How Vitamin C Works and Why You Can't Absorb Unlimited Amounts

Vitamin C is one of the most-supplemented nutrients in the world and one of the most misunderstood. The megadose theory has been around for 50 years. The pharmacokinetics that disprove its practical basis have been around for 30 years. Most people taking 1,000mg per day are generating expensive urine.

The Absorption Ceiling

Your gut absorbs vitamin C through two sodium-dependent vitamin C transporters, SVCT1 and SVCT2. These are protein channels that actively pump vitamin C across the intestinal wall in a process that requires sodium and ATP. They’re saturable, meaning they can only process so much per unit of time, and they can’t be overwhelmed with extra vitamin C to absorb more.

The NIH’s pharmacokinetics study by Levine et al. (1996, Proc Natl Acad Sci) mapped this precisely. At 100mg per day, absorption is around 80%. Plasma vitamin C reaches near-saturation. At 200mg, absorption drops to about 73%, and plasma levels are essentially fully saturated. At 500mg, fractional absorption is around 63%. At 1,250mg, it falls to around 46%. A significant fraction of high doses is excreted by the kidneys before it ever reaches tissues.

Above 200mg per day, you’re absorbing diminishing amounts and excreting the rest. At very high doses, unabsorbed vitamin C reaches the colon and causes osmotic diarrhea.

The implication: plasma vitamin C saturates at around 200mg daily intake. Taking 1,000mg doesn’t give you 10 times the circulating vitamin C. It gives you roughly the same plasma level, with more appearing in urine.

What Linus Pauling Got Right and Wrong

Pauling, who won Nobel Prizes in Chemistry (1954) and Peace (1962), proposed in 1970 that humans have an evolutionary need for much higher vitamin C than the RDA suggests. His reasoning was that most mammals synthesize their own vitamin C internally, and those that do produce amounts equivalent to several grams per day when scaled to human body size. Humans and other primates lost the last enzyme in this synthesis pathway due to a mutation. Pauling argued this mutation assumed a high-vitamin-C diet would compensate.

This evolutionary argument isn’t wrong. But his clinical claims, that gram-level vitamin C would prevent and cure colds and even cancer, didn’t hold up in controlled trials. The absorption ceiling means high doses don’t produce proportionally higher tissue concentrations in most circumstances. Intravenous vitamin C does reach much higher plasma concentrations and is an active area of cancer research, but that’s a fundamentally different delivery mechanism than oral dosing.

Immune Function: What the Evidence Shows

The 2013 Cochrane review by Hemilä and Chalker (PMID: 23440782) is the clearest summary of the immune evidence. It pooled data from 29 trials involving over 11,000 participants. Regular vitamin C supplementation did not reduce cold incidence in the general population. But it did reduce cold duration modestly, by about 8% in adults and 14% in children.

More strikingly, in subgroup analyses of people under heavy physical stress such as marathon runners, skiers, and soldiers on Arctic exercises, supplementation reduced cold incidence by about 50%. This suggests vitamin C has a meaningful immune role specifically when oxidative stress and physical demands are extreme.

Vitamin C supports immune function in established ways: it accumulates in white blood cells at concentrations 50-100 times higher than in plasma, it’s used during immune responses and gets depleted quickly during infection, and it appears to support both innate and adaptive immune responses (Carr & Maggini, 2017, Nutrients). But “supports immune function” doesn’t automatically translate to “prevents colds in healthy people with adequate intake.”

Collagen Synthesis and Why It Matters More Than People Think

Collagen is the most abundant protein in the human body, making up skin, tendons, ligaments, blood vessel walls, corneas, and cartilage. Every collagen molecule requires vitamin C to form correctly.

The mechanism is specific. The enzymes prolyl hydroxylase and lysyl hydroxylase stabilize collagen’s triple helix structure by adding hydroxyl groups to proline and lysine residues in the protein chain. Both enzymes require vitamin C as a cofactor. Without vitamin C, new collagen is unstable and breaks down. This is scurvy: collagen fails, blood vessels leak, wounds don’t heal, teeth fall out.

You don’t need high doses for collagen synthesis. The enzymes run at full capacity well before plasma vitamin C saturates. This is one reason the Levine team concluded 200mg/day was adequate, not because the body stores more at that level, but because tissue saturation achieves full function.

Iron Absorption Enhancer

Vitamin C’s role as an iron absorption enhancer is underappreciated and has real practical significance, especially for people eating plant-heavy diets.

Non-heme iron (from plants) exists as ferric iron (Fe3+). The intestinal transporter DMT1 only accepts ferrous iron (Fe2+). Vitamin C reduces Fe3+ to Fe2+, making it transportable. It also chelates iron and keeps it soluble in the slightly alkaline environment of the upper small intestine, where it would otherwise precipitate.

Eating a source of vitamin C with a plant-based iron source can double iron absorption from that meal. A cup of lentils with lemon juice over them, or a bean burrito with salsa, absorbs more iron than the same food without the vitamin C source. The iron absorption science article covers this in more detail.

This article is for educational purposes only. It’s not medical advice. Talk to your doctor or a registered dietitian before making significant changes to your diet.

The antioxidant chemistry of vitamin C connects to antioxidants explained, which covers how antioxidants actually work at the molecular level. If you’re interested in how vitamin C’s iron-enhancing effect works in practice, iron absorption science covers the full picture of heme vs. non-heme iron and what increases or blocks absorption.