Flu season is upon us, which means it's time for the wave of advertisements promoting $8 juices or even more expensive supplements to "boost your immunity" or "support immune function."
But those are marketing terms, not scientific ones. And there's no proof that those products are going to keep you from getting sick.
When you're exposed to a virus like the influenza virus, a number of factors determine whether you actually get sick, and if so, how severely. One is pre-existing immunity, either from being previously exposed to a similar strain or through a vaccine, says Gregory Poland, a spokesman for the Infectious Diseases Society of America and a professor of medicine at the Mayo Clinic. Just last week, a paper published in Science reported that the flu strain you were first exposed to can affect your protection against new strains that jump from animals to humans.
Your immune status also matters; people who have untreated HIV or have recently received a bone marrow transplant, for example, cannot fight off infections like healthier people can.
Age, too, is a factor, with the very young and the very old suffering worse bouts of the flu.
And yes, what you eat does matter. "We know for a healthy immune system you need a healthy diet," says Joan Salge Blake, a registered dietitian nutritionist and clinical associate professor at Boston University. You need protein as well as micronutrients including vitamins C, A, and E and zinc, she says.
The ideal way to get those nutrients, however, is to eat a healthful, varied diet, including sufficient protein and a variety of fruits and vegetables, says Poland. If you're already doing that, it's unlikely that you have major nutritional deficiencies. One exception is vitamin D, which is necessary for bone health and can be hard to get from food alone, though there's not a consensus on the cutoff for a vitamin D deficiency.
Even those who aren't eating the most healthful diet (i.e. most of us) are likely getting a lot of nutrients through fortified packaged foods like cereal. Nutrient deficiency does happen, but it's relatively uncommon in the U.S. According to the Centers for Disease Control, less than 10 percent of the population is deficient in micronutrients, though Poland says certain groups of people are at risk, including vegans who are not careful about their food choices and older people who eat scant, unvaried diets.
"If you are malnourished, your immune system is going to suffer," says Salge Blake.
If you are not nutrient deficient or malnourished, though, taking megadoses of vitamins is not going to supercharge your immune system or prevent you from catching the flu or other respiratory viruses. Vitamin C, often touted as a way to stay healthy in the winter, doesn't seem to reduce the incidence of colds, though there is some evidence it may cut their duration and it might be helpful for people who experience short periods of heavy physical activity, according to a 2013 Cochrane review.
Juices sound attractive; after all, they are made from real foods. But Salge Blake says the best way to get the nutrients supplied by fruits and vegetables is to actually eat the fruits and vegetables themselves. That way you get the fiber, which slows the absorption of natural sugars and carries its own health benefits.
Drinking juices also makes it easy to consume too many calories, and obesity suppresses immune function.
What can the average person do who wants to make sure their immune system is as healthy as possible? In addition to a healthful diet and sufficient sleep, Poland recommends exercise, staying up to date on flu and pertussis vaccinations, staying away from people who are obviously sick, and washing your hands.
Katherine Hobson is a freelance health and science writer based in Brooklyn, N.Y. She's on Twitter: @katherinehobson.