Prebiotics and Probiotics

(adapted from "Food & Nutrition" 2nd Edition, edited by Professor Mark Wahlqvist)

Although eating food containing specific bacteria has been popular for hundreds of years, only recently has this become accepted scientifically. Yoghurt is probably the best known example of a fermented food that provides beneficial bacteria to the intestine. Probiotics are claimed to prevent and control diarrhoea, lessen the effects of lactose intolerance, relieve constipation, lower cholesterol, prevent and control thrush (a vaginal yeast Infection) and even prevent bowel cancer, as well as stimulate the immune system.

The intestine
The intestine contains millions of bacteria -- many of them friendly and essential for good health. They aid digestion, stimulate the immune system and inhibit the growth of food-poisoning and disease-causing bacteria. However, because some bacteria aren't good for us, keeping the different types in balance plays an important role in our general health and wellbeing. Healthy people have a good balance of intestinal bacteria.
The gastrointestinal tract normally contains large numbers of bacteria (natural microflora) including 10x7-8 organisms in the oral cavity (predominantly Streptococcus, Veillonella, Neisseria), 10x2-3) organisms in the stomach and small intestine (Lactobacillus, Streptococcus), and 10x10-11 organisms in the large intestine and colon (Bifidobacterium, Bacteroides, Eubacterium, Peptostreptococcus). Microorganisms in the right side (proximal) of the colon grow at a fast rate due to a good supply of nutrients, resulting in short chain fatty acid (SCFA) production thereby causing a decrease in pH. In contrast, in the left side (distal) colon bacteria grow more slowly due to a restricted supply of nutrients and therefore the pH often approaches neutrality. Several factors (such as poor diet, stress, antibiotics, aging) can tip the scales in the direction of the pathogenic bacteria and perhaps probiotic bacteria can help in this situation. Two similar terms are used in this area: probiotics and prebiotics.

Probiotics & Prebiotics
Probiotic foods are those foods which contain a live microbiological culture either as a result of fermentation or as an intentional addition to beneficially affect the host by improving the intestinal microbial balance. Probiotics are cultures of bacteria that are healthful for normal intestinal function; they often prevent harmful bacteria from causing disease.
In contrast, a prebiotic is a nondigestible component which beneficially affects the host by selectively stimulating the growth and/or activity of one or a limited number of colonic bacteria thereby improving the health of the host. In other words, prebiotics are nutrients that the bacteria use as a fuel source; these include dietary fibre and carbohydrates (e.g lactose in the lactose intolerant) that resist absorption in the upper, small intestine, reaching the large intestine where most of your bacteria thrive.
Foods metabolised by colonic bacteria include resistant starch and non-starch polysaccharides. By definition, these foods are classified as dietary fibre because they are not digested nor absorbed in the small intestine.
However, in order for a component to be classified as a prebiotic, it must satisfy the following criteria;
a prebiotic must not be hydrolysed nor absorbed in the upper gastrointestinal tract;
be a substrate for growth or activity of one or a limited number of beneficial colonic bacteria;
be therefore able to alter the colonic microflora towards a healthier composition and
to induce luminal or systemic effects which are beneficial to the health of the host.

While many components of dietary fibre such as resistant starch and non-starch polysaccharides provide substrates for fermentation by colonic bacteria, to date only fructooligosaccharides have met all of the criteria mentioned previously to qualify as a prebiotic. Fructooligosaccharides are short- and medium-length chains of b-D fructans. Short-chains are referred to as oligofructose and medium-length chains are known as inulin. Foods containing oligofructose and inulin include garlic, onion, artichokes and asparagus. Commercially available ingredients include Raftilose (inulin hydrolysate) and Raftiline (inulin) derived from chicory roots.

Potential Health Benefits and Mechanisms
It has been proposed that in the intestine these bacteria may:
1. bind, block or remove carcinogens
2. inhibit bacteria which directly or indirectly convert procarcinogens to carcinogens
by enzyme activity
3. activate the host's immune system to antitumorigenesis
4. reduce the intestinal pH, thereby altering microbial activity, solubility of bile acids,
mucus secretion
5. alter colonic motility and transit time.

More studies are needed to confirm the potentially protective effects of fermented
food products on carcinogenesis in animals and humans. Unfortunately most of the health claims have only a limited scientific foundation. Most of the evidence to date is anecdotal, or comes from studies conducted with only a small number of people or with animals. Also, a probiotic's effect will vary depending on your age, race, diet, health and medical history, to name but a few.

The potential benefits of probiotic foods include:

prevention and treatment of diarrheoa caused by rotavirus, especially in children (the evidence for this is quite good),
immune system enhancement
reducing some allergic reactions
treating and preventing respiratory infections, especially in children (emerging good evidence)
decreased faecal mutagenicity
decrease in the levels of pathogenic bacteria
decreased faecal bacterial enzyme activit
prevention of the recurrence of superficial bladder cancer
the restoration of the correct balance of natural microflora after stress, antibiotic treatment, alcohol use and chemotherapy

For fermentation of dietary fibre and oligosaccharides in the colon with the production of volatile fatty acids (VFA) which lower the pH of the colonic contents and provide a substrate for the colonic mucosa which may help to reduce the risk of developing colon cancer.

Further research is required to elucidate the mechanisms involved and to test the proposed theories. A medical news story in the September 20, 2000 edition of the Journal of the American Medical Association highlights some of the research going on with beneficial bacteria. .

In addition, you will not get all potential benefits from just one type or strain of organism. Examples of probiotic microorganisms used in foods include Lactococcus lactis, Lactobacillus sp., Streptococcus, Enterococcus, Bifidobacterium sp., Pediococcus, Propionibacteria sp. The choice of strain of microorganism is important to avoid removal of micronutrients from the food, to avoid production of adverse components such as vasoactive amines and to avoid opportunistic lactic acid bacterial pathogens. As an example of the wide range of strains that are available, the genus Lactobacillus contains up to 60 species (including L. acidophilus and L. casei -- the ones most commonly added to yoghurts and drinks). Within each of these species are separate strains, of which there can be hundreds. Although very closely related, each strain may have different effects on health.
Probiotic foods must contain living microorganisms in appreciable numbers at the end of the product's shelf-life. To have any effect in the colon, the bacteria need to survive food processing and storage in large numbers, then survive the passage through the acids and digestive enzymes in the stomach and small intestine, and still survive once they reach the colon. The evidence regarding the survival of bacteria through to the colon is limited.
In Australia, fermented milk products, including yoghurt, must contain a minimum viable count of 10x6 organisms per gram at the end of the shelf-life. To have the desired effect, scientists believe at least a million of each probiotic bacteria per gram of yoghurt or drink are needed e.g if a yoghurt contains three different types of probiotic bacteria, it should contain at least a million of each of them per gram. The yoghurt Vaalia contains 3 different types of bacteria at these desirable levels; yoplus has 2 different bacteria and LC1 and Yakult have 1 bacteria at these levels.

Safety of probiotics currently on the market? It appears that the worst that can happen
if you choose to eat or drink these products is that they do nothing extra for you. You will just get the calcium and other nutrients you would normally get from yoghurt or a yoghurt drink. However, a couple of the fermented drinks on the market are not that high in calcium. If you would like to give them the benefit of the doubt, a good starting point is to choose a product you can rely on to have consistently high enough levels of bacteria to have any effect.


Last Updated: April 5, 2002.