What are probiotics?

Quick Answer
Helpful, “friendly” bacteria that offer specific health benefits.
Expert Answers
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Lactobacillus acidophilus is a friendly strain of bacteria used to make yogurt and cheese. Although humans are born without it, acidophilus soon establishes itself in the intestines and helps prevent intestinal infections. Acidophilus also flourishes in the vagina, where it protects girls and women from yeast infections.

Acidophilus is one of several microbes known collectively as probiotics (meaning “pro life”), indicating that they are bacteria and yeasts that help rather than harm. Other helpful microbes include the yeast Saccharomyces boulardii and the bacteria L. bulgaricus, L. reuteri, L. plantarum, L. casei, B. bifidum, Streptococcus salivarius, and S. thermophilus.

The digestive tract contains billions of bacteria and yeasts. Some of these internal inhabitants are more helpful to the body than others. Acidophilus and related probiotics not only help the digestive tract function but also reduce the presence of less healthful organisms by competing with them for the limited space available. For this reason, the use of probiotics can help prevent infectious diarrhea.

Antibiotics can disturb the balance of the digestive tract by killing friendly bacteria. When this happens, harmful bacteria and yeasts can move in and flourish. This can lead to vaginal yeast infections (Candida albicans). Conversely, it appears that the regular use of probiotics can help prevent vaginal infections and generally improve the health of the gastrointestinal system. Persons taking antibiotics should probably take probiotics too and should continue probiotics for some time after treatment with antibiotics.


Although probiotic bacteria are helpful and perhaps even necessary for human health, no daily requirement exists. Probiotics are living creatures, not chemicals, so they can sustain themselves in the body unless something comes along to damage them, such as antibiotics.

Cultured dairy products such as yogurt and kefir are good sources of acidophilus and other probiotic bacteria. Supplements are widely available in powder, liquid, capsule, or tablet form. Grocery stores and natural food stores sell milk that contains live acidophilus. In addition to probiotics, related substances known as prebiotics may enhance the colonization of healthy bacteria in the intestinal tract.

Therapeutic Dosages

Dosages of acidophilus are expressed not in grams or milligrams (mg) but in billions of organisms. A typical daily dose should supply about three to five billion live organisms. Other probiotic bacteria are used similarly. The typical dose of S. boulardii yeast is 500 mg twice daily (standardized to provide 3 x 1010 colony-forming units per gram), to be taken while traveling or at the start of using antibiotics and to be continued for a few days after antibiotics are stopped.

Because probiotics are not drugs but rather living organisms that a person is trying to transplant to the digestive tract, it is necessary to take the treatment regularly. Each time this is done, beneficial bacterial colonies in the body are reinforced, so that they can gradually push out harmful bacteria and yeasts.

Probiotics, as living organisms, may die on the store shelf. A study reported in 1990 found that most acidophilus capsules on the market contained no living acidophilus. The situation has improved in subsequent evaluations, but some products remain substandard. The container label should guarantee living organisms at the time of purchase, not just at the time of manufacture. Another approach is to eat acidophilus-rich foods such as yogurt, in which the bacteria are most likely still alive.

To treat or prevent vaginal infections, one can mix two tablespoons of yogurt or the contents of a couple of capsules of acidophilus with warm water. This mixture can then be used as a douche.

Finally, in addition to increasing one’s intake of probiotics, a person can take fructo-oligosaccharides, supplements that can promote thriving colonies of helpful bacteria in the digestive tract. Fructo-oligosaccharides are carbohydrates found in fruit. Fructo means “fruit” and an “oligosaccharide” is a type of carbohydrate. Taking this supplement is like putting manure in a garden; it is thought to foster a healthy environment for the bacteria. The typical daily dose of fructo-oligosaccharides is between 2 and 8 grams.

Therapeutic Uses

Evidence from many double-blind, placebo-controlled trials suggests that probiotics may be helpful for many types of diarrhea and for irritable bowel syndrome. Additionally, probiotics have shown some promise for preventing or treating eczema.

Probiotics may be helpful for controlling symptoms and maintaining remission in ulcerative colitis. However, probiotics may be less useful for inducing remission. For example, when probiotics were added to standard medications used for induction of remission, no additional benefits were seen in a study of people with mild-to-moderate ulcerative colitis.

Probiotics might help prevent colds, possibly by improving immunity. On a related note, one small double-blind study found evidence that the use of the probiotic bacterium L. fermentum improved the effectiveness of the influenza vaccine. The probiotic supplement was taken in two doses. One was taken two weeks before the vaccine, and the other was taken two weeks after.

