Immunity supplements do almost nothing to stop respiratory infections

The pandemic turbocharged an old question: can vitamins and minerals protect healthy people from viral respiratory infections such as COVID-19? Many nutrients, including vitamins A, C, D and E, zinc, iron, folic acid, B12, beta-carotene and long-chain polyunsaturated fatty acids, have recognised roles in immune function, and the supplement industry trades heavily on that fact. What has never been clear is whether taking them actually prevents infections in people who are already healthy, as opposed to correcting a genuine deficiency. This review set out to answer that question for primary prevention in the general population.

The authors conducted a sweep of all the studies on nutrient supplementation for preventing any viral respiratory tract infection in healthy people. Their search produced 15,163 records, of which 93 papers based on 115 studies met the inclusion criteria. That gave them 199,055 subjects from 37 countries, of whom 191,636 were children and 7,419 were adults. Sixty-three studies were suitable for the pooled statistical analyses, which were performed separately for children and adults and then stratified by world region to see whether effects varied by geography.

The headline result is a null: supplementation in the general population had either no effect or at most a very limited one on preventing respiratory infections. Two regional exceptions emerged. Zinc showed a protective effect only in studies performed in Asia, though the studies behind that result disagreed with each other considerably. Vitamin D in adults significantly decreased infection incidence, particularly in North America, but showed no benefit in Europe or Oceania. Strikingly, only 10 of the 115 studies, 8.7%, examined whether participants' baseline nutritional status mattered, and in just one of those zinc worked better in children who started with low zinc levels.

This is one of the larger evidence sweeps on supplements and infection prevention, and it lands firmly on the sceptical side of a long-running debate. It is consistent with the idea that supplementing a nutrient mainly helps people who actually lack it, a question most trials never examined: fewer than one in ten studies stratified results by nutritional status. The regional patterns, zinc in Asia and vitamin D in North America, hint that background deficiency rather than the supplement itself may explain the exceptions. Whether targeted supplementation of genuinely deficient people would show clearer benefits remains the big open question.

For a healthy, reasonably well-nourished adult, this is a reason to be sceptical of products marketed as immune boosters or shields against colds and flu. The pooled evidence suggests any protective effect for the general population sits somewhere between very small and non-existent. The benefits that did appear were confined to specific groups in specific regions, which may reflect local nutrient deficiencies rather than anything universal. Knowing whether you are actually deficient in a nutrient is far more informative than any blanket habit of supplementing just in case.