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What do you know about ... probiotics?

The human body is host to around 1014 bacteria, contributing to at least a kilo of total body weight. Most of these (about 80%) live in the intestines and are collectively known as the gut flora. They reside on the gut's mucous membrane, some attached to the gut wall. The majority (80%) of these bacteria are harmless, if not beneficial, and include more than 500 different species.
The gastrointestinal tract of neonates is sterile and does not contain any microorganisms. However, the gut flora is acquired rapidly after birth. The first micro-organisms to come into contact with the gastrointestinal tract are bacteria from the mother. These are followed by bacteria from milk and other environmental factors. Colonisation is complete after approximately one week, but the numbers and species of bacteria fluctuate markedly during the first three months. In breastfed babies, bifidobacteria dominate as they thrive and proliferate on the proteins supplied in breast milk. Bifidobacteria make the infant gut acidic, which creates a barrier against infection from other bacteria to which the child has yet to develop natural immunity. Bifidobacteria fail to thrive on the proteins provided by modified cows' milk used in formula feeds, with the result that many formula-fed babies are more prone to gastrointestinal infections.(2)
Once the child is fully weaned and is consuming solid foods, the gut flora changes as new substances are introduced, which promote the growth of different bacteria. By the age of two years, the floral composition of a child's gut will resemble that of a normal adult; the bacteria that predominate are species of Bacteroides, Bifidobacterium, Eubacterium and Peptostreptococcus. The number of gut bacteria naturally decreases with age, falling by approximately 1,000-fold each decade after the age of 60.

Composition and regulation
A healthy adult's intestinal flora, although extremely complex, can stay remarkably stable over time. The composition of the intestinal microflora in adults is determined by interactions between the host, microorganisms and environmental factors, such as the supply and nature of food. The intestinal microflora is regulated by saliva, gastric acid, bile, pancreatic juices, gastrointestinal motility and the continual process of shedding and regenerating epithelial bowel cells. It is also self-regulating (colonisation resistance) - preventing intruders becoming established, competing for nutrients and living space, and inhibiting pathogenic microflora through bactericidal effects. The beneficial bacteria compete with harmful or even pathogenic microorganisms for nutrients and living space, by making the gut environment more acidic and by producing antibacterial agents. For optimum "gut flora balance", the beneficial bacteria, such as the Gram-positive lactobacilli and bifidobacteria, should predominate over the potentially harmful bacteria.(3)

Function
The gut flora plays an important role in health. The bacterial flora of the intestinal tract has a number of functions, most of which are beneficial to the host.
It has been established that a healthy intestinal microflora exhibits the following effects:(4)

  • Modulation of the immune system.
  • Increasing resistance to the settlement of pathogenic bacteria.
  • Stimulation of intestinal peristalsis (important for regular stools).
  • Digestion of hard-to-digest components by the production of short-chain and other fatty acids.
  • Synthesis of some vitamins, such as vitamin K.

Disturbance (dysbiosis)
Alterations in the number and/or variety of species of bacteria can be caused by changes to the host or its environment. This may happen during treatment with oral antibiotics, food poisoning with pathogenic bacteria (eg, salmonella), viral infections, stress, reduced bowel motility and lack of gastric acid. Older people may be particularly prone to the latter two causes; lack of mobility can reduce bowel motility, and with age there is a natural decline in gastric acid secretion and numbers of bifidobacteria in the large intestine.
Under any of these circumstances, normal intestinal flora may be disturbed and unwanted bacteria may colonise in the bowel. When the intestinal microflora is disrupted, changes may occur over the entire length of the gastrointestinal tract from the mouth to the anus. The resulting symptoms can be wide and varied in nature, commonly including bloating, diarrhoea, constipation, and gastrointestinal pain and discomfort. Restoring or maintaining a balanced flora could be a way of managing such conditions.(5)
A great concern in hospitals is the susceptibility to Clostridium difficile of elderly people on antibiotics. This is a pathogen that would normally be controlled by a healthy gut flora.(6)

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Probiotic therapy
Probiotic bacteria are generally of human origin and considered nonpathogenic. It is essential that they retain viability during processing and transit through the gastrointestinal tract. Once they have passed through the acidic conditions of the stomach (where many other live bacteria found in products like bioyoghurts are destroyed), they must be able to adhere to the gut wall, proliferate and colonise the site, and they must also be able to exhibit antagonistic activity towards enteric microbial pathogens.
While over 20 criteria have been put forward, there is general agreement regarding the key selection criteria for probiotic bacteria for use in human foods:(7)

  • They must be of human origin.
  • They must be nonpathogenic.
  • They must be acid and bile tolerant.
  • They must have the ability to withstand technological processes and remain viable during their shelf-life period.

Evidence of health benefits
The probiotic microorganisms currently used are predominantly, although not exclusively, lactic acid bacteria (LAB), such as lactobacilli or bifido-bacteria.
Probiotics can be in the form of foods, such as fermented milk drinks and live or bio-yoghurt (if labelled as probiotic). There are also supplements, such as powders and capsules. In Japan and parts of mainland Europe, some probiotics are licensed for therapeutic use. However, in the UK, probiotics are either foods or supplements, sold in supermarkets and health stores. The manufacturers recommend that the probiotic product be incorporated as part of a daily, healthy diet. As most probiotic foods and drinks come in pleasant tasting or convenient formats, such a minor lifestyle change is readily achievable.
For health maintenance, probiotic products with concentrations in the region of 106 per ml are generally recommended.(8) Unfortunately not all brands have been found to contain the numbers of bacteria stated on their labels. A recent report commissioned by the Food Standards Agency found that many products using the term probiotic did not contain sufficient bacteria to have an effect on the gut flora balance or contained bacteria that did not meet the criteria listed above.(9)

Clinical conditions for which use of probiotics may be helpful
In the past few years, research into the use of probiotics as a treatment for various diseases has intensified. A number of double-blind clinical trials have investigated the effect of probiotics in many clinical conditions, a fair number of which have found positive results.
Probiotics have been established to have the
following health effects:(10)

  • Reduction in the duration of rotavirus diarrhoea.
  • Alleviation of the symptoms of lactose intolerance.
  • Diminishing the recurrence of superficial bladder tumours.
  • Reduction of harmful intestinal microbial enzyme activity.
  • Enhancement of the immune system.
  • Decreasing faecal mutagenicity.