Although probiotics are widely used to prevent or treat vaginal yeast infections, evidence regarding potential benefit remains incomplete and inconsistent. A small trial of fifty-five women with vulvovaginal candidiasis did demonstrate that daily L. rhamnosus and L. reuteri supplementation for four weeks, combined with single-dose fluconazole (an antifungal medication), decreased symptoms compared with fluconazole alone. Another large and well-designed trial, however, failed to find a Lactobacillus preparation helpful for preventing yeast infections caused by antibiotics. One study did find that probiotics might reduce levels of C. albicans in the mouth.

The bacterium Gardnerella vaginalis can cause a different type of vaginal infection. As with vaginal yeast infections, probiotics have shown some promise for this condition, but evidence remains inconclusive. A trial of sixty-four women taking a single dose of tinidazole for bacterial vaginosis suggests that women who also took daily probiotics capsules had better cure rates at four weeks. A larger trial involving 120 women with a history of bacterial vaginosis found that taking one capsule daily of the probiotics L. rhamnosus, acidophilus, and S. thermophilus reduced recurrence.

The bacterium Helicobacter pylori is the main cause of ulcers in the stomach and duodenum. Antibiotics can kill H. pylori, but more than one type of antibiotic must be used at the same time. Even in this case, the bacteria are not necessarily eradicated. Probiotics may be helpful for this. Evidence suggests that various probiotics can inhibit the growth of H. pylori. While this effect does not appear to be strong enough for probiotic treatment to eradicate H. pylori on its own, preliminary evidence, including several small double-blind trials, suggests that various probiotics may help standard antibiotic therapy work better, reducing side effects and possibly increasing the rate of eradication.

Some preliminary double-blind trials suggest that probiotics might improve cholesterol profile. Various probiotics might be helpful for allergic rhinitis (hay fever).

Milk fermented by probiotics may slightly improve blood pressure levels. While there is some evidence that probiotics can help reduce symptoms of milk allergies, one study found that adding probiotics to cow’s milk formula for infants was not helpful. One study found that the use of probiotics during pregnancy and breast-feeding may decrease the likelihood that a highly allergic nursing mother will pass her allergic tendencies to her breast-fed infant.

One double-blind, placebo-controlled study of seventy people with chronic constipation found some evidence of benefit with L. casei Shirota. Another study found that L. rhamnosus was helpful for chronic constipation in children. A small trial also found benefits in children, this time with a mixture of bifidobacteria and lactobacilli. In another study, a combination of B. lactis and B. longus showed promise for improving bowel regularity in residents of nursing homes. Finally, in a six-week, double-blind, placebo-controlled trial of 274 people with constipation-predominant irritable bowel syndrome, the use of a probiotic formula containing B. animalis significantly improved stool frequency.

A preliminary double-blind, placebo-controlled study found evidence that Lactobacillus GG might potentially be helpful for treating antibiotic-resistant bacteria. This small study followed twenty-three people with severe kidney disease who tested positive for vancomycin-resistant bacteria in the stool. (Vancomycin is one of the main final-option antibiotics for the treatment of resistant bacteria.) The use of a yogurt product containing L. rhamnosus appeared to be more effective at ridding the gastrointestinal tract of these bacteria than did placebo. However, the study had a number of flaws, especially its small size. Note also that participants in this study did not have active infection with antibiotic-resistant bacteria; they were carriers for it. One should not attempt to use probiotics as the sole treatment for active infection with resistant bacteria. The preventive use of probiotics does not appear to help prevent the development of resistant bacterial strains that may arise during antibiotic treatment.

Probiotic treatment has also been proposed as a treatment for canker sores and as a preventive measure against colon cancer, but there is no solid evidence that it is effective. One study found that giving probiotics to certain critically ill people could help prevent multiple organ failure.

Probiotics have shown some promise for helping to prevent cavities by antagonizing cavity-causing bacteria. An open study found hints that probiotics might be helpful for mouth sores caused by Behçet’s syndrome. One small, placebo-controlled study found that the use of L. helveticus might improve sleep quality in the elderly, for reasons that are not clear.

Probiotics have shown some promise in the treatment of infections with the yeast C. albicans. Probiotics are also proposed for the treatment of a theoretically related, but markedly controversial, condition known as candida hypersensitivity syndrome (also known as chronic candidiasis, chronic candida, systemic candidiasis, yeast hypersensitivity syndrome, and candida). As described by some alternative medicine practitioners, yeast hypersensitivity syndrome is a common problem that consists of a population explosion of the normally benign candida yeast that live in the vagina and elsewhere in the body, coupled with a type of allergic sensitivity to it. Probiotic supplements are widely recommended for this condition because they establish large, healthy populations of friendly bacteria that compete with the candida. However, there is no evidence that yeast hypersensitivity is a common problem and virtually no evidence that it exists at all.

