Probiotics, or the "live active cultures" found in yogurt, are healthy bacteria that keep the gut and intestinal tract free of disease-causing germs. Although they're available in supplement form, a study from the University of Vienna in Austria found that a daily 220g dose of yogurt was just as effective in boosting immunity as popping pills. Be sure to pick up containers free of excess added sugar. Plain varieties (which you can flavour with cinnamon and fresh fruit) are your best bets, but anything with less than 8 total grams of sugar is still a wholesome option.
Your optimal dose: One 220g serving daily.
2. Oats and Barley
These grains contain beta-glucan, a type of fibre with antimicrobial and antioxidant capabilities more potent than echinacea, reports a Norwegian study. When animals eat this compound, they're less likely to contract influenza, herpes, even anthrax; in humans, it boosts immunity, speeds wound healing, and may help antibiotics work better.
Your optimal dose: At least one of your three daily servings of whole grains.
This potent onion relative contains the active ingredient allicin, which fights infection and bacteria. British researchers gave 146 people either a placebo or a garlic extract for 12 weeks; the garlic takers were two-thirds less likely to catch a cold. Other studies suggest that garlic lovers who chow more than six cloves a week have a 30% lower rate of colorectal cancer and a 50% lower rate of stomach cancer.
Your optimal dose: Two raw cloves a day and add crushed garlic to your cooking several times a week. (Psst! These garlic-breath-remedies can help keep your breath fresh.)
Australian Healthy Food Guide
Selenium, plentiful in shellfish such as oysters, lobsters, crabs, and clams, helps white blood cells produce cytokines—proteins that help clear flu viruses out of the body.
Your optimal dose: Two servings a week (unless you're pregnant or planning to be).
People who drank 5 cups a day of black tea for 2 weeks had 10 times more virus-fighting interferon in their blood than others who drank a placebo hot drink, in a Harvard study. The amino acid that's responsible for this immune boost, L-theanine, is abundant in both black and green tea—decaf versions have it, too.
Your optimal dose: Several cups daily. To get up to five times more antioxidants from your tea bags, bob them up and down while you brew.
5. Chicken Soup
When University of Nebraska researchers tested 13 brands, they found that all but one (chicken-flavored ramen noodles) blocked the migration of inflammatory white cells—an important finding, because cold symptoms are a response to the cells' accumulation in the bronchial tubes. The amino acid cysteine, released from chicken during cooking, chemically resembles the bronchitis drug acetylcysteine, which may explain the results. The soup's salty broth keeps mucus thin the same way cough medicines do. Added spices, such as garlic and onions, can increase soup's immune-boosting power.
Your optimal dose: Have a bowl when you're feeling crummy.
Australian Healthy Food Guide
Zinc deficiency is one of the most common nutritional shortfalls among adults, especially for vegetarians and those who've cut back on beef, a prime source of this immunity-bolstering mineral. And that's unfortunate because even mild zinc deficiency can increase your risk of infection. Zinc in your diet is very important for the development of white blood cells, the intrepid immune system cells that recognize and destroy invading bacteria, viruses, and assorted other bad guys, says William Boisvert, PhD, an expert in nutrition and immunity at The Scripps Research Institute in La Jolla, CA.
Your optimal dose: A 90g serving of lean beef provides about 30% of the Daily Value (DV) for zinc. That's often enough to make the difference between deficient and sufficient.
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8. Fortified Cereals
Not a beef person? Many fortified bowls of cereal pack an entire day's worth of zinc. For example, a serving of Kellogg's Smart Start, has about 15 mg of the mineral. For a point of comparison, a 90g serving of beef has about 7 mg of zinc.
Your optimal dose: Aim for one bowl daily.
9. Orange Fruits And Veggies
You may not think of skin as part of your immune system. But this crucial organ, covering an impressive 16 square feet, serves as a first-line fortress against bacteria, viruses, and other undesirables. To stay strong and healthy, your skin needs vitamin A. "Vitamin A plays a major role in the production of connective tissue, a key component of skin," explains Prevention advisor David Katz, MD, director of the Yale-Griffin Prevention Research Center in Derby, CT. One of the best ways to get vitamin A into your diet is from foods containing beta-carotene, like sweet potatoes, carrots, squash, canned pumpkin, and cantaloupe.
Your optimal dose: A half-cup serving only carries 711kJ but packs 40% of the daily recommended intake of vitamin A (in the form of beta-carotene).
For centuries, people around the world have turned to mushrooms for a healthy immune system. Contemporary researchers now know why. "Studies show that mushrooms increase the production and activity of white blood cells, making them more aggressive. This is a good thing when you have an infection," says Douglas Schar, DipPhyt, MCPP, MNIMH, director of the Institute of Herbal Medicine in Washington, DC.
Your optimal dose: Shiitake, maitake, and reishi mushrooms appear to pack the biggest immunity punch; experts recommend at least 10g-30g a few times a day for maximum immune benefits. Add a handful to pasta sauce, sauté with a little oil and add to eggs, or heap triple-decker style on a frozen pizza.
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In an 80-day Swedish study of 181 factory employees, those who drank a daily supplement of Lactobacillus reuteri—a specific probiotic that appears to stimulate white blood cells—took 33% fewer sick days than those given a placebo. Though not every brand of kefir uses this specific strain, many Lifeway products do, including their Perfect12 and Lowfat Kefir beverages.
Your optimal dose: Try working kefir into your diet at least a few times per week.
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12. Fatty Fish
Salmon, mackerel, and herring are rich in omega-3 fats, which reduce inflammation, increasing airflow and protecting lungs from colds and respiratory infections. One animal study also found that the nutrient may help ward off various strains of the flu.
Your optimal dose: Consume two or three 4-ounce servings per week. In you're pregnant or breastfeeding, avoiding eating shark, swordfish, king mackerel, and tilefish, which are high in mercury.