Vitamin C

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.

Dosage:

RDA: 90mg/day for men, 75mg/day for women. Smokers need an additional 35mg/day. These are floor levels to prevent deficiency, not optimal levels for health or performance

Practical daily supplementation: 250-500mg daily, split into 2 doses if possible. This gets most people to plasma saturation without wasting money on unabsorbed vitamin C

Higher demand situations: 500-1,000mg daily for those under chronic stress, training hard, recovering from illness or surgery, or dealing with poor dietary intake.

Acute illness protocol: 1-2g/day split across 3-4 doses for the duration of illness. Higher single doses have diminishing absorption, so splitting matters here. Start within 24 hours of symptom onset for best effect

Best forms: Plain ascorbic acid is fine and the cheapest option, it has equivalent bioavailability to naturally occurring vitamin C in food. If ascorbic acid bothers your stomach, buffered forms like sodium ascorbate or calcium ascorbate are gentler. Liposomal vitamin C genuinely does show higher bioavailability, a scoping review of 10 studies found 1.2-5.4x higher peak plasma levels and 1.3-7.2x higher total absorption compared to standard forms. Worth the premium if you're trying to push levels higher, but for most people at moderate doses, regular ascorbic acid is fine.

Vitamin C content of common foods

Red bell pepper (1 medium, ~120g): ~150mg
Broccoli (150g cooked): ~100mg
Brussels sprouts (150g cooked): ~100mg
Strawberries (150g): ~90mg
Pineapple (150g): ~80mg
Kiwi (1 fruit, ~75g): ~70mg
Orange (1 medium, ~130g): ~70mg
Mango (150g): ~60mg
Potato (1 medium baked, ~150g): ~20mg
Tomato (1 medium, ~125g): ~20mg
Spinach (150g cooked): ~18mg
Hibiscus tea (240ml brewed): ~18-25mg

Timing: Take with meals to reduce GI discomfort. If taking iron supplements, take vitamin C at the same time for enhanced absorption. If training, avoid mega-doses (1g+) within 2-3 hours of your workout to avoid blunting adaptations. No specific difference in requirements between men and women outside of pregnancy/lactation, where needs increase to 85-120mg/day

Here's what you can expect:

If you're genuinely deficient, the first things you'll notice are improvements in energy and a general sense of feeling better, typically within 1-2 weeks. Gum health and bruising tend to improve early as collagen synthesis normalises. Skin quality improvements (firmness, brightness, reduced dullness) take longer, usually 4-8 weeks of consistent intake. Immune improvements show up as fewer colds or faster recovery, which you may notice over 1-3 months. If you're taking it for cortisol support, you should see measurable reductions in stress hormones within 4-8 weeks at 500-1,000mg daily. If your levels were already adequate from diet, you won't feel much difference from supplementation.

Side effects & risks:

GI discomfort is the most common issue, nausea, stomach cramps, diarrhea. This is dose-dependent and usually only shows up above 1,000mg in a single dose. Splitting doses and taking with food resolves it for most people. Buffered forms (sodium ascorbate, calcium ascorbate) are easier on the stomach than plain ascorbic acid

Kidney stones: This is the risk that gets the most attention. Vitamin C is metabolised to oxalate, and high doses increase urinary oxalate. A 2022 review found only limited evidence that supplementation could cause kidney stones, and epidemiological studies haven't shown a clear association in healthy individuals. However, if you have a history of calcium oxalate kidney stones, or a family history, be cautious with doses above 1,000mg daily. Stay well hydrated if supplementing at higher doses

G6PD deficiency: High-dose vitamin C (particularly IV) can cause hemolytic anemia in people with glucose-6-phosphate dehydrogenase deficiency, a genetic condition more common in people of African, Mediterranean, and Southeast Asian descent. If you have G6PD deficiency, avoid high-dose supplementation and definitely avoid IV vitamin C

Iron overload conditions: Because vitamin C enhances iron absorption, people with hemochromatosis or other iron overload disorders should be cautious, supplemental vitamin C can accelerate iron accumulation

Drug interactions: Vitamin C can reduce the effectiveness of certain chemotherapy drugs and may interact with blood thinners (warfarin) by reducing their effect. If you're on any medications, particularly anticoagulants or cancer treatments, check with your doctor before supplementing at high doses

Pro-oxidant effect at extreme doses: At very high concentrations, particularly in the presence of free iron, vitamin C can theoretically act as a pro-oxidant rather than antioxidant. This is primarily a concern with IV vitamin C, not with oral supplementation at normal doses

Interference with blood glucose tests: High-dose vitamin C can cause falsely elevated or falsely low readings on continuous glucose monitors and fingerstick glucometers depending on the device. If you're monitoring blood sugar, be aware of this.

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Blood markers

Plasma vitamin C (normal range 23-114 µmol/L), check if you suspect deficiency or want a baseline. Levels below 11 µmol/L indicate deficiency. Levels between 11-23 µmol/L are suboptimal. Note that plasma levels mostly reflect recent dietary intake, not long-term status.

Serum iron, ferritin, and TIBC, if supplementing vitamin C specifically for iron absorption purposes, check iron markers at baseline and 2-3 months to confirm it's actually improving your levels. Particularly relevant for women with heavy periods or plant-based diets.

Urinary oxalate, only necessary if you have a history of calcium oxalate kidney stones and are supplementing above 1,000mg daily. Not needed for most people.

Serum cortisol (AM draw), if supplementing vitamin C for stress and cortisol management, check at baseline and 8 weeks to assess response.

For most people, no blood work is needed before supplementing vitamin C at moderate doses (250-500mg). If you're supplementing at higher doses (1g+) long-term or using it therapeutically for cortisol or iron, baseline markers and a follow-up at 2-3 months is reasonable.