In one small, twelve-week study, Lactobacillus GG failed to prove more effective than placebo for the treatment of rheumatoid arthritis. Another study failed to find Lactobacillus GG helpful for dyspepsia (generic stomach pain of unknown origin) in children.

A one-year-long open trial of 150 women failed to find Lactobacillus probiotics effective for preventing urinary tract infections, compared with cranberry juice or no treatment. Other studies, however, including a large (453-participant), three-month, double-blind, placebo-controlled study of a special healthful Escherichia coli probiotic did find benefits.

A substantial study failed to find a mixture of L. casei, L. bulgaricus, and S. thermophilus in yogurt and milk helpful for asthma in children. However, another study found that the combination of a probiotic (B. breve) and a prebiotic (galacto- and fructo-oligosaccharide) may help reduce wheezing in infants with eczema.

Scientific Evidence

Travelers’ diarrhea. According to several studies, it appears that the regular use of acidophilus and other probiotics can help prevent travelers’ diarrhea, an illness caused by eating contaminated food, usually in developing countries. One double-blind, placebo-controlled study followed 820 people traveling to southern Turkey and found that the use of Lactobacillus GG significantly protected against intestinal infection.

Other studies using S. boulardii, including a double-blind, placebo-controlled trial enrolling three thousand Austrian travelers, found similar benefits. The greatest benefits were seen in travelers who visited North Africa and Turkey. The researchers noted that the benefit depended on consistent use of the product, and that a dosage of 1,000 mg daily was more effective than 250 mg daily.

Infectious diarrhea. Probiotics may also help prevent or treat acute infectious diarrhea in children and adults. A review of the literature published in 2001 found thirteen double-blind, placebo-controlled trials on the use of probiotics for acute infectious diarrhea in infants and children; ten of these trials involved treatment, and three involved prevention. Overall, the evidence suggests that probiotics can significantly reduce the duration of diarrhea and perhaps help prevent it. The evidence is strongest for the probiotic Lactobacillus GG and for infection with a particular virus, rotavirus, which causes severe diarrhea in children.

One double-blind, placebo-controlled trial of 269 children (aged one month to three years) with acute diarrhea found that those treated with Lactobacillus GG recovered more quickly than those given placebo. The best results were seen among children with rotavirus infection. Similar results with Lactobacillus GG were seen in a double-blind study of seventy-one children. However, 224 young Chinese children with severe, acute diarrhea found no benefit from lactose-free formula supplemented with bifidobacteria and S. thermophilus, suggesting that probiotics may not be as useful for cases of severe, dehydrating diarrhea. Also, L. rhamnosus GG is not always associated with improvement. When given for ten days to 229 infants from rural India who were hospitalized with acute diarrhea, the probiotic did not reduce the severity of the diarrhea during that time period.

In addition, a double-blind study evaluated the possible benefits of the probiotic L. reuteri in sixty-six children with rotavirus diarrhea. The study found that treatment shortened the duration of symptoms and that the higher the dose, the better the effect. Similar benefits were seen in a placebo-controlled trial of 151 infants and children given the probiotic E. coli Nissle 1917 (a safe strain of E. coli) for twenty-one days for nonspecific (presumably viral) cases of mild to moderate diarrhea.

A double-blind, placebo-controlled study of eighty-one hospitalized children found that treatment with Lactobacillus GG reduced the risk of developing diarrhea, particularly rotavirus infection. A double-blind, placebo-controlled study found that Lactobacillus GG helped prevent diarrhea in 204 undernourished children.

Other studies generally (though not consistently) indicate that the probiotics B. bifidum, S. thermophilus, L. casei, Lactobacillus LB, and S. boulardii (individually and combined with L. reuteri and L. rhamnosus) may also help prevent or treat diarrhea in infants and children. One study found that bacteria in the B. bifidum family can kill numerous bacteria that cause diarrhea.

A large (211-participant), double-blind, placebo-controlled study found that adults with diarrhea also can benefit from probiotic treatment. Another study found that the regular use of probiotics could help prevent gastrointestinal infections in adults.

Antibiotic-related diarrhea. The results of many double-blind and open trials suggest that probiotics, especially S. boulardii and Lactobacillus GG, may help prevent or treat antibiotic-related diarrhea. For example, one study evaluated 180 people, who received either placebo or 1,000 mg of Saccharomyces daily with their antibiotic treatment, and found that the treated group developed diarrhea significantly less often. A similar study of 193 people also found benefit.

