Vitamin C

Updated June 3, 2026

Vitamin C does real biochemical work the moment it hits the bloodstream. It builds collagen by serving as a required cofactor for prolyl and lysyl hydroxylase, the enzymes that stabilize the triple helix in skin, tendon, bone matrix, cartilage, and the basement membrane of every blood vessel. It synthesizes norepinephrine through dopamine-beta-hydroxylase, which is why the adrenal cortex and the brain concentrate vitamin C at levels many times higher than plasma. It recycles vitamin E back to its active form. It scavenges free radicals in plasma and inside cells, and it quenches peroxynitrite, the damaging product of nitric oxide reacting with superoxide. The official adult RDA is 75 mg for women and 90 mg for men, with a tolerable upper limit of 2,000 mg per day, and that floor is real for preventing scurvy and maintaining tissue collagen. The more interesting question is what happens above the floor, which is where Linus Pauling's 1970 "Vitamin C and the Common Cold" launched a debate that still runs.

The biochemistry is settled. Vitamin C is a water-soluble antioxidant and an essential enzyme cofactor. Plasma levels plateau at modest oral doses because intestinal absorption uses saturable sodium-vitamin-C transporters that cap around 200 to 400 mg per dose. A 1000 mg tablet gives you roughly 50 percent absorption. A 5000 mg dose absorbs a much smaller fraction of the total, with the rest passing through. That cap is the whole pivot point of the high-dose argument.

Linus Pauling, Robert Cathcart, and more recently the cardiologist Thomas Levy (whose book "Curing the Incurable: Vitamin C, Infectious Diseases, and Toxins" anchors the modern liposomal-and-IV protocol) all argue that the plasma cap is exactly the constraint to bypass. IV delivery skips the gut entirely. Liposomal formulations wrap the molecule in phospholipid bilayers and absorb through a different pathway, reaching tissue concentrations that ordinary tablets cannot. Levy's clinical recommendations sit in the 6,000 to 15,000 mg daily range in divided doses, dropping to around 2,000 mg of liposomal if regular tablets cause loose stools.

That loose-stool threshold is itself useful. Cathcart in the 1970s built a framework called bowel tolerance titration: dose up until your gut tells you to stop, then back off. He observed that healthy adults tolerate 5 to 15 grams per day, while sick adults sometimes tolerate 30 to 200 grams, with the body apparently consuming more when fighting infection. The bowel tolerance phenomenon itself is well-established. The infection-fighting conclusions Cathcart drew remain contested.

Controlled trials give a narrower but consistent picture. A 2013 Cochrane review found that regular supplementation of 200 mg or more daily shortens cold duration about 8 percent in adults and 14 percent in children. For people under heavy physical stress (marathon runners, soldiers), supplementation slightly reduces incidence too. The immune side is one of the cleaner stories. Neutrophils concentrate vitamin C actively and use it for chemotaxis and the oxidative burst, then need to clear the oxidants they generate. T-cell proliferation and natural killer cell activity both fall under depletion and recover with repletion. That is the cellular floor under the cold-duration numbers. For sepsis, the CITRIS-ALI trial (Fowler 2019) tested IV vitamin C and did not replicate the dramatic clinical improvement smaller earlier trials suggested. For cancer adjunct use, IV vitamin C remains under active investigation with mixed but ongoing results.

Beyond infection, the vascular work shows up at ordinary doses. Vitamin C preserves tetrahydrobiopterin, the cofactor endothelial nitric oxide synthase needs to make nitric oxide instead of superoxide. Meta-analyses of 500 mg daily for eight weeks show small but real drops in blood pressure, around 3.8 mmHg systolic in hypertensive subjects. Arterial walls, heart valves, and capillary integrity all depend on the same hydroxylation reactions that keep skin and tendon intact.

The adrenal angle deserves a sentence. Adrenal cortex tissue holds vitamin C at roughly 50 times plasma levels, and it depletes under prolonged stress as cortisol and catecholamine synthesis pull it down. Whether supplementation blunts cortisol in healthy people is debated, but the requirement under stress is established.

Form choice matters more than most users realize. Plain ascorbic acid is cheap and acidic, which is part of why it irritates the gut at high doses. Buffered sodium ascorbate or mineral ascorbates trade acidity for added sodium or minerals, useful if you want gram-level oral doses without the GI penalty. Liposomal vitamin C achieves higher plasma levels than equivalent oral doses, with the magnitude varying by product quality. IV vitamin C is a separate category through integrative clinics. Most users do well taking 500 to 1000 mg with breakfast and another dose with dinner, split because of the absorption cap. Those chasing the Levy-Cathcart-Pauling protocol go higher and divide more aggressively.

Where vitamin C works is unusual. Plasma and inside cells, not fat tissue. Glutathione (covered in the NAC and GlyNAC articles) handles the intracellular work, supported by cysteine and glycine. Vitamin E lives in cell membranes. The body recycles all three together, so topping up vitamin C also supports the rest of the antioxidant network. Vitamin C donates electrons to regenerate oxidized tocopherol back to active alpha-tocopherol, which is why the two are commonly studied as a pair.

A few cautions. Very high doses can raise oxalate excretion. Matters for anyone with calcium oxalate kidney stone history. G6PD deficiency causes severe reactions to IV high-dose vitamin C. Hemochromatosis worsens because chronic high-dose vitamin C drives iron absorption. Pregnancy doses above 2 grams sit in thin territory. None of this is medical advice. If you take large doses, consider IV vitamin C, or have kidney stones, hemochromatosis, G6PD deficiency, or any chronic condition, see a clinician first.

What you actually get. Reliable collagen synthesis at 500 to 1000 mg, the substrate for skin, vessel, tendon, and bone. A topped-up plasma antioxidant pool with vitamin E and glutathione kept in their active forms. Better endothelial nitric oxide handling and a small but real blood pressure effect over weeks. Functional neutrophils and T-cells under load. Shorter colds with consistent daily dosing. A real absorption boost from liposomal forms when you need tissue levels the gut cap will not allow. And a framework, from Pauling and Cathcart and Levy, for going much higher when illness or protocol calls for it. Bowel tolerance is the natural ceiling. Reading "Curing the Incurable" alongside the Cochrane reviews and the CITRIS-ALI paper gives the full picture. Either the conservative RDA floor or a multi-gram protocol is defensible, depending on what you are after.