Vitamin C (Ascorbic Acid)

Vitamin C is the most accessible immune booster for daily use. A meta-analysis of 29 trials confirmed that consistent supplementation of 200-500mg daily reduces cold duration by 8% in adults and 14% in children, with greater benefits under physical stress. This outperforms #10 Probiotics, which lack cold-specific evidence, by directly supporting collagen synthesis and iron absorption without the need for live cultures. For under $10 per month, it provides clinically proven tissue saturation, making it 30% more affordable than the typical rival multivitamin per serving.