Vitamin C
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- Smoking destroys vitamin C at 2–3× the normal rate — smokers need significantly more dietary vitamin C to maintain the same plasma levels as non-smokers. Cigarette smoke generates massive quantities of reactive oxygen species that oxidize ascorbate. Smokers have plasma vitamin C levels approximately 25–50% lower than non-smokers on identical dietary intakes. The Institute of Medicine RDA for smokers is therefore 35 mg/day higher than for non-smokers (105 mg/day for women who smoke, 125 mg/day for men who smoke) — but even these amounts may be insufficient to achieve the same plasma levels as non-smokers. This makes vitamin C status particularly important to measure and optimize in current or recent smokers.
- Vitamin C is essential for collagen hydroxylation — without adequate C, collagen is structurally defective and arterial wall integrity is compromised. Prolyl and lysyl hydroxylase enzymes — which require vitamin C as a coenzyme — add hydroxyl groups to specific proline and lysine residues in procollagen. These modifications are required for the triple helix to form properly and for collagen fibrils to cross-link into stable connective tissue. The most life-threatening consequence of collagen defects (scurvy) is perivascular hemorrhage as weakened blood vessel walls rupture under normal pressure. At subclinical levels, suboptimal collagen quality affects arterial wall stiffness and resilience, wound healing rates, and joint cartilage maintenance.
- Vitamin C substantially enhances non-heme iron absorption — relevant for anyone eating a predominantly plant-based diet. Non-heme iron (from plant sources) must be reduced from Fe³⁺ to Fe²⁺ in the gut for absorption. Vitamin C is the most potent known enhancer of this reduction — consuming 100 mg of vitamin C with a plant-based iron-containing meal can increase non-heme iron absorption by 2–4×. This interaction is particularly relevant for vegetarians, vegans, women with heavy menstrual blood loss, and anyone with borderline iron stores. Conversely, tea, coffee, and calcium consumed with iron-rich meals inhibit absorption; the practical advice is to include vitamin C-rich foods at iron-containing meals.
- The Linus Pauling hypothesis — that very high-dose vitamin C prevents or treats cancer and cardiovascular disease — remains highly contested despite decades of research. Observational studies consistently find that higher dietary vitamin C is associated with lower cardiovascular and cancer risk, but these associations almost certainly reflect the overall dietary pattern (fruit and vegetable consumption) rather than vitamin C specifically. Randomized controlled trials of vitamin C supplementation have generally not demonstrated the dramatic health benefits Pauling proposed. The current scientific consensus is that adequate vitamin C (achievable from diet) supports health, but pharmacological megadoses (gram-level supplementation) do not clearly provide additional benefit beyond what food-level intake achieves for most people.
- Vitamin C at high supplemental doses can mask iron overload by enhancing absorption — relevant for people with hemochromatosis or ferritin above normal. High-dose vitamin C supplementation substantially increases dietary non-heme iron absorption. In people with hereditary hemochromatosis (HFE gene mutations causing excessive iron absorption) or those with already-elevated ferritin, megadose vitamin C supplementation can accelerate iron loading. For anyone with ferritin above the normal range, high-dose vitamin C supplements (above 500 mg/day) should be used cautiously and with monitoring. This interaction does not apply to dietary vitamin C from food.
Subclinical Vitamin C Insufficiency: More Common Than Appreciated
Scurvy in its classical form — gum disease, perifollicular hemorrhage, impaired wound healing — captures attention because it is so dramatic and historically significant. But the population health burden from subclinical vitamin C insufficiency is far greater and largely invisible. NHANES surveys consistently find that 5–15% of US adults have plasma vitamin C in the deficient range, with many more in the subclinical insufficiency range.
The groups most affected are predictable: smokers (who oxidize ascorbate at 2–3× the rate of non-smokers); older adults with reduced dietary diversity; people living in food-insecure environments; and anyone relying heavily on processed foods rather than fresh produce. Given that a single red bell pepper or orange provides a day's worth of vitamin C, clinical deficiency in affluent populations reflects either severe dietary restriction, malabsorption, or smoking-driven oxidative depletion — not dietary unavailability.
The collagen synthesis impairment that begins before overt scurvy has clinical relevance: subclinical vitamin C insufficiency affects arterial wall collagen quality, wound healing rates, and capillary fragility in ways that may contribute to cardiovascular aging, impaired recovery from injury, and accelerated skin aging — none of which trigger a clinical diagnosis until the deficiency is advanced.
Vitamin C in Cardiovascular Biology
Vitamin C's relevance to cardiovascular aging operates through three mechanisms. First, endothelial nitric oxide — the primary vasodilatory signal in arteries — is protected from oxidative inactivation by ascorbate; adequate vitamin C helps preserve NO bioavailability and endothelial function. Second, LDL oxidation (a central step in atherogenesis) is inhibited by vitamin C in the aqueous phase of plasma and lipoproteins. Third, collagen integrity in arterial walls depends on vitamin C-dependent hydroxylation reactions — suboptimal C produces collagen with reduced mechanical strength.
