Everything You Need to Know About Probiotics

It's official: After centuries of trying to sanitize and sterilize bacteria off the face of the planet, Americans have embraced the notion that some microbes might actually be good for us.

That's something Europeans have known for a long time. People in 35 countries around the world regularly down an immune booster called Actimel; its French manufacturer, Danone, sells more than 3 billion single-serving bottles a year, or more than 8 million a day. And the Japanese have been tossing back a bacteria-laden drink called Yakult for more than 70 years.

Now Americans are running to the grocery store and supplement aisle to catch up. Dannon, the U.S. subsidiary of Danone, now sells Actimel here under the name DanActive. And their Activia yogurt exploded onto the US market last year, racking up an unheard-of $130 million in first-year sales.

Scientists have long known that bacteria play an important role in maintaining your health. Some 100 trillion microbes call the human body home; in your gastrointestinal tract, 500 to 1,000 different types of bacteria help crowd out harmful germs, speed the digestion of food, and keep your immune system functioning properly.

But even though your body already hosts millions of "good" bugs, you could probably use a few more. In one 2005 study, researchers gave 94 employees of a Swedish company a daily dose of Lactobacillus reuteri (100 million "colony forming units," or CFU) for 80 days. The workers used less than half as many sick days as did 87 employees who took a placebo. In other words, a probiotic a day kept the doctor away.

Probiotics will someday be recognized as "a new essential food group," predicts Gary Huffnagle, PhD, a professor of internal medicine at the University of Michigan Medical School and a prominent probiotics researcher. "I believe we'll eventually have research-based minimum daily requirements for probiotics," he says, "just as we do for many vitamins and minerals."

For now, it's clear that probiotics can help you get and stay healthy—if you take the right ones. Here's a guide to what they are, what they do—and how to get the biggest bang from your bugs.

What Can Probiotics Do for Me?

The simplest answer is: They can protect your stomach from a variety of ills. Research has shown that several types of bacteria are effective against diarrhea caused by viruses or antibiotics.

And a British study found that women who took a daily dose of 100 million CFU of Bifidobacterium infantis (also called B. infantis or Bifantis) reduced their symptoms of irritable bowel syndrome (IBS) by 20 percent more than those who took a placebo.

Gastroenterologist Charlene Prather, MD, MPH, an associate professor of internal medicine at Saint Louis University School of Medicine, now "prescribes" a probiotic capsule called Align to some of her patients with severe IBS. The condition had left some of them unable to travel or even attend kids' soccer games, but the probiotic "helps them do more of the things they want to do," Prather says.

Probiotics also seem to boost your immune system. A handful of studies show they can shorten or prevent illness. Recent research points to a possible reason: A probiotic brew increased activity of natural killer cells—part of the immune system's early defense team against invaders.

That may be why two Lactobacillus strains (L. reuteri and L. rhamnosus) seem to help control vaginal infections. Last year, two reports showed that a supplement combining those strains, sold as Fem-dophilus, helped get rid of bacterial vaginosis. Women taking medicine plus the supplement were twice as likely to be cured as women who took only the drug.

Studies suggest that other strains ease eczema and allergies. In Finland, researchers halved the incidence of eczema among babies by giving L. rhamnosus (also called Lactobacillus GG) to 77 mothers late in pregnancy, and then to the breastfeeding moms or to the infants for their first 6 months. Researchers suspect that the strain might help adults, too.

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