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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