Boosted or Busted? Can You Tell Fact From Fiction When It Comes to These Common Immune-Health Beliefs?

Immune health has become front-of-mind for many for obvious reasons over the last few years. And with that comes a lot of advice on the best way to prepare your body's defenses for any lingering bugs. Our lifestyle, food choices and more can all have a big impact on our immune system. But what helps and what hurts? Here we take a look at four common myths about immune health and see what the science says.

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1. Your body's defenses weaken with age.

True. Research shows that as more candles top your birthday cake, immune function begins to decline, leaving adults over age 65 more vulnerable to severe illness from viral and bacterial infections. (There's a science-y term for this process: immunosenescence.) You have fewer circulating immune cells, and changes to those you do have make them slower to respond to infectious invaders. To bolster your defenses, exercise regularly, eat a well-balanced diet, avoid smoking and drinking too much alcohol, and stay up to date on vaccines.

2. Probiotics support healthy immune function.

Mostly true. The microbiome plays a key role in a strong, resilient immune system. A recent review found that probiotic supplements (which contain strains of "good" gut bacteria) decreased the risk of becoming sick with a respiratory infection and shortened its duration among those who did come down with one. Probiotics may activate immune cells that fight viruses, reduce inflammation, and kick out "bad" bacteria in your GI system that could open the door to illness. However, this mechanism isn't fully understood and higher-quality clinical studies are needed. Plus, the benefits may only apply to specific strains of bacteria, and not all probiotics are necessarily effective.

3. Honey will "cure" your allergies.

Too early to tell. The theory is this: Honeybees gather pollen from the very plants that cause your itchy eyes, so consuming a small daily dose of the local honey—and subsequently these pollens—may stimulate your immune system and reduce allergies, explains Miguel P. Wolbert, an allergist and immunologist at the Allergy & Asthma Care Center in Evansville, Indiana. But the pollens that cause sneezing and congestion—such as ragweed—are wind borne, while the pollens bees collect are too heavy to fly in the breeze. Wind borne pollens can fall onto flowers, get picked up by bees and end up in honey, says Wolbert, "but it's likely to be a very, very small amount." Not enough to make a difference. And, so far, no clinical evidence shows that honey alleviates allergy symptoms.

But don't nix honey quite yet. It may help soothe your cough. Researchers at Penn State University tested honey against dextromethorphan—the active ingredient in most cough medicines—as a cough suppressant for children and found honey to be more effective. Sweetness may be honey's "active ingredient." The brain part that registers sweet tastes and the part that causes coughing are located near each other so sensing sweetness may affect coughing, says researcher Ian M. Paul, M.D. One (major) disclaimer: Don't give honey to a baby younger than one year old: honey may contain spores of a bacteria that causes botulism, which an infant's immature immune system can't handle.

4. A megadose of vitamin C can squash a cold.

Mostly false. This vitamin does play an important role in immune function. But at the first sign of sniffles, don't run to the drugstore to load up on C—high-dose supplements won't prevent or shorten the duration of a cold, according to a review published in Frontiers in Immunology. (Plus, there's only so much vitamin C your body can absorb in one sitting; you'll simply urinate out any excess.) Some exceptions: Competitive athletes (intense physical stress can lower immunity) and those with metabolic disorders or heart disease may want to take up to 1 gram of vitamin C daily—these groups are more susceptible to viral infections and may benefit from an extra boost of C, the review notes. Everyone else can skip the supplement and opt for C-rich foods instead, like strawberries, bell peppers, kiwi, broccoli, citrus and potatoes.

Similarly, zinc may also help cut the number of days you're sick. In a study in the Journal of Infectious Diseases, adults who took zinc in lozenge form (13.3 mg every 2 to 3 hours for as long as their cold lasted) within a day of noticing the telltale signs kicked the bug about 3 days sooner than those who got a placebo. Though that dose exceeds the recommended daily max of 40 mg, it's safe for a 3- to 5-day period, says study author Ananda Prasad, M.D., Ph.D. Scientists think zinc binds to cell receptors in the mouth and throat, blocking the cold virus from attaching and spreading. Zinc comes in many forms, but only lozenges have been shown to be effective. Skip nasal sprays and swabs: they may damage your sense of smell. And zinc from food (beef, dark poultry meat, shellfish) probably won't help, either, as you can't get enough that way.

5. Sleep has little impact on immunity.

False. Rest is one of the top recommendations for recovering from a cold, but it's also vital to preventing them. Immune function is closely tied to sleep and circadian rhythms (your body's internal clock), and inadequate shut-eye can leave you more susceptible to infection. In one experimental study, participants who got less than 6 hours of sleep a night for one week were four times more likely to get sick after exposure to a cold virus than those who got at least 7 hours. Other research has found that adequate rest may improve immune response following vaccinations. Just one more reason to get more zzz's!

6. Dairy will make you more congested.

False. Some people avoid dairy products when they're sick because they are thought to further increase mucus secretions, but scientific evidence has yet to support this. In a blind test using a soy-based drink with similar sensory characteristics as milk, subjects reported the same changes in mucus production as they did with cow's milk.

So go ahead and have that glass of milk or a latte—the vitamin D in it may help boost your immune system. In a study of more than 300 Japanese children, those who took daily vitamin D supplements (1,200 IU) were 40 percent less likely to get a common flu virus than kids who took a placebo. Studies indicate that the nutrient may help immune cells identify and destroy bacteria and viruses that make us sick, says Adit Ginde, M.D., M.P.H., a public health researcher at the University of Colorado School of Medicine in Denver. Because a cup of milk delivers about 115 IU of vitamin D, and other food sources (oily fish, such as salmon or sardines) also only deliver small amounts, you may need to consider a supplement.