If you buy yoghurt, you've probably noticed the recent appearance of yoghurt packs saying they contain probiotics that can help digestion or make your immune system work more efficiently.
The Australian Institute of Sport has a webpage on probiotics, which says they have beneficial health effects, especially on intestinal microbial balance.
Probiotics are also said to be useful in treating diarrhoea, irritable bowel syndrome, atopic dermatitis, bladder cancer and other diseases.
Garry Huffnagle, Professor of internal medicine and microbiology/immunology at the University of Michigan Medical School and author of The Probiotics Revolution answers some questions:
What are probiotics?
Live micro-organisms - almost always bacteria - that when ingested can confer a health benefit. They can be ingested in food, beverages or supplements. There is also a population of probiotic bacteria that we ingested as infants. You can make the numbers of those go up or down by the food you eat.
Why should people care about these internal bacteria?
What we're learning now is that the bacteria that live in our body normally function together like an organ. There are 100 trillion bacteria that live inside us. That's 10 times more bacterial cells than our cells. This collection of microbes has an important impact on our digestion, on how our GI tract works, our metabolism, and on how our immune system works.
How can bacteria affect our immune system?
Bacteria provide chemical signals that make it slow down once an immune response has been generated. When you think of an immune response, you think about getting an infection, and then the immune response revs up. But once the infection is gone, the response reduces unless something stops it. That's how you get this widespread problem of chronic inflammatory disease. The microflora don't play much of any role in what makes the immune system start, but they do play a role in what makes it slow down.
How do these friendly bacteria slow down the immune system?
The white cells that constitute the immune system talk to each other through chemical messengers. We're learning that the microflora also make chemical messengers, and those messengers are important signals to the immune system to slow down. There are other bacteria that live inside us that can make chemical messengers, but they are bad signals. Probiotics elbow out the bad bacteria.
Why have probiotics become a big deal now?
The most widespread right now are the ones that are being marketed by yoghurt companies. One of these products contains a probiotic that has been shown in clinical trials to improve immune function in some people. It's called Lactobacillus casei. There's another brand that contains a different probiotic bacteria called Bifidobacterium animalis. A third kind contains Lactobacillus reuteri.
All of these are actually good if you have any problems with diarrhoea, especially following a bout of the stomach flu.
Are probiotics available in any foods besides yoghurt?
Dairy foods are the main source. Besides yoghurt, you can find them in cheese and fermented milk drinks. You can also find them in sauerkraut, kimchi and pickles. A down side of modern society is that you can make these products without bacteria. If pickles have vinegar on the ingredients label, they were not made with bacteria.
What's on the horizon?
Probiotic bacteria love chocolate. The acid environment of orange juice also seems to be pretty good at keeping probiotics alive.
Probiotics do not survive in dried foods.
I have seen cereals that claim they have probiotics. It may contain probiotics but they're not alive, they haven't survived the processing.
Are there any risks associated with probiotics?
If you are immuno-compromised or if you are very ill, like hospital ill.
Then you need to be careful.