In the midst of our germ-phobic culture, a quiet revolution is taking place: People have begun clamoring for the health-bestowing benefits of bacteria. Otherwise known as probiotics—micro-organisms that restore the balance of intestinal bacteria and increase resistance to harmful germs—these bacteria have become one of the quickest growing supplements in the U.S. market. According to NutraIngredients.com, a daily online news service, global sales of probiotic supplements are expected to almost double from 2011 to 2016, from 2.7 billion in 2011 to 4 billion in 2016. The strongest growth segment comes from the United States. Troll the aisles of any supermarket and you can now find probiotic-infused chocolate, butter, cereal, tea and juice.

Still, probiotics remain something of an alien supplement, with their impossible-to-pronounce strains and astronomical numbers of CFUs, or colony-forming units. But a respectable body of research suggests that the right strains of probiotics (there are thousands of strains, some more effective than others), taken in a sufficient dosage, can have a significant impact on digestive health, irritable bowel syndrome (IBS), diarrhea and colic—and can even shorten the duration of colds.

How do probiotics work? There's no definitive theory, but one working hypothesis, says Shira Doron, MD, assistant professor in the division of infectious diseases at Tufts Medical Center in Boston, “is that certain probiotic strains colonize the gastrointestinal (GI) tract, thereby inhibiting colonization of other bacteria that might be harmful.” The following three areas have shown the most significant impact of probiotics.

Diarrhea

A review of 82 trials of probiotics, published in the May 2012 issue of the Journal of the American Medical Association, showed that probiotics reduce the risk of diarrhea caused by taking antibiotics. The most common probiotics reviewed were from the genus Lactobacillus (commonly found in yogurt and cheese). This particular strain was found to reduce the rate of antibiotic-associated diarrhea by an impressive 42 percent. Another study, published in the September 2008 issue of the American Journal of Gastroenterology, found that a probiotic supplement containing multiple strains, administered daily for 28 days, reduced diarrhea symptoms in IBS patients compared to the placebo group.

Respiratory illnesses

Probiotics not only aid digestion but may stimulate the immune system as well. A growing body of research shows that intestinal bacteria play an essential role in immune defenses. A study published in the April 2011 European Journal of Nutrition suggests that probiotics can reduce the severity and duration of common cold and flu symptoms.

Allergy prevention

Exposing pregnant mothers and infants to probiotics may also help stimulate immune system growth and perhaps prevent allergies in children. A study published in the March 2008 issue of Pediatric Allergy and Immunology showed that levels of key proteins associated with tissue inflammation were 50 percent higher in the blood of probiotic-treated infants than in the blood of placebo-treated infants. Inflammation, thought to stimulate the immune system, is credited with reducing allergic reactions.

While the research on probiotics is promising, there is “a leap between the studies and consumers’ ability to take the evidence and use it to promote their own health,” says Doron. Sometimes studies don't mention the specific strains or dosage used, making it difficult to replicate the results. Additionally, no studies have demonstrated that probiotics affect general maintenance of health, Doron points out. 

On the bright side, probiotics have no associated side effects. Certain strains work better for particular ailments, so it’s worth doing some research to find an appropriate strain. Your doctor also may help sift through the medical literature to find an up-to-date strain. When it comes to probiotics, think “microsize me.” These infinitesimal microbes have strength in numbers and may wield a surprising impact on your health.