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Making sense of probiotics

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VOL: 97, ISSUE: 07, PAGE NO: 40

Diane Palmer, BSc, RN, PGCE, is lecturer in nursing, University of Hull

Pamela Barker, RN, is nutrition nurse specialist, Scarborough and North East Yorkshire Healthcare NHS Trust

The term probiotic was first described by Fuller (1991) as 'a live microbial feed supplement which beneficially affects the host animal by improving its microbial balance.'

The term probiotic was first described by Fuller (1991) as 'a live microbial feed supplement which beneficially affects the host animal by improving its microbial balance.'

Marketed as 'functional foods' in the form of yoghurt-style desserts and fruit-flavoured low-fat milk drinks, probiotics can be found in the refrigerators of many shops and supermarkets. They are not absorbed by the upper gastrointestinal tract so they can be used to manipulate the gut flora, promoting a favourable balance of beneficial bacteria such as Lactobacillus acidophilus.

Gastrointestinal tract bacteria
More than 400 different species of bacteria live in the human gastrointestinal tract. These gut microflora support digestion through the fermentation process, stimulate immune responses and protect against pathogenic bacteria. Two of the main beneficial bacteria responsible for this process are lactobacilli and bifidobacteria.

Maintaining a slightly acidic pH produces a favourable environment for these bacteria and an unsuitable environment for many harmful organisms found in the gastrointestinal tract. Disturbing the pH in the gastrointestinal tract, possibly through poor diet, alcohol, stress or drugs such as antibiotics, destroys many of these beneficial bacteria (Tannock, 1983; Bartlett et al, 1978).

Lactobacilli reside mainly in the large intestine, synthesising vitamins and breaking down fibre and undigested starch to provide fuel for the cells living in the bowel wall. These bacteria create an environment that reduces the ability of pathogenic bacteria, yeast and fungi to invade the immune system and damage tissue. Studies have concentrated on L. acidophilus and bifidobacteria, which ferment sugars into organic acids, with certain strains thought to stimulate the immune system.

Yoghurt is produced from the fermentation of lactic acid in milk by lactobacilli and streptococci. Some manufacturers produce pasteurised milk with added live cultures, such as lactobacilli, and most supermarkets sell yoghurts and milk supplements that claim to have health benefits. But not all bacterial strains have the same properties: some have greater resistance than others to gastric acid and high adhesion properties to the gastric mucosa.

Care must therefore be taken to ensure that the dietary supplement selected is capable of producing the desired probiotic effect, which can be difficult. Not all manufacturers indicate which live cultures have been used, different cultures have been shown to be of benefit in different circumstances, and in some cases cultures do not to survive until the product's sell-by date.

A large number of clinical trails are in progress to investigate the use of probiotic supplements, both for prophylaxis and treatment. Some of the evidence produced to date is tenuous, mainly because of low numbers of participants. For this reason manufacturers tend to focus on the health promotional properties of their product, particularly as a stimulant to the immune system. The concept of probiotics, however, is not new. Zoologist Elie Metchnikoff (1907) first identified this phenomenon when he observed that yoghurt eaters tended to live longer than other people.

Indications for use
The main benefits of probiotics are their ability to stimulate the immune system and act as a barrier to intestinal pathogens and bacteria (Kailasapathy and Chin, 2000). They have also been associated with an improvement in the symptoms of people with lactose intolerance (Kim and Gilliland, 1983).

Many people routinely eat yoghurt to combat yeast infections and lactobacilli have been found to reduce the recurrence of candida, which leads to vaginal infections (Hilton et al, 1992). Lactobacilli have also been associated with the correction of erythromycin-induced gastrointestinal imbalances and is effective in the treatment of diarrhoea, especially that caused by Clostridium difficile and campylobacter (Gorbach et al, 1987).

One study identified the strain Lactobacillus johnsonii as capable of inhibiting certain pathogenic bacteria, such as Escherichia coli and salmonella, from adhering to the intestinal mucosa, inhibiting the damage associated with pathogens by at least 80% (Bernet et al, 1994). L. johnsonii has proved useful in preventing holidaymakers' diarrhoea, but as it needs to be incubated in the intestinal cell before the addition of the pathogen to the gut it must be introduced as a prophylactic before and during travel. Saavedra et al (1994) found that formula feeds supplemented with Bifidobacterium bifidum and Streptococcus thermophilus reduced the incidence of acute diarrhoea in infants.

But not all probiotics can be used to treat enteric pathogens, and although they are not generally recognised as pathogenic probiotics have been reported to cause infections in a small number of cases, possibly because of pre-existing bacterial overgrowth in the gut (Sussman et al, 1986). People who are severely immunocompromised, for example those who have had an organ transplant or have HIV/AIDS, may be particularly at risk.

An exciting development in the use of probiotics is their association in laboratory studies with cancer prevention (Reddy, 1998). Cultures of bifidobacteria have shown an increase in the hosts immune response (Sekine et al, 1995), while an increase in the number of bifidobacteria has been shown to increase apoptosis in the colon (Hague et al, 1993). Apoptosis is normal cell death, and the failure of this mechanism has been linked to the development of abnormal cells and cancer.

General health promotion
The evidence in favour of routinely taking probiotics for general health promotion is still being gathered and there is controversy over the benefits of the routine use of probiotics to prevent infection in healthy people (Larkin, 1999). The routine use of probiotics during times of stress or when travelling abroad may benefit some people, but to make the most of their health promotional properties manufacturers recommend regular use. The life span of the probiotic in the gut is about 10-15 days.

Product availability
Probiotics are easily administered and are widely available. They are not routinely used in hospitals or health care environments, but a few clinical studies are evaluating their use as treatments.

Most people will not experience any complications as a result of the routine use of probiotics, apart from 10-15 days of increased flatulence at the outset. But those who are receiving health care support should consult their GP before using these products and nurses should remember that they are not routinely prescribed as a treatment.

It is difficult to imagine that a simple, cheap and readily available product such as yoghurt can promote the dramatic health benefits suggested in some of the literature, but the evidence from animal and human studies indicates that probiotics can have therapeutic effects. The challenge is to identify which particular strains are responsible for positive effects on specific parts of the body.

We can expect many more studies and a great deal of academic analysis on this potentially exciting and lifesaving development. The challenge is to filter the valuable from the invaluable data and to ensure that patients make choices based on evidence rather than manufacturers' marketing strategies.

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