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Bugs in Our Guts—Not All Bacteria Are Bad
How Probiotics Keep Us Healthy

(Released September 2006)

  by Leila Kiani  


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Probiotics Disease and Health Effects


Élie Metchnikoff, the father of modern immunology, spoke optimistically about the possible health benefits of lactic acid-bacteria (LAB) (5). For centuries, folklore suggested that fermented dairy products containing live active cultures are healthful. Recent controlled scientific investigation supports these traditional views, suggesting that probiotics are a valuable part of a healthy diet (6). Research suggests that probiotic bacteria may mediate a variety of health effects through numerous proposed mechanisms (Figure below). Alleviation of lactose intolerance symptoms and anti-diarrhea effects are the best substantiated effects. Anticancer and immune modulation effects are encouraging, but need more through substantiation in humans.

Modulation of the gut microflora (populations and activities) and influence on mucosal immunity are mechanisms of probiotic function with potential to broadly influence human physiology (2, 3). For example, the ability of probiotic bacteria to support the immune system could be important to the elderly or other people with compromised immune function. (It is important that immune compromised individuals ask their doctor before taking any dietary supplement, including probiotics) (6). Much active research focuses on the development of target-specific probiotics containing well-characterized bacteria that are selected for their health-enhancing characteristics. These new probiotics are entering the marketplace in the form of nutritional supplements and functional foods, such as yogurt products (5).

diagram of probiotics benefits
various health benefits from probiotics consumption

A brief assessment of probiotics effects targeted toward several endpoints, with emphasis on results from human studies where possible, follows (3, 6, 2):

♦ 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 health effect of probiotics. Probiotics can prevent or ameliorate diarrhea through their effects on the immune system. Moreover, probiotics might prevent infection because they compete with pathogenic viruses or bacteria for binding sites on epithelial cells (2, 16). In the pediatric population, probiotics appear to benefit viral diarrhea, possibly by increasing the antibody secretory IgA and decreasing viral shedding, suggesting an immunological mechanism ((2).

Studies evaluating the effect of probiotics on travelers' diarrhea are equivocal. Travelers' diarrhea (3 times or more) occurs in residents of developed countries after traveling to subtropical and tropical zones. Drinking Lactobacillus GG strain significantly decreased the incidence of diarrhea in travelers. Studies with more reliable results are still needed through appropriate selection of traveling regions (7).

♦ Inflammatory Bowel Disease

Inflammatory bowel diseases, such as ulcerative colitis and Crohn's disease, are serious intestinal disorders that can ultimately necessitate the surgical removal of the colon. The causes of such diseases are unknown, 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 the diseases' remission stages. Several controlled clinical trials have shown that high levels of certain probiotic strains can extend the disease-free remission period. Additional research in this area is progressing in Europe and the US (6).

Probiotics bacteria have been shown to improve the clinical outcome in many intestinal disease targets (table below) (3).

chart of intestinal conditions mediated by probiotic bacteria

♦ Cancer

In general, cancer is caused by mutation or abnormal activation of genes that control cell growth and division. Many processes or exposures can increase the occurrence of abnormal cells,.among them chemical exposures. Cancer-causing chemicals (carcinogens) can be ingested in a normal diet or generated by metabolic activity of microbes that live in the gastrointestinal system. It has been hypothesized that probiotic cultures might decrease the exposure to chemical carcinogens by (a) detoxifying ingested carcinogens; (b) altering the environment of the intestine and thereby decreasing populations or metabolic activities of bacteria that may generate carcinogenic compounds; (c) producing metabolic products (e.g., butyrate) which improve a cell's ability to die when it should die (a process known as apoptosis or programmed cell death); (d) producing compounds that inhibit the growth of tumor cells; or (e) stimulating the immune system to better defend against cancer cell proliferation (6).

Research suggests that the consumption of probiotic cultures may decrease cancer risk. Researchers testing the effect of the consumption of fermented milks, probiotic bacteria, and components or extracts of bacteria have found (6):

  • A reduction in the incidence of chemically induced tumors in rats.
  • A reduction of the activity of fecal enzymes, postulated to play a role in colon cancer in human and animal subjects.
  • Degradation of nitrosamines.
  • A weakening of mutagenic activity of substances tested in the laboratory.
  • Prevention of damage to DNA in certain colonic cells.
  • In vitro binding of mutagens by cell wall components of probiotic bacteria.
  • Enhancement of immune system functioning.
chart of probiotics that may reduce caner risk

Taken together, these results suggest that probiotic cultures may positively influence the gastrointestinal environment to decrease the risk of cancer. However, more studies are needed to confirm cancer reduction in humans, and conducting these would be very expensive. Only one study has done so, in this case the effect of consumption of Lactobacillus casei fermented milk on recurrence of superficial bladder cancer. The recurrence-free period for the Lactobacillus-consuming group was found to be almost twice as long as the control group.

♦ HIV and Immune System Stimulation.

The immune system provides the primary defense against microbial pathogens that have entered our bodies. The immune system is extremely complex, involving both cell-based and antibody-based responses to potential infectious agents. Immunodeficiency can result from certain diseases (e.g., cancer, AIDS, leukemia) or, to a lesser extent, from more normal conditions such as old age, pregnancy, or stress. Autoimmune diseases (e.g., allergies, rheumatoid arthritis, inflammatory bowel diseases) also can occur due to misdirected immune system activity (6).

Probiotic cultures have been shown in a variety of test systems to stimulate certain cellular and antibody functions of the immune system. Animal and some human studies have shown an effect of yogurt or lactic acid bacteria on enhancing levels of certain immunoreactive cells (e.g. macrophages, lymphocytes) or factors (cytokines, immunoglobulins, interferon)]. In addition, some studies have shown improved survival of pathogen-infected laboratory animals consuming probiotic cultures as compared to animals consuming a control diet. Results accumulated so far suggest that probiotics may provide an additional tool to help your body protect itself (6).

Animal models and human studies (table 3) provide a baseline understanding of the degree and type of probiotic-induce immune response. From these studies, it appears that probiotic bacteria are able to enhance both non-specific and specific immune responses by activating macrophages (3).

chart of probiotic effects in immunocompetent humans

♦ Allergy.

Allergy is on the rise in industrialized nations (see.CSA's "ABC's of Allergies" ( for more information). It is estimated that the incidence of asthma in the United States doubled between 1980 and 2000. Scientists have proposed the 'hygiene hypothesis' to explain the rise in allergic conditions such as asthma and eczema. This hypothesis is based on observations that lower allergy incidence is associated with environments that have greater numbers of microbes, such as day care centers, farms, or in homes with siblings or pets. Sanitary living environments and the consumption of processed foods have limited the number of microbes in the diet. The hypothesis suggests that the exposure of infants to microbes before the age of six months helps the immune system mature to better tolerate allergens later in life (6).

Of course, increasing exposure to microbes must be done safely. This hypothesis led researchers in Finland to conduct a study evaluating the effects of a Lactobacillus strain on incidence of atopic eczema in 132 infants at high risk of developing eczema. The study was double-blinded and placebo-controlled. Pregnant mothers two-to-four weeks before delivery and newborn babies through six months of age were given Lactobacillus rhamnosus GG. Infants were followed through two years of age and incidence of recurring atopic eczema was recorded. The study reported a 50% drop in incidence of recurring atopic eczema in the group receiving the probiotic supplement. A follow up study of these same children indicated that these trends were still present at four years of age. These results suggest that exposure to the right types of microbes early in life may decrease allergy risk (6).

Thanks to Deborah Whitman for all of her help, without which this Discovery Guide would never have been written

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