A minimum of three more studies involving adults found that various species of Lactobacillus, taken either alone or in combination, were beneficial, even in cases of Clostridium difficile infection, the most serious type of antibiotic-induced diarrhea. However, a study of 302 people found no benefit with Lactobacillus GG. A review of four probiotic studies found insufficient evidence for their effectiveness in the treatment of C. difficile. Finally, although taking probiotic organisms in the proper concentration may be beneficial for antibiotic-induced diarrhea, one study found that consuming fresh yogurt during antibiotic treatments had no significant effect on the incidence of diarrhea.

Other forms of diarrhea. Two double-blind, placebo-controlled studies enrolling almost seven hundred people undergoing radiation therapy for cancer found that the use of probiotics significantly improved radiation-induced diarrhea. Similar evidence supports the use of L. rhamnosus and a special, nonpathogenic form of E. coli. However, of eighty-five women receiving pelvic radiation for cervical or uterine cancer, those who consumed a liquid yogurt preparation enriched with L. casei had no less diarrhea than those who took a placebo drink. Small double-blind studies suggest that S. boulardii might be helpful for treating chronic diarrhea in people with human immunodeficiency virus infection, in persons hospitalized and being fed through a tube, and in people with Crohn’s disease.

Premature infants weighing less than 2,500 grams (5.5 pounds) are at risk for a life-threatening intestinal condition called necrotizing enterocolitis (NEC). In a study that pooled the results of nine randomized, placebo-controlled trials involving 1,425 infants, probiotic supplementation significantly reduced the occurrence of NEC and death associated with it. A subsequent study found similar benefits in very low-birth-weight infants weighing less than 1,500 grams (3.3 pounds).

Inflammatory bowel disease. The conditions Crohn’s disease and ulcerative colitis are in the family of conditions known as inflammatory bowel disease. Chronic diarrhea is a common feature of these conditions.

A double-blind trial of 116 people with ulcerative colitis compared probiotic treatment with a relatively low dose of the standard drug mesalazine. The results suggest that probiotic treatment might be just as effective as low-dose mesalazine for controlling symptoms and maintaining remission. Evidence of benefit was seen in other trials too.

One preliminary study found S. boulardii helpful for mild diarrhea in stable Crohn’s disease. However, two studies failed to find benefit with Lactobacillus probiotics, and in an analysis of eight randomized, placebo-controlled studies, probiotics were ineffective at maintaining remission in persons with Crohn’s disease.

Probiotics might be useful for people with ulcerative colitis who have had part or all of their colon removed. Such people frequently develop a complication called pouchitis, the inflammation of part of the remaining intestine. A nine-month, double-blind trial of forty people found that a combination of three probiotic bacteria could significantly reduce the risk of a pouchitis flare-up in people with chronic pouchitis. Participants were given either placebo or a mixture of probiotics, including four strains of lactobacilli, three strains of bifidobacteria, and one strain of S. salivarius. The results showed that treated people were far less likely to have relapses of pouchitis.

Another study found that probiotics used right after surgery can help prevent pouchitis from developing. One study, however, failed to find benefit with L. johnsonii in people with Crohn’s disease who had undergone a similar operation. Finally, some evidence hints that probiotics might reduce the joint pain that commonly occurs in people with either kind of inflammatory bowel disease.

Irritable bowel syndrome. People with irritable bowel syndrome (IBS) experience crampy digestive pain and alternating diarrhea and constipation and other symptoms. Although the cause of IBS is not known, one possibility is a disturbance in healthy intestinal bacteria. Based on this theory, probiotics have been tried as a treatment for IBS, with some success.

In a six-week, double-blind, placebo-controlled trial of 274 people with constipation-predominant IBS, the use of a probiotic formula containing B. animalis significantly reduced discomfort and increased stool frequency. Also, 266 women with constipation who consumed yogurt containing B. animalis and the prebiotic fructo-oligosaccharides twice daily for two weeks experienced significant improvement compared with women consuming regular yogurt as a placebo. Finally, in another trial of 298 persons with IBS, eight weeks of treatment with beneficial E. coli reduced typical symptoms compared with placebo.

Benefits were seen in eight other small, double-blind trials too, using L. plantarum, L. acidophilus, L. rhamnosus, L. salivarius, and Bifidobacterium, and using proprietary probiotic combinations of various strains. Benefits have also been seen with combination prebiotic and probiotic formulas and prebiotics alone. Other studies have failed to find probiotics more effective than placebo.

Two studies that pooled previous randomized trials on the use of probiotics for IBS came to similar conclusions: Probiotics appear to offer some benefit, most notably for global symptoms and abdominal discomfort. However, these two studies were unable to determine what probiotic species were most effective.

