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
