Which supplements genuinely support your immune system?
Some supplements genuinely support your immune system, but the effect is greatest when you have a deficiency: so first check whether your vitamin D or zinc levels are adequate, and supplement in a targeted way.
Correcting deficiencies is the most well-supported starting point. Broad agreement across multiple life stages shows that a deficiency in vitamins A, B6, B12, C, D, E, folate, zinc, iron, selenium, magnesium or copper clearly weakens the immune system and increases the risk of infections. Supplementation that corrects such a deficiency reliably helps. Additional supplements on top of an already adequate intake offer a far less certain benefit.
Vitamin D has the strongest evidence when it comes to infection prevention. Several randomised trials pooled together show a modest but measurable reduction in infection risk, particularly in people who were deficient. In people with normal blood levels, the benefit is smaller and less certain. Vitamin D also plays a role in autoimmune diseases: low levels are linked to conditions such as MS and lupus, but randomised trials in type 1 diabetes and bowel conditions show only modest benefits. Supplementation as a treatment for these conditions is not yet sufficiently supported by evidence.
Zinc shortens illness duration but does not prevent infections. Pooling of randomised trials shows that zinc can shorten the duration of respiratory symptoms by around 47% in people who are already ill. The likelihood of getting an infection in the first place does not demonstrably decrease. The effect is most pronounced in people with a zinc deficiency; those who already get enough are likely to notice less benefit.
Vitamin C offers a modest benefit when used preventively. With a daily intake of 100 to 200 mg, the risk of a respiratory infection drops by around 4%, and those who do fall ill are on average 9% less ill for a shorter time. Gram-doses taken once an infection has already started are less well studied and the evidence base for this is weaker. Two authors of a related review are affiliated with a manufacturer of vitamin products; the findings themselves have also been confirmed in other studies, but keep that conflict of interest in mind.
This answer is based on multiple reviews and one pooled analysis of randomised trials on vitamin C, D and zinc. The evidence for correcting deficiencies is the strongest and most consistent. For vitamin D in autoimmune diseases the evidence is thinner: the effects seen in observational research are larger than those found in randomised trials, suggesting that other factors play a role. With vitamin C, two review authors have a potential commercial interest; however, the findings are also supported by other sources.