Your coworker sneezed on the printer. A friend’s kid is down with the flu. You’re about to take a 12-hour plane ride—in a middle seat. So you stock up on purported immune-boosting foods like oranges, ginger, and garlic, hoping to ward off bugs.
Good for you. A healthy diet can keep your immune system (and every other system) humming along smoothly, says Katherine Zeratsky, a registered dietitian nutritionist who works at the Mayo Clinic. Flavorful ginger and garlic don’t hurt either, especially if you use them to season whole foods. But if you think these foods will transform you into some sort of antiviral superhero, you’re wrong. The link between food and immunity is far less clear than most of us have been led to believe. (At Outside, we’ve been guilty of overstating that connection in the past, too.*)
No food has the power to magically boost immunity, according to immunologist Leonard Calabrese, director of the Center for Clinical Immunology at the Cleveland Clinic’s Department of Rheumatic and Immunologic Diseases. The idea that “dietary manipulation alone can reduce your colds or flu during a given season—I know of no evidence of that,” he says. Of all the choices you can make—eating well, sleeping enough, exercising, and reducing stress—food probably plays the smallest role in fighting off infection, he says.
Being deficient in an essential nutrient, like zinc or vitamin E, can decrease your resistance to invading microbes, but that does not necessarily mean that increasing your intake of that nutrient will directly impact your immune system if your levels are normal, says Simin Nikbin Meydani, the lab director and senior scientist at Tufts University’s Nutritional Immunology Laboratory.
You would have to carefully track the effect of increased intake over time to definitively demonstrate the relationship, and reliable nutrition research is difficult to conduct. Consider a six-week clinical trial that Meydani’s group recently conducted on whole grains (which, though they aren’t widely considered immune boosting, showed modest improvements in biomarkers of immunity and gut bacteria). She and her colleagues had to prepare every meal for about 80 participants—half with whole grains, half with refined—and collect fecal samples to analyze bacteria. That’s an expensive proposition, and food manufacturers who might fund these studies typically have smaller research and development budgets than pharmaceutical companies, she says.
The current evidence for common curative foods is far from definitive, so before you start chugging superjuices, check out the research—or lack thereof—behind the most popular cold-fighting foods.
Citrus Fruits and Juices
Yes, they’re rich in vitamin C—which does play a key role in immune health, potentially by speeding up the rate in which pathogen killers called T cells develop. But again, just because vitamin C plays a part in our immunological defense doesn’t mean consuming more than the recommended amount will make us stronger, says Colleen DeBoer, a clinical dietitian at Northwestern Medicine’s Lake Forest Hospital. A comprehensive review by medical research nonprofit Cochrane from 2013 found no data to justify megadoses of vitamin-C supplements, with the possible exception of people under extreme stress, such as marathoners and soldiers in subarctic climates (and you’re not going to get that much from a morning glass of OJ or a few clementines anyway).
In the lab, extracts from this spicy root prevented respiratory bugs from infecting cells from human airways. And in some animal studies, ginger and compounds derived from it reduced influenza and other infections. This type of research is a step toward understanding a compound’s effects, but “we need to do these studies in humans in order to be able to show that it’s effective,” Meydani says. Human trials show some support for ginger when it comes to other ailments, specifically nausea, but not for infectious diseases.
Garlic’s infection-fighting properties also don’t pass scientific muster, at least not yet. A Cochrane review of research published in 2014 found just one rigorous study of garlic as a cure for the common cold. The single trial did show a benefit in terms of warding off illness; people who took a garlic tablet every day for three months reported half as many colds as people who took a placebo pill instead. That might warrant further research, but it’s far from definitive. According to Meydani, more studies need to show similar results, with a variety of people and in a variety of circumstances, before researchers can honestly tout garlic’s powers.
In 2000, researchers at the University of Nebraska Medical Center tested a hearty family recipe that included plenty of root vegetables and a whole baking chicken, plus a pack of wings. One way colds cause respiratory symptoms is through inflammation, which occurs when viruses trigger a cascade of immune cells to your airways; in the study, the soup seemed to act as a mild anti-inflammatory, calming the movement of some of these cells, called neutrophils, in chambers in the lab. That—and the mere fact it’s a hydrating hot liquid—might make you feel better temporarily, but there’s no evidence it can shorten the duration of your infection. Besides, a single study in the lab isn’t proof something works in the real world, Meydani says.
Of all the rumored immune enhancers, zinc has the most evidence on its side. A meta-analysis published last year crunched the numbers from seven placebo-controlled trials and found that high-dose zinc lozenges shortened the duration of the average cold by about one-third. But those same high doses can cause unpleasant side effects, including nausea and vomiting, and can also interfere with prescription drugs like antibiotics and blood-pressure medications. They likely work best in people with low zinc levels to begin with. (Approximately 30 percent of older adults have zinc deficiencies, according to research Meydani conducted.) You can ask your doctor about a blood test to determine your zinc levels.
As the cold and flu season drags on, the bottom line is: what you eat on a regular basis matters far more than any food you rush out to buy at the first sign of a sniffle, DeBoer says. A whole-foods diet that includes plenty of vegetables, fruits, whole grains, and fish is likely to provide all the vitamin C, zinc, and other nutrients you need to shore up your defenses, according to Zeratsky. The Mediterranean diet is a good place to start. It might even supply you with antioxidants and other helpful compounds that haven’t been identified yet.
*In 2014, we published a story titled “5 Top Flu-Fighting and Cold-Crushing Foods.” That page on our site now redirects to this one, because we believe that this piece provides more accurate service to our readers.