Absorption and Transport of Thiamin

The bioavailability of thiamin occurring naturally in foods is believed to be high. Dietary sources of thiamine include: yeasts, pork, sunflower seeds, legumes, chicken, fish, beef, wheat germ, cereal products, lentils, potatoes, rice polishing and nuts.

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.

Dietary thiamine is absorbed readily from jejunum and proximal ileum and is transported to tissues where it is converted to the active form, thiamine pyrophosphate (TPP).

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.

Thiamine is carried by the portal blood to the liver. In normal adults, 20-30% of plasma thiamine is protein-bound all of which appears to be TPP. The transport of thiamine into erythrocytes seems to be facilitated diffusion process, whereas it enters other cells by an active process.

Thiamine uptake by active transport is highest in the jejunum and ileum, with both passive diffusion and active carrier-mediated transport.

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

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

The ascorbic acid contained in foods appears to be readily available and absorbed.

Absorption of ascorbic acid in the intestine occurs through a sodium-dependent active transport system. But simple diffusion may also contribute somewhat to uptake of the vitamin.

The reduced and oxidized forms of the vitamin are absorbed by different mechanism of active transport:
*Ascorbic acid uptake by the sodium-dependent vitamin C transporter (SVCT)
*Dehydroascorbic acid uptake by glucose transporters (GLUT)

The transport of ascorbic acid into the ileum is a carrier-mediated process at low mucosal concentrations of ascorbic acid.

Absorption rate can vary from 16% at very high intakes (approximately 12g) to 98% at low intakes.

The degree of absorption as suggested by the urinary excretion of the vitamin appears to be adversely affected by pectin, zinc, copper, and iron.

From the intestinal cells, ascorbic acid diffuses through anion channels into extracellular fluid and enters the plasma by way of the capillaries.

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.

The cellular accumulations of vitamin C in humans are mediated by a variety of specific transporters located at the cell membranes and regulated in a cell-specific manner.

Absorbed ascorbic acid is transported in the plasma as a free anion. Normal plasma ascorbic acid concentrations range from about 0.4 to 1.7 mg/dL 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 ascorbic acid are found in the adrenal and pituitary glands (with each possessing approximately 30-50 mg/100 g of wet tissue) as well as in the eyes, brain and white blood cells.

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.

In absolute terms based on total weight, the liver contains the most ascorbic acid. The maximum pool is estimated at about 2g.
Absorption and Transport of Vitamin C

What is a lipoprotein?

A lipoprotein is the particle transport cholesterol and triglycerides.

All plasma lipoproteins have the same general structure: a hydrophobic core of triglyceride and cholesteryl esters surrounded by a surface layer of apolipoproteins, phospholipids and unesterified cholesterol. 

Structurally, lipoproteins are microscopic spherical particles ranging from 7 to 100 nm in diameter.

Each lipoproteins particle consists of a monolayer of polar, amphipathic lipids that surrounds a hydrophobic core. 

One of the major functions of the liver in lipid metabolism is lipoprotein synthesis. The four major classes of circulating plasma lipoproteins are: chylomicrons, very low density lipoproteins (VLDL), low-density lipoproteins (LDL) and high density lipoproteins (HDL).

While chylomicrons made in the small intestine, the lipoproteins made by the liver. Chylomicrons in the blood deliver dietary fatty acids to cells via the enzyme lipoprotein lipase. The enzymatic reaction releases fatty acids from the chylomicrons’ triglycerides, allowing their uptake into surrounding cells.

LDL lipoproteins are the major transporters of cholesterol in human plasma.

The protein portion of any lipoprotein is called the apolipoprotein. Apolipoproteins play a very important role in the structural and functional relationship among the lipoprotein. Each of the lipoprotein particles contains one or more apolipoprotein.
What is a lipoprotein?