Vitamin C (Ascorbic Acid)

Vitamin C earns 6.5/10 for immune function because the real-world signal is modest but genuine. The Hemila 2013 Cochrane review pooled 29 comparisons and 11,306 people: regular supplementation did not prevent colds in the general population (RR 0.97), but it shortened cold duration by 8 percent in adults and 14 percent in children, and cut incidence roughly in half (RR 0.48) in marathon runners, skiers, and soldiers under heavy physical stress. The honest read is that vitamin C is a maintenance nutrient for immune tissue, not a cold cure. It matters most when you are deficient or under extreme exertion, and starting it only after symptoms appear does little.

Antioxidant capacity is one of vitamin C's defining roles, scoring 6.5/10. It is a water-soluble reducing agent that neutralizes reactive oxygen species directly and regenerates oxidized vitamin E, and plasma and tissue antioxidant markers reliably rise with intake. The boundary is clinical: a biomarker moving is not the same as a disease prevented, and large prevention trials of antioxidant supplements for cancer and cardiovascular disease have largely failed. Per Juraschek 2012, the antioxidant mechanism does translate into a measurable 3.84 mmHg systolic blood pressure drop, which is the cleanest downstream outcome. Treat the antioxidant effect as real chemistry with modest, marker-level clinical payoff rather than a longevity guarantee.

Vitamin C scores 6.0/10 for cardiovascular support. The Juraschek 2012 meta-analysis of 29 randomized trials (median dose 500 mg/day, median 8 weeks) found a pooled systolic reduction of 3.84 mmHg, rising to 4.85 mmHg in hypertensive participants, with a smaller diastolic effect. That is a real, drug-comparable surrogate movement for a cheap nutrient. The caveat the authors themselves flag is that long-term trials powered for hard cardiovascular endpoints (heart attack, stroke) have not shown benefit, so the score reflects blood pressure improvement, not proven event reduction. Best used as an adjunct alongside diet, exercise, and any indicated medication, not as a replacement for them.

For glycemic control vitamin C scores 6.0/10, driven by the Ashor 2017 systematic review and meta-analysis of randomized trials, which found significant reductions in HbA1c and fasting glucose concentrated in people with type 2 diabetes taking 1000 mg/day or more for at least 12 weeks. Effects on fasting insulin and HOMA-IR were inconsistent, and the authors rated overall certainty as low with a call for larger, longer trials. The practical takeaway is that vitamin C is a plausible adjunct for diabetics specifically, where the higher-dose, longer-duration signal lives, and offers little glycemic value to metabolically healthy people. Track HbA1c rather than assume the effect.

Skin gets a 6.0/10 because vitamin C's role here is mechanistically certain even where the cosmetic trial base is thin. As the essential cofactor for prolyl and lysyl hydroxylase, it is required for stable collagen cross-linking, which is why deficiency produces fragile skin, poor wound healing, and perifollicular hemorrhage that reverse on repletion. For oral supplementation in already-replete people, dramatic anti-aging effects are not well demonstrated, and topical L-ascorbic acid serums are a separate product category with their own (modest) photoaging data. The defensible claim is that adequate vitamin C is necessary for normal skin integrity and wound repair, not that extra grams visibly rejuvenate healthy skin.

Wound healing scores 6.0/10 on strong mechanistic and deficiency-reversal grounds. Collagen deposition is the structural backbone of healing tissue, and vitamin C is the obligatory cofactor for the hydroxylase enzymes that stabilize collagen, so deficiency directly impairs wound closure and reopens healed wounds (a classic scurvy sign). Repletion restores normal healing. The boundary is that supplementing beyond adequacy in well-nourished patients has not been shown to accelerate healing further. The strong case is for people who are deficient, malnourished, elderly, post-surgical, or burn patients, where ensuring adequate vitamin C is standard supportive care. For a healthy person eating fruit and vegetables, extra vitamin C offers little additional wound benefit.

