What do they do?

Probiotic bacteria favorably alter the intestinal microflora balance, inhibit the growth of harmful bacteria, promote good digestion, boost immune function, and increase resistance to infection.^{1 2} People with flourishing intestinal colonies of beneficial bacteria are better equipped to fight the growth of disease-causing bacteria.^{3 4} Lactobacilli and bifidobacteria maintain a healthy balance of intestinal flora by producing organic compounds—such as lactic acid, hydrogen peroxide, and acetic acid—that increase the acidity of the intestine and inhibit the reproduction of many harmful bacteria.^{5 6} Probiotic bacteria also produce substances called bacteriocins, which act as natural antibiotics to kill undesirable microorganisms.⁷

Immune function tends to decline with age. Twice daily supplementation with Bifidobacterium lactis (a particular strain of bifidobacteria) in milk was found in a double-blind trial to significantly enhance various aspects of immune function in a group of healthy elderly people.⁸ Benefits were apparent after only six weeks of supplementation. Yogurt has been purported to support immune function, due to its inclusion of lactic-acid bacteria.⁹ While B. lactis is a different organism than that found in yogurt, effects on immunity may be similar.

Regular ingestion of probiotic bacteria may help prevent vaginal yeast infection.^{10 11} A review of the research concluded that both topical and oral use of acidophilus can prevent yeast infection caused by candida overgrowth.¹²

Diarrhea flushes intestinal microorganisms out of the gastrointestinal tract, leaving the body vulnerable to opportunistic infections. Replenishing the beneficial bacteria with probiotic supplements can help prevent new infections. The incidence of "traveler’s diarrhea," caused by pathogenic bacteria in drinking water or undercooked foods, can be reduced by the preventive use of probiotics.¹³

Most people associate lactobacilli with L. acidophilus, the most popular species in this group of probiotic bacteria. However, research shows that other Lactobacillus species may be beneficial as well. For example, L. rhamnosus and L. plantarum appear to be protective intestinal bacteria. They are involved in the production of several "gut nutrients," such as short-chain fatty acids, and the amino acids, arginine, cysteine, and glutamine.¹⁴ These beneficial bacteria may also help remove toxins from the gut and exert a beneficial effect on cholesterol levels.¹⁵

In a double-blind trial, administration of a preparation containing L. plantarum to people with acute pancreatitis reduced the number of complications severe enough to require surgery.¹⁶

One probiotic, Saccharomyces boulardii, has prevented diarrhea in several human trials.¹⁷ Double-blind research studying critically ill patients found this strain of yeast to prevent diarrhea when 500 mg is taken four times per day.¹⁸

Probiotics are important in recolonizing the intestine during and after antibiotic use. Probiotic supplements replenish the beneficial bacteria, preventing up to 50% of infections occurring after antibiotic use.¹⁹

Probiotics also promote healthy digestion. Enzymes secreted by probiotic bacteria aid digestion. Acidophilus is a source of lactase, the enzyme needed to digest milk sugar, which is lacking in lactose-intolerant people.²⁰

Fructo-oligosaccharides (FOS) are naturally occurring carbohydrates that cannot be digested or absorbed by humans. They support the growth of bifidobacteria, one of the beneficial bacterial strains.²¹ Due to this effect, some doctors recommend that patients taking bifidobacteria also supplement with FOS. Several trials have used 8 grams per day. However, a review of the research has suggested that 4 grams per day appears to be enough to significantly increase the amount of bifidobacteria in the gut.²²