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NOTE: Bulletin Board logo is in Liebart art and can be pulled in 12/17/96
INTESTINAL ABSORPTION OF VITAMINS AND OF FOOD-FORM ORGANIC SELENIUM
(SELENOMETHIONINE)
|
Vitamin |
Site of absorption |
Mechanism of absorption |
|
A |
Proximal and distal small intestine |
Facilitated (carrier-dependent)
diffusion at low concentrations, simple diffusion at higher concentrations |
|
D |
Duodenum, jejunum |
Absorption increases with fats;
salts of bile acids increase resorption |
|
D, E, and carotenes |
Duodenum, jejunum |
Simple diffusion; fats and emulsification facilitate
absorption |
|
K1 |
Proximal small intestine |
Active transport; unsaturated fatty acids, vitamin
E, inhibit absorption |
|
K2 |
Terminal ileum |
Simple diffusion |
|
Thiamin |
Jejunum |
Synergistic absorption with sodium; conversion to
absorbable metabolites |
|
|
Duodenum |
Active transport at physiological concentrations,
at higher levels by simple diffusion; thiamine at 10 times daily
doseproduces only minor increase of serum level because of rapid
excretion
|
|
Riboflavin |
Proximal jejunum |
Absorption is probably regulated hormonally, with
sodiumacting synergistically (?) after conversion into coenzyme
form (flavine mononucleotide) in intestinal mucosa |
|
Nicotinic acid Nicotinamide |
Proximal jejunum |
Synergistic absorption with sodium (?), and simple
diffusion
Nicotinamide Rapid intestinal absorption at all concentrations |
|
B6 |
Not known |
Simple diffusion, facilitated by conversion into metabolites
(rat, hamster); pyridoxamine, pyridoxal and pyridoxine pass intestinal
mucosal cells by passive diffusion; at low levels, a partial phosphorylation
and dephosphorylation, transport by passive diffusion |
|
Folic acid |
Jejunum |
Free folic acid is absorbed by simple diffusion at
high concentrations; tenfold daily dose is rapidly absorbed |
|
Biotin |
Proximal small intestine |
Synergistic absorption with sodium; active absorption
or by diffusion (hamster) |
|
Pantothenic acid |
Proximal small intestine |
Probably primarily by simple diffusion B12 (cobalamin) |
|
B12 (cobalamin) |
Ileum |
Cobalamin complex with intrinsi factor (IF) is bound
to specific receptors in ileum; the IF cobalamin complex or free
cobalamin is then transported into the mucosal cell |
|
C (ascorbic acid) |
Ileum |
At physiological levels, active, carrier, and sodium-dependent
absorption that is saturable; in some animals, absorption by diffusion |
|
Se (selenomethionine) |
Small intestine |
Active transport, similar to methionine |
INTESTINAL ABSORPTION OF ESSENTIAL MINERALS
|
Mineral |
Site of absorption |
Mechanism of absorption |
|
Na, K |
Small intestine |
Passive and associated with water absorption; regulation
of sodium/potassium balance in higher animals and humans occurs
in the kidney |
|
Ca |
Small intestine |
Uptake is regulated by parathyroid hormone, vitamin
D, calcitonin, phosphate; citrate, orotate, ascorbate increase Ca
bioavailability; phytic and oxalic acid decrease bioavailability. |
|
Mg |
Small intestine |
Mechanism of absorption in animals and humans is probably
related to that of Ca, but in general less Mg is absorbed |
|
Fe |
Duodenum and small intestine
|
Passive absorption facilitated by stomach hydrochloric
acid, citric acid, ascorbic acid, lactic acid, succinic acid, etc.,
inhibited by polyphenols, phytic acid, coffee, tea, dietary calcium |
|
Cu |
Small intestine, duodenum
|
Active transport with a diffusion component, facilitated
under acidic conditions, by citric and ascorbic acid; diminished
by carbohydrates, notably fructose, excess ascorbic acid, also by
zinc |
|
Zn |
Jejunum, duodenum, colon
|
Active and carrier-mediated, inducible in rodents
and humans (metallothionein); absorption is increased by glucose,
low-molecular-weight compounds, inhibited by phytic acid, calcium,
vitamin D |
|
Mn |
Duodenum |
Absorption increased by low intestinal pH, but not
specifically by ascorbic acid; absorption inhibited by calcium |
|
Cr |
Jejunum, ileum duodenum
|
Facilitated diffusion, absorption very low; absorption
is increased by oxalate, amino acids, transferrin, albumin, starch,
ascorbic acid; absorption decreased by antacids, zinc, iron, vanadium |
|