Does vitamin C prevent colds?

Not in the general population. Regular supplementation slightly shortens duration and shows prevention benefit only in people under extreme physical stress, such as marathon runners and soldiers in the cold.

Should I take zinc all winter?

No. Long-term high-dose zinc causes copper deficiency. Use lozenges only at the first sign of a cold, for under a week.

What is the single best immune intervention?

Sleep, followed by vaccination and vitamin D sufficiency. No supplement matches them.

Immune

Moderate Evidence

Pathogen Resistance

Reduced susceptibility to bacterial, viral, and fungal infections.

TL;DR

Resistance to infection is built on sleep, vitamin D sufficiency and vaccination — not on immune "boosters". Zinc shortens colds when started immediately; most other products do little.

Why It Matters

Host defence depends on barrier integrity, innate immune cells and adaptive memory. The strongest modifiable predictors of infection frequency are sleep duration, vitamin D status, smoking, blood glucose control and stress. "Boosting" immunity is the wrong frame — an over-activated immune system causes autoimmunity and inflammatory damage; the goal is competence, not amplification.

How to Measure

Biomarkers

25(OH)D

Zinc (plasma)

Full blood count with differential

HbA1c

Infection frequency per year

Optimization Protocol

1) Sleep 7-9h — under six hours multiplies experimental rhinovirus infection risk several-fold. 2) Keep 25(OH)D at 30-50 ng/mL; supplementation reduces acute respiratory infection risk most in deficient people. 3) Stay up to date on relevant vaccinations. 4) Zinc lozenges 75mg/day of elemental zinc started within 24 hours shorten colds by roughly one day; do not use long term. 5) Manage blood glucose and stop smoking, both of which impair neutrophil function. 6) Wash hands; the boring interventions have the strongest evidence.