Vitamin C for Immune Function
Vitamin C is essential for normal immune function, but for well-nourished adults routine supplements do not prevent colds and offer at most a small reduction in cold duration and severity. Real benefit is concentrated in people who are deficient or under heavy physical stress.
Why this grade7-layer evidence engine
The mechanistic case is solid: a widely cited review (PMID 29099763) describes how vitamin C supports the epithelial barrier, neutrophil function, and lymphocyte activity, and notes that deficiency clearly impairs immunity. That review carries a high industry conflict of interest (a Bayer-employed author), and biological plausibility has not translated into strong clinical benefit for healthy people.
The largest clinical evidence pulls the grade down to preliminary. A Cochrane review of 11,306 participants (PMID 23440782) found routine supplementation did not lower cold incidence in the general population (RR 0.97), with prevention halved only in marathon runners and soldiers (RR 0.48); duration fell just 8% in adults and 14% in children. A 2023 meta-analysis (PMID 38082300) showed gram-level dosing cut severity about 15%, an effect absent at typical multivitamin doses, and from the same dominant author group. COVID-19 trials disagree, with one meta-analysis significant (PMID 36235869) and one not (PMID 36553979).
Regulators and clinics reinforce a food-first, cautious stance. EFSA authorizes only a nutrient-function claim, FDA permits no immune claim, and the NHS and WHO advise getting vitamin C from diet. Harvard states megadoses above 500 mg/day have no meaningful effect on colds in healthy people, while Mayo and Cleveland Clinic see possible benefit mainly for smokers, athletes, or cold-exposed groups.
Scoring transparency
All scores computed by a 7-layer evidence engine — fully auditable▸View the full decision path (audit trail)
- compute_raw_score — 加權公式: L2×0.30 + L3×0.25 + L5×0.25 + L11×0.10 + L1×0.10 = 0.65
- tier_from_score — 依分數區間映射至 tier letter
- apply_hec_rules — 高階證據未達主導 (1 positive vs 1 negative)，由 raw_score 決定
- tier_strict_requirement_check — Tier 條件達標，未降階
- detect_disputes — 偵測到 0 個 hard + 0 個 soft dispute
- decide_status — 依 tier + dispute 結果決定 status