How Probiotics Work

To understand how probiotics work, it is important to understand a little about the microbiology and physiology of the human gastrointestinal tract. Human beings, like all animals, play host to many types and high numbers of microbes on our skin, in our mouths, in women's vaginal tracts, and all the way through our gastrointestinal tract. In fact, it has been estimated that there are more microbes associated with the human body (about 1 trillion bacterial cells) than there are human cells in it. In addition to this very large number of bacteria, there also is a very large diversity of bacteria. It has been estimated that more than 400 different species, or types, of bacteria make their homes on humans.

Taking this into consideration, it is not surprising that microbes have been found to play an important role in human health. Most of these bacteria are not harmful, and in fact contribute positively to normal human growth and development. But some of these bacteria can have negative influences. It is therefore important that the balance of microbes be maintained to favor the beneficial bacteria over the potentially harmful ones.

Health Effects Of Probiotics

There have been hundreds of papers published on the health benefits associated with probiotic cultures. The field of probiotics is accelerating rapidly. A recent search of the bibliographic database, Medline, is a perfect example of how the science has grown over the past ten years. Prior to 1990, only five citations appeared in a search for "probiotic" with zero citations for probiotic when the search was limited to clinical trials. This is a stark contrast to a search of publications from 2002 through March 16, 2004, which returned 774 probiotic citations and 77 citations limited to clinical trials.

Some of these publications describe clinical studies designed to determine how probiotic cultures may influence a variety of health conditions. These are very complicated questions, and research is still actively being conducted to clarify the role of probiotics in human health.

When considering the health effects of probiotics, it is important to recognize that different strains, species and genera of bacteria may have different effects. For the most part, specific clinical studies on probiotics are done with one defined strain or a defined blend of strains. The following discussion, therefore, should be taken as a general description of probiotic activity, keeping in mind that any one effect may have been documented with only one or a limited number of probiotic strains.

Although any one study may not give a complete picture of effectiveness, an evaluation of the body of research done on probiotic cultures suggests that certain strains consumed at adequate levels positively influence human health. The following describes some of the proposed health benefits of consumption of probiotic cultures.

Scientific Support For Efficacy

In the United States, it is essential to have scientific substantiation if a statement (known as a structure/function statement) about the effect of probiotics on the normal functioning of the human body is made on a food or dietary supplement product or during promotion of the product. The burden of proof rests with the manufacturer. Although the FDA does not require premarket approval of such statements, the manufacturer must provide scientific justification for use of any health statements if asked by the FDA.

The following provides a rationale for probiotic impact on a variety of health targets:

Diarrhea: Many types of diarrheal illnesses, with many different causes, disrupt intestinal function. The ability of probiotics to decrease the incidence or duration of certain diarrheal illnesses is perhaps the most substantiated of the health effects of probiotics. A paper published in 2002 reviewed nine studies on the effect of Lactobacillus as therapy for diarrhea in children. This paper (Van Niel, et al. 2002. Pediatrics 109:678-684) concluded that "Lactobacillus is safe and effective as a treatment for children with acute infectious diarrhea." Although this meta-analysis can be criticized for combining data from different species and strains of Lactobacillus into one analysis, the positive nature of the conclusion suggests that at least for this indication and for these strains, positive results have been obtained.

Antibiotics: One group assessed for the impact of probiotics is people on antibiotic therapy. The purpose of antibiotics is to kill harmful bacteria. Unfortunately, they frequently kill normal bacteria as well, often resulting in disruption of the bacterial flora, leading to diarrhea and other intestinal disturbances. Replenishing the flora with normal bacteria during and after antibiotic therapy seems to minimize disruptive effects of antibiotic use. A paper published in 2002 reviewed seven studies (881 total patients) on the impact of probiotics (Lactobacillus rhamnosus GG or Saccharomyces boulardii) on antibiotic-associated diarrhea (Cremonini, et al. 2002. Aliment. Pharmacol. Ther. 16:1461-1467). The paper concluded that evidence suggests that probiotic can be used to prevent antibiotic associated diarrhea, but that no strong effect on the ability of probiotics to treat diarrhea exists. Not all studies have shown positive results in the prevention of antibiotic associated diarrhea or other symptoms associated with antibiotic therapy.

Travelers Diarrhea: Studies evaluating the effect of probiotics on travelers' diarrhea are equivocal. There is a need for further research in this area for more convincing findings.

Irritable Bowel Syndrome: Irritable bowel syndrome (IBS) is a functional bowel disorder that can be characterized by symptoms of abdominal pain, cramps, gas, bloating, diarrhea and constipation. Surveys estimate the prevalence rate ranging from 10-20% of the adult population and the condition is diagnosed 3 times more often in women than men (FDA Consumer Magazine, July-Aug, 2001). Only a few controlled studies have been conducted evaluating probiotics and IBS. Some symptom relief (primarily from diarrhea or abdominal pain or bloating) has been reported in studies published to date.

Inflammatory Bowel Disorder: Inflammatory bowel diseases such as ulcerative colitis and Crohn's disease, are serious intestinal diseases that can ultimately lead to the surgical removal of the colon. The cause of these diseases are not known but it has been hypothesized that an intolerance to the normal flora (bacteria) in the gut leads to inflammation and resulting pathology. The role of gut flora in the progression of these diseases has led some researchers to study the impact certain probiotic bacteria might have on maintaining the state of reduced inflammation that occurs during remission stages of the diseases. Several controlled, clinical trials have shown that high levels of certain probiotic strains can extend the disease-free remission period. Studies also have documented this effect on remission of pouchitis. Additional research in this area is progressing in Europe and the US.

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