Vitamin C Supplement Guide: Immune Function, Collagen Synthesis, and the Right Dose

Vitamin C (ascorbic acid) is a water-soluble vitamin that cannot be synthesized in the body and must be obtained through diet or supplementation. Its main roles include serving as a cofactor for collagen synthesis (wound repair, skin, tendons, and bone maintenance), potent antioxidant activity, enhancing iron absorption, and supporting immune cell function. It accumulates in high concentrations in neutrophils and lymphocytes and is consumed as these cells mount immune responses. Deficiency causes scurvy (due to collagen impairment), though severe deficiency is rare in modern Japan.

Multiple meta-analyses, including Cochrane reviews, find that preventive effects on the common cold in the general population are limited at around 8%. However, in groups performing intense endurance exercise (marathon, triathlon, etc.), vitamin C supplementation has been found to reduce post-exercise upper respiratory infection risk by approximately 50% (Peters et al. 1993). Intense exercise causes transient immune suppression, during which vitamin C demand surges. For those who train intensely on a regular basis, this represents a meaningful benefit.

~50% reduction

Upper respiratory infection risk after intense exercise (athletes)

~8% reduction

Cold risk reduction in general population

Vitamin C is essential for catalyzing the hydroxylation of proline and lysine, which stabilizes collagen molecules. Adequate vitamin C is required for synthesizing collagen, the primary structural component of tendons, ligaments, and cartilage. Some research suggests that consuming vitamin C in combination with collagen peptide supplements before exercise enhances collagen incorporation into tendons and ligaments. People with high training loads may have greater vitamin C demands due to elevated oxidative stress.

Japan's recommended daily intake is 100 mg/day, though research commonly uses 200–1,000 mg/day as supplements. As a water-soluble vitamin, excess vitamin C is excreted in urine, but intestinal absorption drops sharply above 1,000 mg, and digestive symptoms such as diarrhea may occur. Intakes above 2,000 mg/day may increase kidney stone risk. Plasma saturation concentrations are approached at 200–500 mg/day, with limited additional benefit above that. A practical approach is to meet baseline needs through diet (vegetables and fruits) and supplement according to training volume.

200–500 mg

Dose range approaching plasma saturation