Lately, your digestion feels off; in fact, you've caught a few more colds than usual and your overall vitality seems a bit compromised. Must be time to detox, right? The truth is that solely focusing on flushing toxins from your digestive system is a bit like cleaning the pool filters but forgetting to add chlorine. “We tend to think of gut maintenance as only removing poisons and neglect to think of what we need to add to our system to keep it healthy,” says Gary Huffnagle, PhD, professor of internal medicine, microbiology, and immunology at the University of Michigan Medical Center in Ann Arbor, and co-author of The Probiotics Revolution (Bantam, 2007). That's where probiotics (or friendly bacteria) come in.

At any given time, there are about 100 trillion bacterial microbes living inside your body — enough microscopic beings to fill up a quart jar — most of which reside in the digestive tract. “The digestive system is like a rainforest — teeming with life,” explains Elizabeth Lipski, PhD, CCN, a nutritionist in private practice in North Carolina and author of Digestive Wellness (McGraw-Hill, 1995). When friendly bacteria levels outnumber the bad, the body is in stasis. But the by-products of modern life often throw this delicate balance out of whack. “The combination of a typical Western diet, the high stress levels of modern life, and an over-reliance on antibiotics is the equivalent of clear-cutting parts of our internal ecology,” Lipski explains. In other words, if you're a typical member of Western society, it's likely time to reforest your internal landscape.

There are thousands of probiotic strains, or friendly flora, found naturally in everything from breast milk to pickles. When ingested they actively promote overall health in many ways. Probiotics take up room and resources in the digestive tract and make it inhospitable to unfriendly microbes. They encourage regularity: In a 2006 Spanish study, daily probiotic consumption increased the frequency and volume of bowel movements, and a 2007 study found that the probiotic strain Lactobacillus rhamnosus GG greatly reduced acute diarrhea in children. Good bacteria also manufacture a range of B vitamins, which the body cannot create or store on its own and which offset the effects of stress; vitamin K, which bolsters bone density; and enzymes that aid metabolism.

And the benefits of probiotics extend well beyond the realm of nutrition and digestion. “Because the digestive system is our first line of defense against harmful bacteria carried in through food, drink, or air, probiotics help the immune system function correctly,” Huffnagle says. In recent studies, probiotics reduced the duration and severity of colds, cut down recurrences of eczema outbreaks, increased the effectiveness of the flu vaccine, and decreased the risk of diabetes.

If you suffer from constipation or diarrhea, frequent colds, yeast infections, or inflammatory or autoimmune conditions (such as IBS, allergies, asthma, or rheumatoid arthritis), it's likely your probiotic levels need a boost. But even if you're relatively healthy, increasing your intake of probiotics through diet and supplements tips the balance in your favor.

Revamp your diet

Natural sources of probiotics include fermented foods, such as sauerkraut, or cultured foods, such as yogurt. Pay attention to your intake of prebiotics, too. Prebiotics are probiotics' favorite foods and help them thrive (see “What are prebiotics?” page 31). But maintaining a healthy gut requires more than just eating a cup of yogurt with berries now and then. “In order to keep your bacteria levels in balance with diet alone, you have to commit to eating fermented and cultured foods every day,” Lipski says. While that may be possible during quiet times, when you can eat the majority of your meals at home, one spurt of busyness can derail your efforts. Luckily, probiotics come in easy-to-take supplements (for help choosing the right one, see "Probiotic shopping tips").