This from the Linus Pauling institute:
Vitamin C (L-ascorbic acid) is available in many forms, but there is little scientific evidence that any one form is better absorbed or more effective than another. Most experimental and clinical research uses ascorbic acid or its sodium salt, called sodium ascorbate. Natural and synthetic L-ascorbic acid are chemically identical and there are no known differences in their biological activities or bioavailabilities “.
Note, however, they refer always to L-ascorbic acid. This being one of the two stereo-isomeric forms, the other being D-ascorbic acid, which is of less use. Wikisay:
“Chemically, there exists a D-ascorbic acid which does not occur in nature. It may be synthesized artificially. It has identical antioxidant properties to L-ascorbic acid, yet has far less vitamin C activity (although not quite zero).
“This fact is taken as evidence that the antioxidant properties of ascorbic acid are only a small part of its effective vitamin activity. Specifically, L-ascorbate is known to participate in many specific enzyme reactions which require the correct epimer (L-ascorbate and not D-ascorbate).”
which seems pretty much on the nail.
Commercial synthesis of ascorbic acid seems to be always as Vitamin C, using a mix of biological and chemical processes which seem to ensure production of only the L form:
Of pure fact rather than his more famous fiction, Isaac Asimov wrote:
“As it happened, Vitamin C was finally isolated by someone who was not particularly looking for it. In 1928, the Hungarian-born biochemist Albert Szent-Gyorgi, then working in London in Hopkins’ laboratory and interested mainly in finding out how tissues made use of oxygen, isolated from cabbages a substance which helped transfer hydrogen atoms from one compound to another. Shortly afterward Charles Glen King and his co-workers at the University of Pittsburgh, who were looking for vitamin C, prepared some of the substance from cabbages and found that it was strongly protective against scurvy. Furthermore, they found it identical with crystals they had obtained from lemon juice. King determined its structure in 1933, and it turned out to be a sugar molecule of six carbons, belonging to the L-series instead of the D-series. It was named “ascorbic acid” (from Greek words meaning “no scurvy”).” (Asimov 1972 p. 690-700)
Asimov I. 1972. Asimov’s Guide to Science. New York. Basic Books Inc. 945 p.
And Wikisay, re body concentrations and the physiology:
With regular intake the absorption rate varies between 70 to 95%. However, the degree of absorption decreases as intake increases. At high intake (1.25 g), fractional human absorption of ascorbic acid may be as low as 33%; at low intake (<200 mg) the absorption rate can reach up to 98%.
Ascorbate concentrations over renal re-absorption threshold pass freely into the urine and are excreted. At high dietary doses (corresponding to several hundred mg/day in humans) ascorbate is accumulated in the body until the plasma levels reach the renal resorption threshold, which is about 1.5 mg/dL in men and 1.3 mg/dL in women. Concentrations in the plasma larger than this value (thought to represent body saturation) are rapidly excreted in the urine with a half-life of about 30 minutes. Concentrations less than this threshold amount are actively retained by the kidneys, and the excretion half-life for the remainder of the vitamin C store in the body thus increases greatly, with the half-life lengthening as the body stores are depleted. This half-life rises until it is as long as 83 days by the onset of the first symptoms of scurvy.
Although the body’s maximal store of vitamin C is largely determined by the renal threshold for blood, there are many tissues that maintain vitamin C concentrations far higher than in blood. Biological tissues that accumulate over 100 times the level in blood plasma of vitamin C are the adrenal glands, pituitary, thymus, corpus luteum, and retina. Those with 10 to 50 times the concentration present in blood plasma include the brain, spleen, lung, testicle, lymph nodes, liver, thyroid, small intestinal mucosa, leukocytes, pancreas, kidney, and salivary glands.
Ascorbic acid can be oxidized (broken down) in the human body by the enzyme L-ascorbate oxidase. Ascorbate that is not directly excreted in the urine as a result of body saturation or destroyed in other body metabolism is oxidized by this enzyme and removed.
And I’ve not said a word about therapeutic use, for anything from scurvy prevention up to the orthomolecular megadose applications. These are there, of course, this was the background and answer to those who use Vitamin C “wholly as biology makes it” extractions. For such a simple chemical I feel this has no validity, particularly as the Dextro form is not co-synthetised with the Laevo rotatory isomer and so the latter as sold is pure.