Dr. Greene, my daughter often gets yeast infections. I've heard that active-culture yogurt can prevent or treat vaginal yeast infections and also diarrhea. Is this true? Or is it just an old wives' tale?

One of those ideas is true and one is not, but the larger truth about yogurt may surprise you!

In school, my daughter is studying the diversity of ecosystems from the Great Barrier coral reef near Australia to the equatorial rainforests of the Amazon River basin. Her eyes widened when I described to her the amazingly complex ecosystem hidden in the dark recesses of our intestines.

As many as 500 different species of bacteria form a society there, in a delicate balance with each other and with their 'earth' - us. We are outnumbered in our own bodies¹ - there are more bacterial cells in us and on us than there are human cells. The beneficial bacteria produce critical enzymes that we need for our health. When everything is going well, we all benefit.

But sometimes things don't go so well. When we take an antibiotic, it can be like clear-cutting rainforest land. Entire species are eliminated. It's a devastating emergency for the species there.

The short-term results of taking antibiotics include diarrhea (comparable to erosion) and yeast infections (as more primitive species takes over the vacated niches).

The opposite of antibiotics are probiotics - a term coined in 1965 to describe substances that favor the growth of beneficial microorganisms in the body.²

The idea of giving yogurts and fermented milks to promote health has been around for millennia³  - far older than most old wives' tales. Buttermilk, feta cheese,⁴ and active-culture yogurts are among the foods that have been used. Almost a century ago, Russian microbiologist Elie Metchnikoff suggested that consuming the live microbes infermented milk products may be, at least in part, responsible for the longevity of certain ethnic groups.⁵

Is this quackery or solidly scientific? A flood of recent research made probiotics a hot topic at the World Congress on Pediatric Gastroenterology, Hepatology, and Nutrition in August 2000. Two species of probiotics, Bifidobacterium and Lactobacillus, have been the most studied

Some benefits have been well established for some time. Bacteria that produce the enzyme lactase will help reduce lactose intolerance.⁶  Probiotics can treat other enzyme deficiencies (e.g. sucrase maltase deficiency) as well.⁷

Could probiotics be used to counteract episodes of diarrhea that are caused by antibiotics? In a placebo-controlled study, Lactobacillus was given to children along with antibiotics, resulting in fewer cases of diarrhea and milder diarrhea for those who did get it.⁸

How about treating other types of diarrhea? Active-culture foods are somewhat effective at preventing and treating bacterial diarrhea, including Clostridium,⁹ Shigella, Salmonella,¹⁰  and the dreaded E.