How the absorption of vitamin C takes place
Since the human body cannot synthesize any vitamin C at all, we must include it in the daily diet. Dietary vitamin C in the reduced form of ascorbic acid is absorbed across the intestinal cell border throughout the small intestine by active transport and passive diffusion, with the assistance of the carriers SVCT 1 and 2. While SVCT 1 permits intestinal absorption of vitamin C in excess of cellular needs, the vitamin C itself limits the amount that can be absorbed.
The oxidized form of vitamin C, dehydroascorbic acid is also found in some foods. Or, alternatively, some oxidation of ascorbic acid can take place in the intestinal tract. Oxidized vitamin C is readily absorbed by glucose transporters (GLUT), primarily GLUT 1 and 3. After absorption has been completed, with the help of the dehydroascorbic enzyme reductase, dehydroascorbic acid is quickly reduced back to the ascorbic acid form of vitamin C for recycling purposes. Glutathione (GSH) is involved in this process.
Vitamin C does not need to be digested prior to absorption. Ideally about 80-90% of consumed vitamin C is absorbed from the intestines. However, vitamin C absorption capacity is inversely related to intake; it tends to reach 80-90% efficiency with fairly low intake of the vitamin, but these percentages are markedly reduced with daily intakes exceeding 1 gram. Given the usual dietary intake of 30-180 mg/day of vitamin C, absorption is typically in the 70-90% range, but increases to as much as 98% with a very low intake (less than 20 mg). Conversely, at intakes of above 1 g, absorption tends to be less than 50%. Little purpose is served by taking mega doses (i.e., 12 g) since only about 16% of all that vitamin C would be absorbed. In other words, the body won’t keep what it cannot use or store. The whole process is very fast; the body takes what it needs in about two hours, and within three to four hours, the unused portion is out of the bloodstream.
Vitamin C can be utilized even more rapidly by heavy drinkers and smokers and other conditions of stress. Vitamin C blood levels of smokers are much lower than those of non-smokers taking in the same amount of the vitamin. Many other substances and conditions can also increase the body’s need for vitamin C, either by interfering with absorption or increasing the utilization of the vitamin C on hand, leading to rapid depletion of any stored vitamin C. The following are some examples.
Antibiotics (especially sulfa drugs)
Aspirin and other pain medications
Environmental toxins (e.g., DDT, petroleum products, carbon monoxide)
Exposure to heavy metals (e.g., cadmium, lead, mercury)
Vitamin C leaves the intestinal cells through ion channels and enters the plasma via the capillaries. Normal plasma concentrations of vitamin C (primarily in the form of ascorbic acid) are between 0.4 and 1.7 mg/dL. Uptake of ascorbic acid into the cells takes place via SVCT1 or SVCT transporters, while cellular uptake of dehydroascorbic acid takes place via GLUT transporters.
Ordinarily, only small amounts of dehydroascorbic acid are found in the plasma due to the rapid uptake into the cells via GLUT transporters. However, the GLUT system is stimulated by insulin and inhibited by glucose, so it tends to work less efficiently for diabetics with poor glucose control. Diabetics with high glucose levels have high plasma levels and low cellular levels of dehydroascorbic acid.
Otherwise, vitamin C is usually found in higher concentrations in the tissues than in the plasma. In fact, the vitamin C concentration of white blood cells can be 80x the vitamin C plasma concentration. Even so, the body has a limited capacity to store vitamin C. The most common storage sites are the adrenal glands (about 30 mg), pituitary gland, brain, eyes, ovaries, and testes. Vitamin C is also found, though in lesser amounts, in the liver, spleen, heart, kidneys, lungs, pancreas, and muscles. All together, the body can store about 5 grams, or about 30 mg per pound of body weight. This means that the average individual probably needs at least 200 mg/day in order to maintain maximum body stores. However, the figure is probably much more for those who smoke, drink alcohol, or are under extreme stress, as well as the elderly, and those with certain medical conditions such as severe allergies or diabetes.
Metabolism and Excretion
Plasma concentrations of vitamin C increase with increased intake, but only to a point. Any intake of 500 mg or greater is usually excreted.
The unused vitamin C can be excreted intact, or it can be converted first to dehydroascorbic acid. This oxidation occurs mainly in the liver, but also in the kidneys. Unused vitamin C is excreted in the urine.
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