In May, President Trump said he had been taking a zinc supplement daily to protect himself against the novel coronavirus—a practice that has gained renewed attention in light of the president's Covid-19 diagnosis. Here's what research says on whether zinc can help to prevent Covid-19.

Cheat sheets: Evidence-based medicine 101

The facts on zinc

Zinc is the second-most common trace mineral found in the human body. It's a micronutrient that affects all organs and cells, and it is needed for the body to properly smell and taste.

Zinc also plays a key role regulating metabolism and the immune system. Over the years, several studies have shown that people with low levels of zinc are more likely to develop infections and certain health conditions, including chronic fatigue syndrome, diabetes, kidney disease, pneumonia, and recurring sepsis.

According to an NIH fact sheet, severe zinc deficiency "depresses immune function"—and "[e]ven mild to moderate degrees of zinc deficiency can impair macrophage and neutrophil functions, natural killer cell activity, and complement activity." That's because "[t]he body requires zinc to develop and activate T-lymphocytes," NIH explains. However, NIH states that many of the issues associated with zinc deficiency "can be corrected by zinc supplementation."

It's safe to say that all of those facts lead to one conclusion: According to David Hafler, professor of neurology and immunobiology at Yale School of Medicine, "It's very clear: If you are zinc deficient, your immune system will not function as well."

What research shows on zinc and Covid-19

Some research has indicated that zinc may help fortify the immune system against Covid-19, the disease caused by the novel coronavirus. 

For instance, last month, researchers from Spain reported finding an association between low zinc levels in the blood and poor health outcomes among patients with Covid-19.

For their study, the researchers examined data on 611 patients who were experiencing symptoms of Covid-19 and were admitted to a tertiary university hospital in Barcelona, Spain, from March 15 to April 30. According to the researchers, 249 of those patients died in the hospitals.

The researchers found that, among the group of patients who died, the patients' average zinc-blood level was 43 micrograms per deciliter. In comparison, among the group of patients who survived, the patients' average zinc-blood level was 63 micrograms per deciliter, which is close to the level considered to be normal. After the researchers adjusted for age, sex, illness severity, and treatments among the patient groups, they found that each unit increase in zinc-blood level was associated with a 7% lower risk of in-hospital death.

The researchers concluded that their findings showed "[l]ower zinc levels at admission correlate with higher inflammation in the course of infection and poorer outcome," and "[p]lasma zinc levels at admission are associated with mortality in Covid-19." However, they added, "[f]urther studies are needed to assess the therapeutic impact of this association."

Separately, researchers in an article recently published in the Journal of Medical Virology theorized that zinc—which may inhibit RNA viruses, including coronaviruses—could have an antiviral effect against the novel coronavirus. The researchers based their hypothesis in part on a 2010 study that found zinc in combination with an ionophore, a chemical that transports an ion across a cell membrane, inhibited the replication of the coronavirus that causes severe acute respiratory syndrome, commonly known as SARS.

In addition, in a study of hospitalized Covid-19 patients in New York, researchers found that patient treated with zinc in combination with the antimalaria drug hydroxychloroquine and the antibiotic azithromycin were about half as likely to die or be transferred to a hospice when compared with patients who did not receive zinc.

What the findings mean

While some research has suggested that zinc may have benefits when it comes to protecting patients against severe Covid-19, the World Health Organization has said more research is needed to determine whether zinc products can improve health outcomes among Covid-19 patients. And because there's only limited evidence showing zinc is associated with positive outcomes among patients with Covid-19, FDA has sent warning letters to five companies that claimed zinc products can prevent or treat the disease.

Further, public health experts have noted that it's important to keep in mind that nutritional research has limits. For instance, because people eat a variety of foods with different nutrients, it's difficult to definitively determine whether one certain nutritional supplement has affected a person's health.

It's also worth keeping in mind that most healthy adults are not zinc deficient, and most people with adequate levels of zinc in their blood won't absorb excess amounts and will excrete them. Therefore, if a person is not zinc deficient, taking a zinc supplement likely won't provide them with any additional protection against Covid-19 if zinc does, in fact, offer some.

"There's no question that zinc is important. … But once you have the minimal amount of zinc, there's no evidence that adding more boosts the immune system," Hafler said.

Overall, public health experts say the best things people can do to protect themselves against the novel coronavirus and Covid-19 are those that experts have been calling for throughout America's coronavirus epidemic: Wear a face mask or covering, wash your hands frequently, and maintain physical distance from people who don't live in your household or who may be infected with the virus (MacKeen, "Scam or Not," New York Times, 9/28; Finley, Wall Street Journal, 10/5; European Society of Clinical Microbiology and Infectious Diseases, MedicalXpress, 9/23).