By Wesley Canfield

A year ago, I wrote about probiotics and prebiotics in this column. So what's new in the world of good bacteria?

Probiotics, the bacteria “for life,” normally are found in the human intestine and provide health benefits to us.

Not all of the more than 500 species found in humans are beneficial individually, but collectively, they certainly are. In fact, bacteria are credited with maintaining bowel regularity, fermenting nondigestible fiber to provide usable energy and reducing cancer risk. They've also been credited with helping to maintain healthy body weight, which I'll write about in a future column.

There has been recent interest in yogurt-based drinks as sources of healthful bacteria, particularly with respect to the maintenance of a healthy immune system, some 70 percent of which is located in the intestine. Let's look at the evidence.

Several recent clinical studies have been conducted using a particular strain of the probiotic bacterium Lactobacillus casei. The first study, conducted by a commercial research group, found that consumption of a food containing that bacterium was associated with reductions in both the duration and incidence of diarrhea in infants and children under three years of age. However, because no measures of immune function were reported, it is not clear whether the benefit involved effect(s) on the immune system, although that is certainly possible.

A second compared fermented milk or yogurt treatment (with and without the bacterium) on the incidence of winter infections in elderly Italians. It found that three weeks of consumption of either food produced no differences in the incidence of gastrointestinal or respiratory infections, although each reduced the duration of infection from an average of 8.7 days to only 7.0 days.

A third study found that the consumption of a fermented milk beverage had a weak stimulatory effect on the bacteria-destroying capacities of white blood cells in healthy adults. The clinical relevance of that finding is confused by the low magnitude of the response, as well as by the fact that stimulation of immunoglobulins (antibodies produced by the intestinal lining) - a finding indicative of immune enhancement - was not observed.

The effects of several other probiotic bacteria on the immune system have been investigated. One bacterial strain, Lactobacillus rhamnosus GG, is among the most frequently used. Some studies have found it to reduce by one to two days the duration of intestinal symptoms in infants.

What does all of this mean to your food choices? As I see it, there is potential for healthful effects of probiotics in enhancing immune function. But the clinical data still are limited; most have come from studies with children, few of which have assessed effects on immune function directly.

It is estimated that, worldwide, more than $10 billion dollars are spent annually for fermented drinks containing probiotics.

Understanding the roles that such foods may have in supporting good health calls for more complete clinical evidence - a priority that has emerged in scientific discussions about the roles of food in supporting good health.

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