From: altar@beaufort.sfu.ca (Ted Wayn Altar)
Subject: calcium: Factors Affecting Absorption
Organization: Simon Fraser University, Burnaby, B.C., Canada
Date: Sat, 23 Jan 1993 21:11:43 GMT



I.  EXOGENOUS FACTORS AFFECTING ABSORPTION

According to a conventional text on nutrition (Hunt & Groff,
ADVANCED NUTRITION AND HUMAN METABOLISM, 1990):

   "a greater absorption does not necessarily mean greater
    retention.  Sometimes increased absorption of calcium is
    offset by its increased excretion in urine and/or digestive
    juices.  Calcium economy in the body is very complex being
    influenced not only by exogenous factors by also by
     endogenous physiologic conditions". p. 266

Of the exogenous factors, Hunt & Groff list the following as
factors making for a decreased or retention of calcium:

Purified PROTEINS (e.g., meat & dairy) increase urinary calcium.

caffeine increases urinary as well as a increased secretion of
    calcium into the gut (hence both urinary and endogenous
    fecal losses.

Unabsorbed dietary FAT interferes with absorption by producing
    "soaps" that simply cannot be absorbed and are excreted in
     feces.  Some "brilliant" nutritionists have therefore
     thought that increasing calcium intake could benefit the
     health of the colon by binding free fatty acids and bile
     acids thereby preventing irritation to the colon by these
     fat-related substances.  Odd that it never occurs to these
     folk that the simplest and best recommendation is simply
     not to eat such excess fat (i.e. meat) in the first place.

SODIUM increases urinary calcium loss.

MAGNESIUM and calcium, both divalent cations, appear to compete
     with each other for absorption whenever there is an excess
     of either present in the gut.  Magnesium is important
     for the secretion of PTH which in turn is important for the
     serum regulation of calcium.  Hence, by consuming too much
     calcium without a commensurate increase in magnesium, one
     paradoxically inhibits proper serum calcium regulation.

FIBER "(specifically cellulose and hemicellulose) decreases
     absorption not only by increasing the bulk of intestinal
     contents and decreasing transit time but also by simulating
     microbial  proliferation, a mineral-requiring process".
     Of course, such microbial proliferation is desirable and
     cereal fibers being high in hemicellulose (a slowly
     fermentable component desirable for microbial
     proliferation), therefore aid in the "detoxification"
     mechanism provided by microbial action scavenging
     degradable nitrogenous substances.  We should also note
     that fiber, oxylates and phytates don't decrease absorption
     of calcium that much (this has been exaggerated while
     the protein interference has been underplayed in the popular
     press) and they in turn help to offset the decrease by the
     fact that a diet high in cereals and vegetables will
     increase the intake of calcium and other minerals.  That
     is, eat your greens (kale, chard, beet tops, rhubarb, wild
     poke) along with a cereal like rice.  Lest one becomes
     overly concerned, studies show (e.g., Anderson,  "The iron
     and zinc status of long-term  vegetarian woman.  Am. J.
     CLIN. NUTR., 1981, 34:1042) no evidence that vegetarians
     suffer mineral deficiencies, even though we have here a
     population maintained on a high-fiber diet.


                                        ted
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