In a study published in August in the journal Pediatrics, scientists looked at the effects of daily probiotic supplements in 300 children aged 3 to 5. The kids were divided into three groups and given two doses a day for six months. One group got a placebo, the second got one type of good bacteria, and the third got a combination of several different types of good bacteria. Over the six-month period, there were fewer fevers, runny noses, and coughing episodes among the children who got either of the supplements containing good bacteria. Those who were given the combination of bacteria were healthiest.

Further, when kids taking the probiotics did get sick, they took less time to recover, required fewer antibiotics, and missed fewer days of child care. The study was funded by a company that makes probiotic products, which could raise questions about its impartiality, but other research is leading to similar conclusions. Solid evidence indicates that probiotics may be effective in reducing the risk and shortening the duration of gastroenteritis in children, decreasing cases of a serious gut infection in premature infants, preventing eczema in children, and preventing diarrhea associated with antibiotics in both adults and children.

The question is why. One theory is that the introduction of good bacteria into the system sets off a competition with bad bacteria, thus stimulating the immune system.

Many physicians consider probiotics safe for healthy individuals because they are identical to germs already found in our bodies. However, they may be harmful to people with certain chronic conditions or compromised immune systems. And more evidence is needed about their usefulness in people with vaginal infections, inflammatory bowel disease, or ailments with bacteria that cause stomach ulcers.

If you decide to take probiotics, here are a few things to keep in mind:

• They are considered food and thus are not regulated as drugs by the FDA. As a result, different brands may vary in quality. A recent investigation found several products with fewer organisms than advertised and even some contaminated products.
• Side effects may include gas or bloating. Severe infections are rare but possible.
• Not all bacteria are created equal. As with antibiotics, different types are better for different conditions.