Eczema. The use of probiotics during pregnancy and after childbirth may reduce the risk of childhood eczema. In a large, long-term, double-blind study, 1,223 pregnant women were given either placebo or a probiotic mixture (containing lactobacilli and bifidobacteria) beginning two to four weeks before delivery. Their newborn children then received either probiotics or placebo for six months. The results showed that the probiotics mixture markedly reduced the incidence of eczema (though not of other allergic diseases). However, in a follow-up to this study, researchers found that the probiotic supplementation was not associated with reduced eczema in children followed through the age of five years. The probiotics also had no effect on allergic rhinitis or asthma.

Another study also yielded marginal results, and a third study involving only Lactobacillus found no benefit for the prevention of eczema. This latter study actually demonstrated a modestly increased risk of wheezing bronchitis in infants who took the probiotic. However, some probiotics combined with prebiotics may help to reduce wheezing in infants with eczema. For example, B. breve and a galacto- and fructo-oligosaccharide mixture showed benefit in one randomized study involving ninety infants.

B. longum and L. rhamnosus supplementation did not reduce the incidence of eczema atopic dermatitis or allergic sensitization at twelve months among Asian infants at risk of allergic diseases. Researchers in another study concluded that not all probiotics are created equal. In this placebo-controlled study, L. rhamnosus reduced the incidence of eczema in the children, but a strain of B. animalis did not.

According to some studies, infants who already have eczema may benefit from probiotics. However, a careful review of twelve studies involving 781 children found no convincing evidence that probiotics can effectively treat eczema in this age group.

If probiotics are beneficial for childhood eczema, they are probably more effective at preventing the condition than at treating it. A carefully conducted review of numerous studies cautiously concluded that probiotics may help reduce the risk of eczema in infants and children, particularly those at high risk. Also, two subsequent reviews found that probiotics were more effective at preventing childhood eczema, particularly when given before birth to the pregnant woman and the fetus.

Immunity. A number of studies suggest that various probiotics can enhance immune function. One twelve-week, double-blind, placebo-controlled trial evaluated twenty-five healthy elderly people, half of whom were given milk containing a particular strain of B. lactis and the other half of whom were given milk alone. The results showed various changes in immune parameters, which the researchers took as possibly indicating improved immune function. Another double-blind, placebo-controlled study of fifty people using B. lactis had similar results.

A seven-month, double-blind, placebo-controlled study of 571 children in day care centers in Finland found that the use of milk fortified with Lactobacillus GG reduced the number and severity of respiratory infections. In another controlled trial, probiotics (L. rhamnosus GG and B. lactis Bb-12) given daily to infants in their formula significantly reduced the risk of acute otitis media and of recurrent respiratory infections during the first year of life, compared with placebo.

Benefits were seen in three other large studies, in which probiotics combined with multivitamins and multiminerals helped prevent colds (or helped reduce their duration and severity) in adults. However, a smaller and shorter study failed to find any effect on respiratory infections. Similarly, L. fermentum given to twenty healthy, elite distance runners for four months during winter training was significantly more effective at reducing the number and severity of respiratory symptoms compared with placebo. One study found that Lactobacillus GG or L. acidophilus may improve the immune response to vaccinations.

Cholesterol. An eight-week, double-blind, placebo-controlled trial of seventy overweight people found that a probiotic treatment containing S. thermophilus and Enterococcus faecium could reduce LDL (bad) cholesterol by about 8 percent. Similarly positive results were seen in other short-term trials of various probiotics. However, a six-month, double-blind, placebo-controlled trial found no long-term benefit. Researchers speculate that participants stopped using the product regularly in the later parts of the study.

Safety Issues

Probiotics may occasionally cause a temporary increase in digestive gas. However, beyond that, they do not present any known risks for most people. In one trial of 140 healthy infants, formula supplemented with long-chain polyunsaturated fatty acids and probiotics appeared as safe as standard formula and did not have any effect on infant growth by the end of the seven-month trial.

However, persons who are immunosuppressed could conceivably be at risk for developing a dangerous infection with the probiotic organism itself; at least one person taking immunosuppressive medications has died in this manner.

In a detailed review of four studies, researchers concluded that the use of probiotics did not benefit persons with severe acute pancreatitis. Furthermore, according to one study, the use of probiotics led to an increased risk of mortality in persons with severe acute pancreatitis and should, therefore, be avoided under these circumstances.

Drug interactions. It may be beneficial for persons taking antibiotics to take probiotic supplements at the same time and to continue the probiotics for a couple of weeks after finishing the course of drug treatment. Doing so will help restore the balance of natural bacteria in the digestive tract.


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