An essential water-soluble vitamin that your body can't make. Nearly all animals produce their own vitamin C, but humans can't. We undertake every part of the synthesis process except, inexplicably, the last step, the production of a single enzyme (L-gulonolactone oxidase). This means you need it daily from food or supplementation. Vitamin C is required for the synthesis of collagen, L-carnitine, and certain neurotransmitters. It's critical for protein metabolism, iron absorption, immune function, and acts as a potent antioxidant. If you train hard, deal with stress, or care about skin and connective tissue quality, vitamin C is foundational.
Most people eating a reasonable diet with some fruits and vegetables aren't going to develop scurvy (vitamin C deficiency). But subclinical deficiency, where your levels are low enough to impair function but not low enough to cause obvious symptoms, is more common than you'd think. Smokers, heavy drinkers, people under chronic stress, anyone training intensely and sweating a lot, and those eating highly processed diets with minimal fresh produce are all at higher risk. The adrenal glands contain among the highest concentrations of vitamin C in the body, and when you're stressed, they burn through it rapidly, your adrenals actually release vitamin C in tandem with cortisol during the stress response. If you're chronically stressed, your vitamin C demand is significantly higher than the RDA suggests.
Collagen synthesis: This is the most practically relevant function for most people. Vitamin C is a required cofactor for prolyl hydroxylase and lysyl hydroxylase, the enzymes that hydroxylate proline and lysine residues in procollagen. Without this hydroxylation step, collagen can't form its triple-helix structure, which is what gives collagen its structural integrity. No vitamin C, no stable collagen. This applies to skin, tendons, ligaments, blood vessels, and bone. If you're supplementing collagen peptides without adequate vitamin C, you're missing half the equation.
Antioxidant function: Vitamin C is the body's primary water-soluble antioxidant. It donates electrons to neutralize free radicals and reactive oxygen species before they can damage DNA, proteins, and lipids. It also regenerates vitamin E (the primary fat-soluble antioxidant) from its oxidized form, maintaining the broader antioxidant network. This matters for skin health (UV protection), cardiovascular function, and recovery from training.
Iron absorption: Vitamin C enhances the absorption of non-heme iron (the form found in plant-based sources and supplements) by reducing ferric iron (Fe³⁺) to ferrous iron (Fe²⁺), which is far more readily absorbed in the gut. If you're supplementing iron or eating iron-rich plant foods, taking vitamin C alongside significantly improves absorption. This is particularly relevant for women, who are more likely to have low iron stores, especially those with heavy periods or plant-based diets. If you're a woman taking iron supplements, pairing them with vitamin C is one of the simplest things you can do to improve absorption.
Cortisol and stress: The adrenal glands contain the highest concentration of vitamin C of any organ in the body. Vitamin C is a cofactor for enzymes involved in cortisol biosynthesis, and your adrenals actively release vitamin C during the stress response. A study on 69 women with elevated cortisol from chronic stress found that 1,000mg of vitamin C daily for 2 months brought cortisol levels from 780 down to 446 nmol/L. In ultramarathon runners, 1,500mg daily for a week before and after a 90km race significantly lowered post-race cortisol and dampened adrenaline and inflammatory markers. Vitamin C doesn't just suppress cortisol, it seems to normalize it, reducing excess output under stress while supporting production when needed.
Immune function: Vitamin C supports immune function by enhancing T-cell proliferation, phagocyte function, and antibody production. It accumulates in immune cells at concentrations much higher than in plasma. The evidence on colds is more nuanced than the popular narrative suggests. A Cochrane review of 29 trials involving 11,306 participants found that regular vitamin C supplementation did not reduce the incidence of colds in the general population. However, it consistently shortened cold duration by about 8% in adults and 14% in children. In people under heavy physical stress (marathon runners, skiers, soldiers), it halved the risk of getting a cold. A more recent meta-analysis found that 1g+ daily reduced cold severity by 15%, with a particularly strong effect on severe symptoms versus mild ones. So vitamin C won't stop you from catching a cold, but if you take it consistently, it can shorten and soften the experience, especially if you train hard.
The training blunting question (important for athletes): This is one vitamin C caveat that most people don't know about. High-dose antioxidant supplementation (typically 1,000mg vitamin C + 400 IU vitamin E) has been shown in several studies to blunt certain cellular adaptations to training. The mechanism: exercise generates reactive oxygen species that act as signalling molecules, triggering mitochondrial biogenesis, upregulation of endogenous antioxidant enzymes, and other beneficial adaptations. When you flood the system with exogenous antioxidants, you dampen that signal. One study found that 1g vitamin C daily hampered endurance capacity and prevented the exercise-induced upregulation of the PGC-1α mitochondrial biogenesis pathway in both rats and humans. For resistance training, the evidence showed that high-dose C + E blunted p70S6K and MAPK signalling after a strength session, though it didn't significantly reduce hypertrophy over 10 weeks of training.
The practical takeaway: if you're training seriously, don't mega-dose vitamin C around workouts. Moderate doses (200-500mg) from food and a reasonable supplement are unlikely to cause issues. The problems in studies emerged at 1,000mg+ combined with vitamin E. If you want higher doses for cortisol or immune support, take them well away from training windows.
Absorption pharmacokinetics: This matters for dosing strategy. At doses up to 200mg, your body absorbs nearly 100% of the vitamin C. At 500mg, absorption drops to about 75%. At 1,000mg+, you're absorbing less than 50%, and the rest is excreted through urine. Plasma levels plateau at around 70-80 µmol/L in healthy adults, which typically happens at intakes of 200-400mg daily. Beyond this point, additional oral vitamin C mostly just gets excreted. This is why splitting doses is more effective than taking one large dose.