"Immune support": what the trials actually show

The phrase "immune support" has no agreed meaning and no regulatory definition, and the products sold under it are heterogeneous. What can be said with evidence is narrower and more useful.

Vitamin D. Low vitamin D is associated with respiratory infection, and supplementation shows a modest reduction in infection in people who are deficient — with the effect concentrated in the deficient. It is not a general cold preventive in the replete.

Echinacea. Pooled analyses find a small and inconsistent effect on cold duration and severity, with heterogeneity between preparations so large that a single result does not transfer between products.

Elderberry. Small trials show reduced duration and severity of influenza-like illness. The trials are small, some analyses have been criticised, and the effect is modest.

Andrographis. Small trials, often in combination with other herbs, showing reduced symptom severity in upper respiratory infection.

Probiotics. A modest reduction in respiratory infection incidence and duration in pooled analyses, with heterogeneity.

Cat's claw, medicinal mushrooms, and the long list of "immune" herbs. Laboratory immunomodulation is demonstrated for many of them. Clinical outcomes in humans are largely not, and the leap from "changes immune markers in a dish" to "you get fewer colds" is the leap the category relies on.

What actually supports immune function, and it is unglamorous. Adequate sleep — sleep restriction reduces vaccine response and increases infection risk in controlled studies, which is a real and measurable effect. Adequate protein and micronutrients (zinc, vitamin D, iron, vitamin C — deficiencies impair immunity and correcting them restores it). Regular moderate exercise. Not smoking. Reducing excess alcohol. Managing chronic stress, which has measurable immune effects. Vaccination, which is the intervention with the largest effect on infectious disease outcomes and is not a supplement.

The category's real function. "Immune support" is a marketing frame that implies a supplement can be a substitute for sleep, nutrition and vaccination, and that you should feel responsible for your illness if you get one. Neither is true. A person who is well-nourished, sleeps, exercises and is vaccinated will still get colds, and the honest answer to "what boosts immunity" is that the things that do are mostly not purchasable in a capsule.