounds known as
phytates, which are found in dietary fiber; and oxalates, which are found in
leafy greens, reduce absorption. The acid environment of the stomach makes
calcium salts more soluble, and therefore easier to absorb; and low stomach
acid reduces absorption.

Calcium absorption and retention become less efficient with age, partly
due to lower estrogen and testosterone levels; and a postmenopausal woman
may only absorb 7 per cent of her dietary intake. The ability to absorb and
retain calcium improves during pregnancy although it seems that some calcium
is drawn from bone stores later in pregnancy. A study done in 1996 in Cincinnati
showed that breastfeeding stimulates increases in calcium absorption and these
increases become apparent after weaning or after menstrual periods restart.

Smoking, high refined sugar intake, caffeine, alcohol and excess salt promote
calcium excretion, thereby increasing the risk of deficiency. High protein diets
also increase calcium excretion, particularly if the protein comes from meat.

 
 
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