What they are
Probiotics are strains of bacteria and yeast that are helpful to the human body. Many species of the bacteria Lactobacillus (including L. acidophilus) and Bifidobacterium, as well as the yeast Saccharomyces boulardii, have proven health benefits.

Whether from supplements or food sources, probiotics help reestablish healthy intestinal flora. This is critical anytime these friendly bacteria are flushed out by diarrhea or decimated by antibiotics. Probiotic bacteria also help control the overgrowth of vaginal yeast (candida) that triggers yeast infections in women. Furthermore, they aid digestion, stimulate immune function, and help prevent diarrhea. Probiotics may also help prevent colon cancer, allergies, dermatitis, and post-surgery infection, as well as treat inflammatory bowel disease (Clinical Reviews in Allergy & Immunology, 2002, vol. 22, no. 3) and Crohn’s disease (Annals of Nutrition and Metabolism, 1996, vol. 40, no. 3).

How they work
In the intestines, bacteria compete for real estate where they can establish colonies. When the intestine hosts large colonies of beneficial bacteria, a stray harmful germ (such as E. coli) cannot multiply to dangerous numbers simply because it can’t find any unoccupied space on the intestinal wall to call home.

Probiotics produce organic compounds (including lactic acid, hydrogen peroxide, and acetic acid) that increase intestinal acidity; this inhibits the reproduction of many harmful bacteria. Probiotics also produce substances called bacteriocins that act as natural antibiotics to kill undesirable micro-organisms. In addition, probiotics enhance immune function by boosting disease-fighting cells (Journal of Nutrition, 2000, vol. 130, no. 2 Suppl).

Traveler’s diarrhea afflicts as many as half of tourists traveling to exotic locales. Studies show that taking probiotic supplements throughout a trip can reduce the risk of being waylaid by “Montezuma’s revenge” (American Journal of Clinical Nutrition, 2001, vol. 73, no. 2 Suppl). In one study of American tourists, taking supplement L. rhamnosus GG throughout their trip cut the chance of getting diarrhea in half (Journal of Travel Medicine, 1997, vol. 4, no. 1).

Anytime your doctor prescribes antibiotics, there’s a good chance you’ll end up with a case of the runs. Antibiotics indiscriminately kill off bacteria both the bad ones causing your condition and the good ones that live in your intestines and (if you’re a woman) in your vagina. As good bacteria die off, harmful ones have a better chance of thriving and causing diarrhea.

S. boulardii can prevent antibiotic-associated diarrhea (International Journal of Antimicrobial Agents, 2002, vol. 20, no. 5).

Diarrhea is particularly troublesome in infants and young children because their small bodies become dehydrated more quickly. Studies show that the Lactobacillus GG strain, in particular, helps end diarrhea episodes in children (Journal of Pediatric Gastroenterology and Nutrition, 2000, vol. 30, no. 1).

Yeast infections which can be triggered by antibiotic use as well as hormone changes and pregnancy also can be prevented and treated by probiotics. Studies show benefits from taking probiotic supplements, eating live-culture yogurt, and even applying yogurt directly in the vagina (Journal of the American Medical Association, 1996, vol. 275, no. 11).

Side effects
Probiotics are very safe. Among the small number of problems that have been reported is an internal fungal infection, called fungemia, which is linked with the use of S. boulardii by critically ill or immuno-compromised patients.

S. boulardii is not associated with any safety issues in children or adults with normal immune function (Intensive Care Medicine, 2002, vol. 28, no. 6).

How to take them
Probiotic-rich food choices include live-culture yogurt, kefir, cottage cheese, and some brands of milk. Labels note which species of bacteria are present.

You can buy probiotic dietary supplements in powder, liquid, capsule, and tablet forms. Many of these products require refrigeration for the bacteria to remain viable. However, recently developed probiotic supplements are stable at room temperature; these are best to pack for trips.

The minimum recommended dose to prevent and treat common conditions is 1 billion live organisms per day. But it is often preferable to use more, such as 2 billion to 6 billion live organisms, taken in divided doses. For yogurt, 1 billion live organisms translate to about 8 ounces per day. This amount can vary widely because manufacturers are developing foods with higher probiotic levels, which are noted on the label.

There are numerous strains of probiotics; it’s best to use those proven in clinical studies to be effective for specific conditions. All probiotics aid digestion and improve immunity. Lactobacillus, Bifidobacterium, and S. boulardii all help with diarrhea. For yeast infections, take Lactobacillus.

Cost
Acidophilus supplements cost $5 to $15 per month; S. boulardii cost as little as $5 per month.

Victoria Dolby Toews, MPH, is the author of User’s Guide to Sexual Satisfaction (Basic Health, 2003).