Probiotics are gut-friendly critters available from supplements and fermented foods and have been shown to have a positive health effect. The trick is to figure out which probiotics work for you, which don't, and how to know the difference.

How probiotics work
Your gut plays host to a delicate balance of good and bad bacteria. Call it a synergy of sorts. If the mix is right, you can have "bad" bacteria in your system and still be healthy. You get sick when the bad bacteria start to outweigh the good, and probiotics ensure that you have an abundant supply of the bacteria you need to stay healthy.

There's evidence that probiotics are effective in treating diarrhea and supporting the immune system, says Mary Ellen Sanders, Ph.D., a probiotic expert and consultant for the food and supplement industry. In fact, many people first hear of probiotics in the context of replacing good bacteria when they use an antibiotic, which wipes out all of your bacteria and can result in diarrhea. Some health care practitioners believe that probiotics can help treat other conditions, such as yeast overgrowth, irritable bowel syndrome (IBS), eczema and lactose intolerance, and that they can protect against cancer.

How to select a probiotic
Not all probiotics are created equal, and this makes selecting one confusing. Different strains are used to treat different health conditions, so the results of studies using specific strains can't be generalized to probiotics as a whole. What's more, some supplement companies know how to process probiotics to ensure their efficacy, while others don't.

"'Probiotic' does not have a legal definition," says Sanders. "There is no third-party verification for probiotics, so the consumer has to figure out how effective a probiotic is, case by case. This is an emerging science. What we know today is only a piece, and we don't necessarily understand fully who will benefit and how."

Sanders suggests logging on to probiotic manufacturers' Web sites, where studies will often be posted that substantiate the companies' probiotic claims. This will help you figure out which probiotic works, and for which condition. "If someone came to me and said, 'What do you suggest for IBS?' I'd say they should go to the product that has been tested for that," says Sanders. This would be better than using a general probiotic that may contain strains that haven't been tested for IBS. When in doubt, Sanders says to go for two strains that are tried and true: Lactobacillus and Bifidobacterium. The product label, whether for a supplement or a food, will tell you which type of probiotic the product contains.

There's also confusion regarding whether a probiotic needs to be refrigerated or is shelf-stable. Both storage techniques can work, says Sanders, depending on how the probiotic was made, so pay attention to the manufacturer's directions on the label and store your probiotic accordingly. This applies to probiotics in food and in supplement form. "The type of storage and the ways these are preserved has everything to do with the specific microbes and the type of stabilization used," says Sanders.

Best ways to take probiotics
There are a number of ways to fit probiotics into your diet. They occur naturally in fermented foods, are added to foods and come in supplement form as a powder or pill. The supplements are easy to include in your daily vitamin routine, but many experts prefer to eat their probiotics. "Ideally, if we had them naturally in our diet we wouldn't need to take them as a supplement," says Alex Kraft, a doctor of naturopathy (ND) and licensed acupuncturist. Sanders agrees. "Foods work for me; I'm a big yogurt eater. I buy yogurt and that is where I get my fermented microbes," she says.

Look for probiotics in foods such as miso soup and sauerkraut. Many companies add probiotics to yogurt, kefir, juice, granola and other cereals, candy bars and cheese. There's no added nutritional value from a probiotic and it doesn't change the way a food tastes, but the bacterial benefits are "critter-licious."