Probiotic, derived from the Greek word meaning “for life,” is defined as a live microbial food ingredient that is beneficial to the health of the host . The concept of using bacteria to prevent and cure disease originated over a century ago with Nobel Prize winner Elie Metchnikoff's view that the regular consumption of fermented dairy products led to the long and healthful lives of Bulgarian peasants . Although Europeans have traditionally accepted the potential health benefits of probiotics, North Americans have not always shared this same enthusiasm .
The lactic acid bacteria (LAB), including Lactobacillus species and Bifidobacteria species, are the most common types of probiotic bacteria. LAB are frequently utilized by the food industry to convert carbohydrates into lactic acid, which produces the sour tastes of yogurt and other foods, and lowers food pH so as to reduce the growth of spoilage organisms. Lactobacilli and other probiotics occur naturally in many fermented foods, including yogurt, sauerkraut, kefir, yakult, cabbage kimchee, and the soybean products miso, natto, and tempeh. L. acidophilus NCFM (Rhodia, Madison, WI), a commonly used form of probiotic bacteria, was isolated and characterized at North Carolina State University over 30 years ago. It is sold commercially as an additive to low-fat milk as Sweet Acidophilus Milk (North Carolina Dairy Foundation, NCSU, Raleigh, NC). Although fermented dairy products are the most common dietary sources of probiotics in North America, probiotics are also available for consumption in freeze-dried capsular forms. Table 1 lists the various probiotic bacteria that have been tested and used or considered for use commercially [4••,5].
Prebiotics are nondigestible food ingredients that benefit the host by selectively stimulating the growth and/or activity of one to a limited number of bacteria in the colon . Beneficial prebiotics are frequently used in combination with probiotics to stimulate their numbers and functional activities. Most prebiotics are classified as nondigestible oligosaccharide carbohydrates . Examples of inulin-type fructan prebiotics are native inulin, enzymatically hydrolated inulin, oligofructose, and synthetic fructo-oligosaccharides . Both inulin and oligofructose are present in various fruits (eg, bananas), vegetables (eg, garlic, leeks, artichokes), legumes (eg, soybeans), and grains (eg, unrefined wheat, oats, barley). Commercially, prebiotics are used as sugar substitutes, fat replacements, food texture providers, and food stabilizers.
“Functional foods” originated in Japan in the 1980s as foods that are beneficial to the consumer . Both probiotics and prebiotics are important components of functional foods. When used in combination to synergistically provide health benefits, probiotics and prebiotics are often referred to as synbiotics.
Mechanisms and Effects of Probiotics and Prebiotics
The human gastrointestinal (GI) tract is colonized by an estimated 400 different bacterial species and 1014 individual bacterial cells. In Western societies, the most prevalent intestinal bacteria are bacteroides, eubacteria, peptostreptococci, bifidobacteria, enterobacteria, streptococci, lactobacilli, clostridia, and staphylococci . Although certain bacteria are beneficial to the host, others may be harmful. The adult bacterial colonic flora is generally stable in composition, but may be influenced by a number of factors, including age, nutritional requirements, immune status, antibiotic use, stress, alcohol use, pH, transit time, and the presence of material in the gut . The consumption of probiotics may help to reestablish balance to the colonic intestinal flora by changing the intestinal pH and producing various antimicrobial substances including bacteriocidins, organic acids, and hydrogen peroxide [11,12]. Beneficial probiotic bacteria may form temporary colonies that compete with pathogenic bacteria for nutrients and survival. Prebiotics not only enhance the benefits of probiotic bacteria, but can also improve the dietary absorption of certain minerals. Probiotics exert their main activities in the small intestines, whereas prebiotics are most active in the large intestines.
Health Claims and Potential Benefits of Probiotics
Probiotic bacteria have been associated with potential health effects that may aid in the prevention and treatment of various medical conditions. These include lactose intolerance, colon cancer, hypercholesterolemia, hypertension, immune function, mineral GI absorption, intestinal bacterial overgrowth, diarrhea, constipation, irritable bowel syndrome, inflammatory bowel disease, allergic conditions, urogenital infections, and osteoporosis. However, much of the scientific evidence on the efficacy of probiotics is weak, and generalized health improvement claims are difficult to substantiate because different strains of probiotic bacteria exert widely disparate effects on human health.
