As the coronavirus that causes Covid-19 spreads, be wary of claims that supplements can make your immune system stronger.

No studies have tested supplements against the new coronavirus strain in a clinical setting. And even if you're only trying to fend off the common cold or flu, the evidence is unimpressive.

Zinc

When it comes to fighting colds, "the data on zinc is better than on any other supplement,” Bruce Barrett, professor of family medicine and community health at the University of Wisconsin, told Nutrition Action in September 2019.

But not just any zinc. Zinc lozenges release zinc slowly, which may prevent viruses from replicating or binding to cells in the throat and nose.

In a handful of small trials, colds were one to three days shorter in adults who sucked on zinc rather than placebo lozenges every few hours throughout their colds. Those trials (most were funded by or had lozenges provided by zinc-supplement makers) had people take at least 75 milligrams a day of zinc acetate or zinc gluconate at the first sign of a cold. (Zinc lozenges that contain citric acid, mannitol, sorbitol, or tartaric acid don’t seem to work.)

In a larger, company-funded trial (co-authored by a company employee), on the other hand, neither zinc gluconate nor acetate cut colds short. It’s not clear why.

“Zinc may work for some people,” noted Barrett. “But the effect is probably modest.” And the lozenges may cause nausea or a lingering bad taste.

Vitamin C

Vitamin C is king of the cold supplements. Does it work?

In seven trials that tested vitamin C’s ability to treat a cold, taking roughly 3,000 milligrams at the first sniffle didn’t make the cold shorter or less severe.

And in 24 trials on a total of nearly 11,000 people, those who took 200 to 2,000 mg a day for an average of three months were no less likely to catch a cold than placebo takers. In those same studies, colds were 8 percent shorter in the adult C takers—about half a day for a weeklong cold. Hardly worth it.

But in five trials of roughly 600 people doing intense physical activity like an ultramarathon, 250 to 1,000 mg a day of vitamin C for two to eight weeks cut the risk of catching a cold in half.

Elderberry

In two small studies, flu-like symptoms resolved faster in people taking elderberry than a placebo. But the studies were poor quality, and one of the studies was funded by a supplement maker. The other didn’t reveal its funding.

Can elderberry prevent illness? In a company-funded trial, 312 adults who took 600 mg a day for 9 days leading up to airline travel and 900 mg a day for 6 to 7 days during and after travel were no less likely to catch a cold than placebo takers.

Probiotics

The data on probiotics is skimpy (and company funded). Consider the research highly preliminary unless it is replicated in multiple new studies.

■ Emergen-C Probiotics+. It’s “the delicious way to fortify your immune system at your core by adding good bacteria into your gut microbiome,” claims the supplement’s website.

Only one good study in adults has been done on the probiotic’s strains: Lactobacillus rhamnosus GG and Bifidobacterium lactis BB-12. In 198 college students, those who took 1 billion colony-forming units (CFU) a day for 12 weeks had no fewer colds than placebo takers, but their colds were two days shorter and less severe.

■ Metagenics UltraFlora Immune Booster. Results varied in two 12-week trials that studied Metagenics’s strains (L. plantarum HEAL9 and L. paracasei 8700:2) at a dose of 1 billion CFU a day. In 272 adults, 67 percent of placebo takers—versus 55 percent of probiotics takers—got at least one cold, but the length and severity of colds was the same in both groups. In another study in 310 adults, probiotics takers had no fewer colds, though their colds were less severe and one day shorter.

■ DanActive and Yakult. According to six trials that lasted three weeks to six months, the dairy drinks are unlikely to prevent colds or flu, but they may shorten both by about a day.

Don’t bother

■ Colloidal silver. In February, televangelist Jim Bakker used his show to claim that the Silver Solution supplement that were sold in his online store could cure the coronavirus “within 12 hours.”

Colloidal silver is a solution of tiny silver particles in water. In 1999, the Food and Drug Administration banned over-the-counter drugs with colloidal silver, since there was no evidence that it’s safe and effective. (Silver can build up in the body, sometimes causing a permanent graying of the skin known as “blue man syndrome.”)

Yet you can still find colloidal silver in OTC supplements like Sovereign Silver (“Immune support,” promises the label) and Silver Solution.

Silver is sometimes used topically to treat burns or wounds. What’s the evidence that swallowing it prevents or treats infections? Zilch.

At the urging of the Center for Science in the Public Interest (Nutrition Action's publisher), the Food and Drug Administration and the Federal Trade Commission have warned the Jim Bakker Show to stop making illegal health claims about colloidal silver. Attorneys General in two states also took action.

■ Oscillococcinum. Like other homeopathic treatments, it’s diluted so much that it’s unlikely to contain any of its “active” ingredient (duck liver or heart).

In two low-quality studies (one coauthored by an employee of Oscillococcinum’s manufacturer), roughly 800 people were randomly assigned to take Oscillococcinum or a placebo within the first day of having flu-like symptoms. After two days, symptoms were gone in 17 to 19 percent of Oscillococcinum takers and 10 to 15 percent of placebo takers. You have to wonder: Did those folks even have the flu?

■ Mushrooms. “Harnesses the power of ancient wisdom by creating an immune blend of 6 diverse mushrooms to support your immune system,” says Nature’s Way Immune Blend.

In one industry-funded study, eating mushrooms daily for a week increased the secretion of an immune system protein compared with a control group. What about colds or flu? No one has looked.

Nutrition Action doesn’t accept any paid advertising or corporate or government donations.

Photos: montecilllllo/stock.adobe.com, Cold-Eeze, Viva Naturals, Sambucol, Emergen-C, Sovereign Silver.