Vitamin C

Vitamin C is one of the most-researched essential nutrients in modern supplement science, with a literature spanning cold and respiratory infection prevention, non-heme iron-absorption enhancement, immune function, wound healing, cardiovascular markers (blood pressure, endothelial function), and antioxidant effects. The body cannot synthesize vitamin C, so dietary intake is required to prevent classical deficiency (scurvy). The supplemental form is almost always ascorbic acid or its sodium/calcium salts; liposomal preparations are marketed for better absorption but have thinner head-to-head trial data.

The outcome dimensions covered most heavily are the common cold (the best-known indication, subject of a repeatedly updated Cochrane review), non-heme iron absorption (the established physiological role that drives the AGA recommendation to pair vitamin C with oral iron), respiratory tract infections beyond the classic cold framing, wound healing (especially in post-surgical and pressure-ulcer contexts), and cardiovascular markers. Newer growth areas include high-dose intravenous administration in oncology and critical care, which is methodologically separate from oral supplementation and addresses different clinical questions than the consumer-supplement use case.

Demographically, healthy adults dominate the cold-prevention literature, with pediatric trials and a small set of trials in athletes and soldiers. The cold-prevention question has been the subject of multiple Cochrane updates, with the literature focused on distinguishing routine supplementation in the general population from supplementation in populations under acute physical stress (athletes, soldiers in cold environments) - a distinction that shapes how the synthesized evidence is interpreted. Most trials do not measure baseline plasma ascorbate, which means pooled analyses cannot cleanly distinguish replete-population effects from deficient-population effects - a recurring methodological limitation that the literature has begun to address.

Vitamin C is generally well-tolerated at typical supplemental doses. The most-common adverse effects at higher doses are gastrointestinal: nausea, diarrhea, and abdominal cramping, more frequent at intakes of roughly 3,000 mg/day and above. The IOM tolerable upper limit for adults is 2,000 mg/day. Long-term very-high-dose use has been associated with an increased risk of oxalate kidney stones in susceptible individuals. Drug interactions are limited but practical: vitamin C enhances non-heme iron absorption (sometimes desired, sometimes not depending on iron status), has been reported, though not confirmed, to lower copper levels at very high intakes, and has a contested interaction with some chemotherapy agents that warrants clinician discussion in oncology contexts. People with hemochromatosis, glucose-6-phosphate dehydrogenase deficiency, or recurrent kidney stones should consult a clinician before high-dose supplementation.

This summary is informational and not medical advice. Consult a clinician before starting or changing any supplement, especially if you take prescription medications.