Probiotics are also being evaluated as adjuncts in the treatment of the following medical conditions, with varying results:

  • Inflammatory bowel disease (IBD).
  • Irritable bowel syndrome (IBS).
  • Food allergy.
  • Atopic diseases.
  • Helicobacter pylori infection.
  • Urogenital infections.
  • Bladder cancer.
  • Oral rehydration therapy.
  • Diarrhoea.
  • Constipation.
  • Infant colic.

Despite the emergence of increasing numbers of studies on the health effects of probiotics, many scientists still remain sceptical as to their efficacy.(11) They argue that much of the effort has been devoted to screening bacterial isolates for properties deemed appropriate for a "probiotic" strain. There are many such strains to choose from, because the intestinal flora is already the home to bacteria with these properties.
Probably the biggest obstacle to allaying scepticism is that the probiotic concept is based on a very poor understanding of the intestinal microflora. The concept concentrates essentially on two groups of bacteria, Lactobacilli and Bifidobacteria, while practically ignoring the vast array of other species that inhabit the intestinal tract of humans and how they may interact. It is the impact of probiotics on the composition of the intestinal microflora, nevertheless, that forms the basis for the probiotic concept.

Important to gain understanding
While there is a current trend to conduct "clinical trials" to prove the efficacy of existing probiotics, some critics argue that it would be more relevant to consider the fundamental principles of the flora in the context of human physiology and immunology to gain a better understanding of the interactions between members of the intestinal microflora and between the microflora and the human host.(12) The intestinal bacteria could then be used as mediators to modulate health effects.
If probiotics are to be used in the treatment of diseases, it is prudent that they are developed in the same way as pharmaceutical drugs, based on fundamental knowledge of processes occurring within the human body. Mechanisms of drug action are known, and explanations of efficacy can be publicised. To attain scientific validity, probiotics should be derived by the application of logic.

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The use of probiotics in older people
As the number of beneficial bacteria decreases with age, older people may be more prone to food poisoning as well as conditions such as IBS. In 1996 an outbreak of Escherichia coli food poisoning resulted in the death of 21 people in Scotland, all of whom were elderly. Some researchers have speculated that some of the deaths may have been prevented if the intestinal gut flora had been more robust.(1) They recommend that older people may benefit from boosting beneficial gastrointestinal bacteria, by helping to protect against acute and chronic bowel diseases. They also stress the importance of using probiotics after a course of antibiotics because antibiotics reduce levels of all strains of bacteria in the gut.

Practical advice for elderly patients
First, consider how they would prefer to take the probiotic product. For some people, a supplement in the form of a capsule is the most easy and convenient way, but for others, who may already take a number of tablets and other supplements, adding another to the daily list can be too much. Food products come in a variety of flavours, mainly as milk, yoghurt drinks and yoghurts. While palatability is important, it is also worth checking the labels to compare fat and sugar contents. Those watching their weight for instance may prefer a lower-calorie option. The advantage of including these probiotic products in the diet is that they make a significant contribution to calcium and protein intake. Lastly, it is important to consider price; a tub of supplements is a cheaper option than a drink or yoghurt, especially if consumed on a daily basis. However, if a probiotic yoghurt is replacing a yoghurt that is already a part of the daily diet, the extra cost is likely to be small.

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References

  1. Gibson G. Available from:http://www.food.rdg.ac.uk/news.htm#81%22
  2. Gil A, Rueda R. Interaction of early diet and the development of the immune system. Nutr Res Rev 2002;2:263-92.
  3. Biancone L, et al. Resident bacterial flora and immune system. Dig Liver Dis 2002;34:S37-43.
  4. Isolauri E, et al. Probiotics: effects on immunity. Am J Clin Nutr 2001;73:444S-50S.
  5. Dugas B, et al. Immunity and probiotics. Immunol Today 1999;20:387-90.
  6. Lawrence SJ, Korzenik JR, Mundy LM. Probiotics for recurrent Clostridium difficile disease. J Med Microbiol 2005;54: 905-6.
  7. Arvanitoyannis IS, Van Houwelingen-Koukaliaroglou M.Functional foods: a survey of health claims, pros and cons, and current legislation. Crit Rev Food Sci Nutr 2005;45:385-404.
  8. Reid G, Hammond JA. Probiotics. Some evidence of their effectiveness. Can Fam Physician 2005;51:1487-93.
  9. Food Standards Agency. Probiotics research. Available from:http://www.food.gov.uk/
  10. Heselmans M, et al. Gut flora in health and disease: potential role of probiotics.Curr Issues Intest Microbiol 2005;6:1-7.
  11. Reid G. Safe and efficacious probiotics: what are they? Trends Microbiol 2006;14:348-52.
  12. Reid G, et al. New scientific paradigms for probiotics and prebiotics. J Clin Gastroenterol 2003;37:105-18.