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