Vitamin C lands at 5.5/10 for general metabolic health. The supporting evidence overlaps with its glycemic and lipid data: Ashor 2017 reported modest HbA1c, fasting glucose, triglyceride, and total cholesterol improvements that scaled with baseline HbA1c, meaning the people who benefit most are those who start metabolically impaired. In replete, healthy adults the metabolic upside is small. The score is conditional on baseline status and dose, and the certainty is low. Define a marker such as fasting glucose or triglycerides before starting and judge the supplement by that marker rather than by the broad antioxidant story, stopping if nothing moves over a few months.

Older adults score 5.5/10, one of the stronger non-core use cases. Vitamin C deficiency is more common in the elderly because of lower dietary intake, reduced appetite, and higher prevalence of smoking and chronic illness, and the consequences (poor wound healing, fatigue, fragile skin, bleeding gums) hit this group hardest. Correcting a real deficit produces clear benefit. The Juraschek 2012 blood pressure effect is also relevant to an age group with high hypertension prevalence. The score is conditional on actually being low, which is more likely with age than in younger adults.

Vitamin C scores 5.0/10 as an anti-inflammatory. It lowers some oxidative-stress and inflammatory markers in trials, consistent with its reducing-agent chemistry, and the blood pressure and glycemic improvements in Juraschek 2012 and Ashor 2017 plausibly run partly through reduced oxidative burden. But the high-dose intravenous trials aimed squarely at inflammation in critical illness, including the Fowler 2019 CITRIS-ALI trial, failed their primary inflammatory and organ-failure endpoints. The honest score reflects real but modest marker-level effects without convincing hard-outcome benefit. It is a supportive nutrient, not a targeted anti-inflammatory therapy.

Respiratory support scores 5.0/10, anchored to the same Hemila 2013 Cochrane evidence that drives the immune rating: modest cold-duration shortening and meaningful prophylaxis only in people under extreme physical stress. Beyond the common cold, high-dose intravenous vitamin C for acute respiratory failure did not improve organ failure in Fowler 2019. There is supportive interest in vitamin C for pneumonia recovery in deficient populations, but the evidence is not strong enough to claim a reliable respiratory benefit in well-nourished people. Use it as a maintenance nutrient rather than a treatment for respiratory infection.

Energy scores 5.0/10 because fatigue is one of the earliest and most reversible symptoms of vitamin C deficiency. Genuinely low people, including smokers and those with poor diets, often report meaningful energy improvement on repletion, and vitamin C is a cofactor for carnitine synthesis, which supports fatty-acid energy metabolism. The catch is that this is a correction effect, not a stimulant effect: replete people should expect little to no energy lift from extra vitamin C. The score sits at the midpoint to reflect a strong benefit for the deficient minority and a negligible one for everyone else. Check plasma status or dietary intake before expecting an energy change.

Healthspan scores 5.0/10. The case rests on vitamin C being essential for collagen, immune competence, iron handling, and antioxidant defense, all of which degrade visibly in deficiency and recover with repletion. Adequate status plausibly supports vascular health (via the Juraschek 2012 blood pressure effect) and connective-tissue integrity into older age. What is missing is evidence that exceeding adequacy compresses morbidity in healthy adults. The honest score credits the strong necessity floor and the modest blood pressure surrogate while declining to inflate the supplement into a healthspan intervention for people who already eat fruit and vegetables.

Recovery scores 5.0/10. Vitamin C supports tissue repair through collagen synthesis and may buffer exercise-induced oxidative stress, which is the rationale athletes use for supplementation. The nuance is a genuine tradeoff: high-dose antioxidant supplementation around training can blunt some of the adaptive signaling that exercise stress triggers, so megadosing may slightly interfere with adaptation. The Hemila 2013 data showing strong cold protection in heavily exercising people is the clearest recovery-adjacent signal. The balanced read is that adequate vitamin C supports repair, especially under heavy training load or in deficiency, while routine megadosing is not clearly beneficial and may be mildly counterproductive for adaptation.