Vitamin C

Why your body can't make vitamin C

Most mammals produce their own vitamin C from glucose. Humans lost that ability millions of years ago when the GULO gene became inactive during primate evolution. Every milligram of vitamin C in your body comes from food or supplements. Your tissues hold roughly 1.5 to 3 grams at any time, concentrated in the adrenal glands, white blood cells, and brain. Because it's water-soluble, your kidneys flush out the excess rather than storing it. Without regular intake, levels drop within weeks.

Subclinical deficiency is more common than most people realize. A 2023 scoping review found that older adults and institutionalized populations frequently have vitamin C levels below optimal thresholds ^[1].

Vitamin C and biological aging

A 2025 population-based study of over 10,000 NHANES participants found that higher serum vitamin C concentrations were inversely associated with phenotypic age acceleration (PhenoAgeAccel). The relationship was nonlinear, with an inflection point at about 1.46 mg/dL of serum vitamin C, above which the protective association plateaued ^[2]. Separately, a cross-sectional analysis of more than 7,000 U.S. adults showed that greater dietary vitamin C intake correlated with longer telomere length, a well-established marker of cellular aging ^[3]. Vitamin C also acts as a cofactor for TET enzymes that regulate DNA demethylation, connecting it directly to epigenetic regulation of gene expression ^[4].

These findings don't prove that taking vitamin C pills will make you age slower. But they do indicate that maintaining adequate blood levels is associated with better aging biomarkers at the population level.

How it protects your cells

Vitamin C is one of the body's primary water-soluble antioxidants. It donates electrons to neutralize reactive oxygen species (ROS) from metabolism and environmental stress. The oxidized form gets recycled back to ascorbate through the glutathione system, so it works catalytically rather than being consumed in a single reaction.

Vitamin C also keeps iron in its reduced ferrous (Fe2+) state, dramatically improving absorption of non-heme iron from plant foods. Adding 50 mg of vitamin C to a meal can increase iron absorption three- to sixfold ^[5]. This matters most for vegetarians and vegans. People with hemochromatosis (iron overload) should avoid high-dose vitamin C for exactly this reason.

Collagen synthesis and tissue repair

Vitamin C is required for the hydroxylation of proline and lysine residues in procollagen, the step that stabilizes collagen's triple-helix structure and gives connective tissue its tensile strength. Without enough vitamin C, collagen production breaks down: wounds heal slowly, gums bleed, and skin loses elasticity. A 2024 clinical trial found that 12 weeks of supplementation with hydrolyzed collagen plus vitamin C reduced collagen fragmentation by 44.6% as measured by confocal microscopy ^[6]. Athletes, post-surgical patients, and anyone recovering from injuries benefit from keeping their vitamin C status at adequate levels.

Immune function: what the data actually shows

White blood cells concentrate vitamin C at 10 to 100 times plasma levels, pointing to a specialized immune role. Vitamin C supports neutrophil chemotaxis, phagocytosis, and pathogen killing. It also promotes T-cell differentiation toward Th1 responses, the branch of immunity that handles intracellular infections.

A 2023 meta-analysis found that regular vitamin C intake reduces the severity of cold symptoms, though the effect on duration was modest: about 8% shorter in the general population, up to 50% in people under extreme physical stress ^[7]. It won't stop you from catching a cold, but it takes some of the edge off.

Dosing, safety, and form

The RDA is 90 mg/day for men and 75 mg/day for women, but these numbers prevent scurvy, not optimize health. Plasma levels saturate at intakes around 200 to 400 mg/day. Above 1,000 mg, absorption efficiency drops sharply and unmetabolized vitamin C is excreted through urine.

Liposomal vitamin C has better bioavailability than standard ascorbic acid. Clinical comparisons show liposomal forms achieve 1.5 to 2.4 times higher plasma levels at equivalent doses ^[8]. For most people, 200 to 500 mg daily from a combination of food and supplements is a sensible target. Food sources with the highest vitamin C density include guava (228 mg per 100g), bell peppers (128 mg), kiwi (93 mg), broccoli (89 mg), and strawberries (59 mg).

The main risk of high-dose supplementation (above 1,000 mg/day) is kidney stones. Vitamin C metabolizes to oxalate, and a prospective study found that men taking vitamin C supplements had roughly double the risk of calcium oxalate stones ^[9]. Women showed no significant increase. People with a history of kidney stones should stay below 1,000 mg daily.