These mechanisms align with observational data: the EPIC-Norfolk cohort study found that plasma vitamin C was inversely associated with all-cause and cardiovascular mortality in 20,000 adults, with the highest quartile of plasma C showing significantly lower event rates than the lowest quartile — an association that persisted after adjustment for other dietary factors. 1
| Plasma Vitamin C | Status | Notes |
|---|---|---|
| < 0.2 mg/dL | Deficient | Risk of clinical scurvy; immediate dietary intervention |
| 0.2–0.4 mg/dL | Insufficient | Subclinical deficiency; impaired collagen synthesis |
| 0.8–1.6 mg/dL | Longevity optimal | Achievable with adequate fruit and vegetable intake |
| > 2.0 mg/dL | Supraphysiological | Typically from high-dose supplementation; monitor if high ferritin |
| Range Type | Value (mg/dL) | Notes |
|---|---|---|
| Standard Clinical Range | Plasma vitamin C: 0.4–2.0 mg/dL (22–114 µmol/L) | Designed to identify disease risk — not longevity optimisation. |
| Longevity-Optimal Target | 0.8–1.6 mg/dL (45–90 µmol/L) |
Associated with reduced all-cause mortality and extended healthspan.
Plasma vitamin C reflects recent dietary intake more than tissue stores — it is susceptible to rapid decline with illness, stress, or even a few days of low vitamin C intake. Leukocyte vitamin C concentration is a better measure of tissue stores but is not routinely available. The standard reference range lower bound (0.4 mg/dL) is calibrated to prevent overt deficiency — functional insufficiency occurs above this level. The longevity-optimal range targets steady-state levels achievable with consistent dietary intake of ~200 mg/day from food. Very high-dose supplementation (> 1,000 mg/day) typically does not further elevate plasma levels above the saturation point (~70 µmol/L) because intestinal absorption becomes fractional at high doses and renal excretion increases; plasma levels plateau despite increasing doses.
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How much vitamin C do I need daily, and can I get enough from food?
The RDA for vitamin C is 75 mg/day for women and 90 mg/day for men (higher for pregnant and breastfeeding women, and for smokers). Achieving the longevity-optimal plasma range of 0.8–1.6 mg/dL typically requires 150–300 mg/day from combined food and supplements. Many commonly consumed foods provide this: one medium orange (70 mg), one cup of broccoli (80 mg), one red bell pepper (150 mg), one cup of strawberries (85 mg), one kiwi (65 mg). A person eating 5+ servings of fruits and vegetables daily almost certainly meets the RDA and likely achieves the longevity-optimal range. People with limited produce intake, smokers, and older adults may need supplementation. The absorption advantage of dietary vitamin C over supplements is modest — both forms are well absorbed at doses below 200 mg.
Does vitamin C really prevent or treat colds?
The evidence is more nuanced than popular belief suggests. Prophylactic high-dose vitamin C supplementation (500–2,000 mg/day) in the general population does not consistently prevent colds in RCT data — the Cochrane review found no effect on common cold incidence. However, regular supplementation consistently reduces cold duration by approximately 8–14% and symptom severity in multiple meta-analyses. A separate, more robust finding: in people under heavy physical stress (marathon runners, skiers, soldiers in subarctic conditions), prophylactic vitamin C significantly reduced cold incidence by approximately 50% in the Cochrane analysis. The practical implication: vitamin C supplementation is worth maintaining for its established roles in collagen synthesis and antioxidant defense, and likely provides modest benefit for cold duration and severity — but it is not a reliable cold prevention strategy for the general population.
What are the symptoms of vitamin C deficiency?
Classical scurvy — now rare in high-income countries — presents with perifollicular hemorrhage (bleeding around hair follicles, producing corkscrew hairs), spongy and bleeding gums, impaired wound healing, easy bruising, fatigue, and joint pain. These reflect systemic collagen defect and capillary fragility. Subclinical insufficiency (below optimal but above the clinical deficiency threshold) is harder to recognize: it may manifest as slow wound healing, increased bruising tendency, easy fatigue, and impaired immune responses. Bleeding gums on tooth brushing that are not explained by dental disease warrant consideration of vitamin C insufficiency, particularly in people with limited produce intake. A dietary history revealing very few fruits and vegetables over several weeks is sufficient to produce measurable plasma vitamin C decline and subclinical deficiency symptoms.
Can I take too much vitamin C?
Vitamin C has a very favorable safety profile — it is one of the least toxic vitamins. The tolerable upper intake level (UL) is 2,000 mg/day for adults. The primary adverse effect of doses above 1,000–2,000 mg/day is gastrointestinal: osmotic diarrhea, bloating, and cramping from unabsorbed ascorbate in the colon. Some individuals experience these symptoms at lower doses. Kidney stone risk from oxalate: vitamin C is metabolized to oxalate, and very high doses (> 1,000 mg/day chronically) can modestly increase urinary oxalate — a concern for individuals with a history of calcium oxalate kidney stones or hyperoxaluria. The iron overload interaction (for people with high ferritin or hemochromatosis) is the other meaningful safety consideration at supplemental doses. For most adults, 200–500 mg/day is the practical sweet spot — achieving or exceeding optimal plasma levels without meaningful adverse effect risk.