Absorption and Transport of Thiamin

Absorption and Transport of Thiamin
The bioavailability of thiamin occurring naturally in foods is believed to be high. Foods containing the highest concentration of thiamin are listed below:
Yeasts
Pork
Sunflower seeds
Legumes

Occasionally, however, anti-thiamin factor may be present in the diet. For example, thiaminases present in the raw fish catalyze the cleavage of thiamin, thereby destroying its activity.

These thiaminases are thermolabile, however and cooking of fish rendered the enzymes inactive. Other anti-thiamin factors that are thermostable may be found in tea and certain fruits and vegetables such as blueberries, black currents, Brussels sprouts and red cabbage.

Absorption of thiamin can be both active and passive, depending upon the amount of the vitamin presented for absorption.

At low physiologic concentrations, thiamin absorption is an active process. This Na+ dependent, carrier mediated absorption occurs primarily in the jejunum but can occur in other portions of the small intestine as well.

When intakes of thiamin are high, the absorption route is predominantly passive. The rate of thiamin absorption is always quite high except in the case of ethanol ingestion and/or folate deficiency.

Ethanol ingestion interferes with active transport of thiamin, and folate deficiency prevents the normal duplication of enterocytes, thereby decreasing absorption, both active and passive.

Within the mucosal cells, thiamin is converted into a phosphate ester, in which form it move into the plasma. To be active as a coenzyme, thiamin must be converted to its pyrophosphate (diphosphate) form.

Conversion to the active coenzyme form requires adenosine triphosphate (ATP) and thiamin pyrophosphokinase, an enzyme found in the liver and brain (and perhaps in other tissue as well).

Another form of thiamin (thiamin triphosphate, or ATP) is synthesized in the brain by action of a thiamin diphosphate (ADP) – ATP phosphoryl-transferase.
Absorption and Transport of Thiamin

Absorption and Transport of Vitamin C in Human Body

Absorption and Transport of Vitamin C in Human Body
Humans are one of the few animals unable to synthesize ascorbic acid (vitamin C). Other primates, flying mammals, guinea pigs, and birds belonging to Order Passeriformes make up the additional animals to whom ascorbic acid is a vitamin.

Ascorbic acid (vitamin C) is synthesized from glucose. Animals that require an exogenous source of ascorbic acid are unable to synthesize gulonolactone oxidase, the last enzyme needed for the conversion of glucose to ascorbic acid.

The ascorbic acid contained in foods appears to be readily available and absorbed. Absorption of ascorbate occurs primarily via an active transport system, but simple diffusion may also contribute somewhat to uptake of the vitamin. Most of absorption occurs in the distal portion of the small intestine with degree of absorption decreasing with increased intake of the vitamin. Absorption rate can vary from 16% at very high intakes (approximately 12g) to 98% at low intakes (<20mg). But over a range of usual intakes from food (20 to 120mg/d), the average for overall absorption is around 90%.

The degree of absorption as suggested by the urinary excretion of the vitamin appears to be adversely affected by pectin, zinc, copper, and iron. At present it is unknown whether the decreased urinary ascorbic acid caused by the presence of the above three minerals reflects a less efficient absorption or an increased oxidation of the vitamin before it can be absorbed.

Absorbed ascorbic acid is transported in the plasma as a free anion, and it readily equilibrates with the body pool of the vitamin. The size of the pool therefore varies with the intake. Ascorbate moves freely into the cells, but the concentration is much greater in some tissues than in others. The highest concentration of the vitamin is found in the adrenal gland (30 to 40 mg/100g wet tissue), with the cortex having higher concentration than the medullar. Other tissues with particularly high concentration are the pituitary gland and the retina. An intermediate level of the vitamin is found in the liver, lungs, pancreas and leukocytes, while smaller amounts occur in the kidneys muscles and red blood cells. Tissue concentration of the vitamin usually exceeds the plasma level by three to ten times, the degree of concentration depending upon the specific tissue.
Absorption and Transport of Vitamin C in Human Body