The truth about immune-boosting supplements

Increasing your intake of zinc and vitamins C and D during winter isn’t a magic bullet. Here’s what the science says about the most commonly touted interventions.

To guard against a winter rise in respiratory infections, some people “boost” their immune system with supplements and nutrient-rich foods. For a subset, that is in addition to social distancing, masking, and flu and COVID-19 vaccinations, which reduce the risk of getting an infection or developing severe symptoms; for others it’s their sole defense.

They’re hoping to use minerals like zinc and vitamins including C and D to amp up the immune response in case an infection strikes. Although such efforts are unlikely to prevent an infection, they could support a person’s immune system. But for an average person who eats well, exercises, and gets enough sleep, supplements may not do a whole lot, says immunologist Scott Read at Australia’s Western Sydney University, unless they're deficient.

If someone wants to consume extra vitamins or minerals to strengthen their defenses, “there’s nothing harmful about that,” says Carol Haggans, a dietitian and consultant at the National Institutes of Health’s Office of Dietary Supplements, as long as the amounts don’t exceed daily limits set by the Food and Nutrition Board of the National Academies of Sciences, Engineering and Medicine. “High intakes can be toxic,” she warns. It’s also worth noting that supplements can interfere or interact with some medications, so it is best to check with your physician before taking them.

For now, here’s what the science says about some of the most common interventions touted to ward off or fight infections.

Zinc

Zinc is vital for the immune system. It is important for the generation of T cells, which recognize and destroy cells infected with bacteria and viruses. It also plays a role in the functions of cells that line the respiratory tract—the first line of defense against invading bacteria and viruses.

Some studies have suggested that zinc supplements taken within the first 24 hours of common cold symptoms can reduce the duration of illness. A recent review found that the use of zinc lozenges or sprays shortened cold symptoms by an average of two days. “But the difference is very small,” says nutritional immunologist Philip Calder at the University of Southampton, England. “You’re not gaining a lot.”

Zinc was also promoted as a guard against severe COVID-19, but experts at the National Institutes for Health say the evidence is insufficient to support its use as a treatment.

It won’t prevent a viral or bacterial infection, says Jarrod Dudakov, an immunologist at the Fred Hutch Cancer Research Center. If someone wants to take zinc supplements for a short time, it’s unlikely to be harmful, he says, but whether it’ll be beneficial is unclear. That’s because people who generally have a good diet get plenty of zinc from meat and seafood, as well as smaller amounts from sources like beans, lentils, nuts, seeds, and whole grains. However, zinc supplementation could be beneficial for people with a zinc deficiency, particularly the elderly and individuals with a poor diet, Dudakov says.

The daily dietary allowance for zinc is 11 milligrams for men and eight milligrams for women who are not pregnant. The NIH recommends against exceeding those doses. Research suggests that excess zinc intake, especially for prolonged periods, can lead to copper deficiencies that can cause neurological and blood-related problems. High intakes can also cause nausea, vomiting, and headaches.

Vitamin C

Like zinc, vitamin C also plays an important role in keeping our immune system healthy. It stimulates the migration of white blood cells known as neutrophils that help the body fight infections. The nutrient also supports other white blood cells called macrophages that kill and ingest pathogens and clear dead host cells, thus reducing inflammation.

Although several animal studies have indicated that vitamin C can prevent or alleviate bacterial and viral infections, it doesn’t reduce the incidence of common colds in human populations. However, like zinc, vitamin C did shorten the duration of symptoms.

In the case of COVID-19 infections, the NIH says there’s insufficient evidence to support using vitamin C for treating patients. But some studies indicate it has the potential to reduce inflammation in critically ill COVID-19 patients.

However, supplementing with vitamin C is unlikely to prevent a viral or bacterial infection in humans. Like other vitamins and minerals, it supports the immune system should an individual become infected, Calder says. “There’s no doubt about that.” Nonetheless, he suggests a food first approach, in which people eat a diet rich in nutritious foods and use supplements only when they’re unable to consume these foods or are deficient. An average healthy person may not gain much from supplements, Calder says. Also, consuming too much vitamin C—exceeding the 90 milligrams daily dietary allowance for men and 75 milligrams for non-pregnant women—could cause kidney stones.

Vitamin D

Like vitamin C, vitamin D has anti-inflammatory properties. Low vitamin D levels have been linked to increased risks of respiratory infections like cold and flu. A recent study suggests that vitamin D supplements can reduce the duration and severity of common colds. In the context of COVID-19, however, the role of vitamin D remains unclear.

An October 2021 analysis found that inadequate vitamin D in the body did not make people more susceptible to COVID-19 or increase their chances of death from the infection. The authors showed that supplements did not improve severe symptoms in COVID-19 patients.

But vitamin D supplements could still have a place, Calder says, especially because it’s hard to get enough vitamin D from food. The body produces vitamin D when skin is exposed to sunlight; but the flesh of fatty fish or fish liver oil are also good sources.

“None of this is going to be a magic bullet,” he says. “It’s getting the immune system in better shape should it be challenged.”

The difficulty, however, is that many people may not necessarily know if they’re deficient in certain vitamins and minerals and to what extent. Although there are good tests for vitamin D and B12, “most nutrients are harder to measure,” Haggans says. The goal should be to eat a variety of nutritious foods, but “if someone wants to take a multivitamin or multi-mineral supplement for insurance, there’s nothing wrong with that. It’s not necessarily going to do any good.”

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