Certain probiotic bacteria exhibit antimicrobial activity via the production of organic acids (eg, lactic acid and acetic acid), hydrogen peroxide, diacetyl, γ-hydroxypropion, aldehyde, and other bacteriostatic/bacterocidal peptides and proteins . These antimicrobial properties may inhibit the growth of a range of microbes, including pathologic bacteria and bacteria associated with spoilage.
GI tract diseases and function
Many probiotic bacteria exert their most powerful effects on improving general health through the GI tract. This therapeutic effect is dependent on the ability of probiotic bacteria to adhere to and colonize the intestinal mucosa.
Antibiotic-induced, infectious, and traveler's diarrheal illnesses
Saccharomyces boulardii yeast and Lactobacillus bacteria have been shown to have a protective effect on the development of antibiotic-associated diarrhea . It seems likely that these and perhaps other probiotic bacteria compete directly with pathogenic bacteria that may overgrow as a result of antibiotic therapy . Additionally, there is strong evidence that Lactobacillus bacteria are effective in the treatment of community-acquired diarrhea including rotavirus infections [16,17]. Although scientific evidence that probiotics prevent traveler's diarrhea is weak, Lactobacillus GG and S. boulardiia have been shown to be moderately effective [18,19].
Managing lactose intolerance
Fermented dairy product probiotics have been shown to enhance tolerance to lactose compounds in individuals with lactose intolerance .
Prevention of small bowel bacterial overgrowth
The small bowel normally harbors few bacteria. However, individuals with various chronic diseases may develop an overgrowth of bacteria in the small bowel. L. acidophilus NCFM has been shown to positively influence the pathologic colonization of the small bowel by inhibition of dimethylamine and nitrosodimethylane-producing bacterial populations in chronic renal disease dialysis patients .
Irritable bowel syndrome
Some evidence suggests that probiotics may reduce the symptoms of irritable bowel syndrome, although it is not known which types and doses of probiotic bacteria are most effective.
Inflammatory bowel disease
Probiotics may have beneficial effects in the management of inflammatory bowel disease, but human clinical trial studies are few.
Prevention of constipation
Probiotics may help to prevent and/or treat constipation through direct effects on intestinal motility and by increasing fecal bulk.
Prevention of colon cancer
Meat-based diet studies on rats demonstrate a reduced incidence of colon cancer in rats receiving probiotic supplementation . Human studies reveal a two- to fourfold reduction in the activity of various fecal enzymes (γ-glucuronidase, nitroreductase, and azoreductase) that are thought to play a role in the conversion of procarcinogens to carcinogens .
Improvement in immune function
Probiotic bacteria may exert beneficial effects through direct stimulation of the immune system , as evidenced by the improved immune response that follows oral antigen administration after supplementation with L. acidophilus NCFM . Additionally, competitive colonization of the intestinal mucosa stimulated by probiotics may help to prevent systemic candidiasis in immunodeficient mice .
Although cholesterol levels were shown to decline when probiotic bacteria are added to a laboratory growth medium [26,27], the direct cholesterol lowering effects of probiotics in humans are equivocal .
The use of systemically or intravaginally applied probiotic bacteria may help to control the incidence of urogenital infections. This effect may occur through the production of hydrogen peroxide, which inhibits the growth of various urogenital pathogens .
Other claims of improvements in health function and reductions in disease risks
Probiotics have also been reported to help in the prevention and treatment of hypertension, osteoporosis, atherosclerotic coronary vascular disease associated with dyslipidemia, obesity, allergic conditions, intestinal mineral absorption, and inflammation.
