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In some cases, you can feel good about “bugs” in your food.

Probiotics – the live micro-organisms found in fermented foods including yogurt, kefir, miso, tempeh and sauerkraut – may help boost your immunity.

In one study, participants who took a probiotic supplement (50 million cultures of Lactobacillus gasseri, Bifidobacterium longum and B. Bifidum) over two winter/spring seasons shortened any colds they got by about two days and lessened their symptoms, compared with those who took a placebo.

Additional research – reviewed in Pharmacological Research – found that L. Casei (often added to yogurt) boosts T-cell production, our immune system’s specific, targeted line of defense. Scientists think probiotics help your GI tract’s natural bacteria to block pathogens from being absorbed.

Research is promising, but it’s unclear why certain strains work or how many we actually get in a product. (Yogurt labeled “Live and Active Culture” guarantees 100 million cultures per gram – about 17 billion cultures in a 6-ounce cup.)

Even if a number is listed, some of the probiotics may die during storage or be destroyed during digestion. We also don’t know how many probiotics are needed to be effective (some say it’s the amount in a spoonful of yogurt, others say a cup).

Bottom Line: Stick to probiotics from fermented foods. You’ll get a dose of healthy bacteria plus other good-for-you nutrients (think: calcium in yogurt, etc.). Supplements might help, but they may not deliver the strain or dose promised.

Brierley Wright is a registered dietitian