Vitamin C: Absorption Limits, Liposomal Forms & Cofactor Roles

Vitamin C (ascorbic acid) is probably the most well-known vitamin, but the nuances of its absorption, dosing, and cofactor roles are more complex than most people appreciate. It's not just about preventing scurvy or fighting colds — vitamin C is a critical cofactor for at least eight enzymes and a key player in collagen synthesis, antioxidant recycling, and mineral absorption.

Absorption: There's a Ceiling

Vitamin C is absorbed in the small intestine via sodium-dependent vitamin C transporters (SVCT1). At typical dietary doses (30-180 mg), absorption efficiency is 70-90%. But as the dose increases, absorption drops dramatically: at 1,000 mg, only about 50% is absorbed; at 3,000 mg or more, absorption may fall below 30%. Unabsorbed vitamin C is rapidly excreted by the kidneys and can cause osmotic diarrhea at very high doses — this is the "bowel tolerance" threshold sometimes referenced in orthomolecular medicine.

Liposomal Vitamin C

Liposomal vitamin C encapsulates ascorbic acid within phospholipid vesicles, which may allow it to bypass the SVCT1 transporter limit and achieve higher plasma levels than standard oral vitamin C. Some studies have shown that liposomal delivery produces blood levels between those of standard oral and intravenous vitamin C, though the research is still limited.

Cofactor Roles

Vitamin C is required for collagen synthesis (as a cofactor for prolyl and lysyl hydroxylases), carnitine biosynthesis, norepinephrine production, and peptide hormone amidation. It also enhances non-heme iron absorption by reducing ferric iron (Fe3+) to the more absorbable ferrous form (Fe2+) in the digestive tract.

Antioxidant Recycling

Vitamin C regenerates vitamin E after it has been oxidized by neutralizing a free radical. It also helps recycle glutathione, the body's master intracellular antioxidant. This creates an interconnected antioxidant network where each component extends the functional life of the others.

Food Sources

Contrary to popular belief, citrus fruits are not the richest source. Bell peppers, kiwi, strawberries, broccoli, brussels sprouts, and guava all contain more vitamin C per serving than oranges. Vitamin C is heat-sensitive and water-soluble, so raw or lightly cooked preparations preserve the most.

High-dose vitamin C has been raised as a concern for oxalate formation in susceptible individuals, as ascorbic acid can be metabolized to oxalate. People with a history of calcium oxalate kidney stones should discuss high-dose supplementation with a healthcare provider.

External resources: NIH — Vitamin C Fact Sheet