Probiotics and the Athlete
Probiotic bacteria are most readily available to the consumer and the athlete in the form of cultured dairy products, such as yogurt. Consumer awareness of the need for dietary calcium and evidence that dairy products can assist in weight loss and weight control has further contributed to an increasing use of cultured dairy products in the population . Among the challenges of administering probiotic bacteria via other delivery vehicles such as capsules, is devising methods to enable the passage of viable bacteria through the low pH of the stomach into the large intestines. Recent scientific developments include the production of a microencapsulated, freeze-dried probiotic form that may be consumed as capsules and as single-serving oral liquid probiotic “shots.” Sales of probiotic shots rose to 8 million dollars in 2004, a 105% increase over sales in 2003, and the compound annual growth rate of these beverages in the United States was 115% between 2000 and 2004 . Among probiotic bacteria-fortified foods that are currently available are certain cereals and beverages.
According to a nationwide survey, approximately 2 million adults in the United States use probiotics for health reasons . Probiotic users are more likely to be female with an average age of 47 years. Reasons given for using probiotics are to maintain health, feel better, promote health, reduce the risk of disease, and treat a specific medical condition such as gastroesophageal reflux disease, irritable bowel syndrome, and constipation . The incidence of probiotic use by athletes is not known.
Potential Ergogenic Effects of Probiotics
A systematic review of the medical literature was conducted to identify and include all English language scientific studies that investigate the potential ergogenic effects of probiotics on human athletic performance. The search of the PubMed database for all years between 1966 and 2007 was conducted on January 31, 2007 and is summarized in Table 2. Overall, 17,177 references were identified that included at least one of the terms probiotics, Lactobacillus, or Bifidobacterium. Of these, 79 articles also included a reference to “human performance,” and four additional articles contained references to “athletics” or “sports.” Abstracts of these 83 articles were reviewed, references were scanned, and related articles were reviewed. Ultimately, only one article was identified that met inclusion criteria [32•]. Using this article, the search was extended to identify related articles. An additional 244 articles were identified. Abstracts of these articles were also scanned and reviewed. This yielded one additional article that met systematic review inclusion criteria . Of the two articles identified, both primarily studied the effects of probiotic bacteria in restoring suppressed immune function to fatigued athletes who engaged in intense athletic training and enhancing immune function in healthy training athletes. Neither article examined the direct ergogenic effects of probiotics in athletes.
The first article was a pre–post intervention study on the effects of L. acidophilus upon immunity in healthy and fatigued athletes. [32•]. Investigators found that fatigued athletes (n = 9) had clinical characteristics similar to those seen in patients who experienced reactivated Epstein Barr virus infections, including significantly less secretion of interferon γ (IFNγ) from blood CD4-positive T-cells as compared with healthy control athletes (n = 18). After 4 weeks of treatment with capsules containing 2 × 1010 colonies of L. acidophilus, the fatigued athletes increased the quantity of IFNγ secretion to levels similar to those of the healthy subjects. L. acidophilus administration to healthy athletes also resulted in increases in mucosal IFNγ concentrations. The authors conclude that probiotic therapy may reverse a T-cell defect in fatigued athletes, and enhance mucosal IFNγ concentrations in healthy athletes. The second article demonstrated that the consumption of fermented milk (500 mL/d) containing L. casei probiotic bacteria for one month limits the routine postexercise observed decrease in natural killer cell activity . Although these articles do not specifically identify an ergogenic role for probiotic therapy they both suggest that immune function may be enhanced in athletes undergoing intense physical training.
This systematic review of the current medical literature did not identify any scientific evidence that the consumption of probiotic bacteria has an ergogenic effect on athletic performance. Probiotics may, however, provide the athlete with secondary benefits by helping to maintain an overall state of good general health, enhance immune function, and restore a suppressed immune function that may result from intensive athletic training. Additional potential health benefits for the athlete involve the maintenance of a healthful GI tract microflora that may possibly prevent and/or treat diarrheal illnesses resulting from travel, infectious agents, and antibiotic use. The review underscores the need for additional clinical trial research to further evaluate the potential health and athletic performance benefits of probiotics.
References and Recommended Reading
Papers of particular interest, published recently, have been highlighted as: • Of importance •• Of major importance
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