Are they just a con? Guy Hagan examines the issues

"It's all a con," exclaimed my medic housemate, as an advertisment for a live yoghurt drink was shown on the television. "All the bacteria would be broken down in the stomach and do nothing," he went on.

Live yoghurt drinks refer to a group of drinks made from fermented milk containing cultures of specific types of bacteria. The bacteria cultured are genera of human origin, such as Lactobacillus acidophilus, L. Casei, Bifidobacterium bifidum, and B. Longum.¹ The advertised philosophy behind live yoghurt drinks is that when they are eaten they can replenish levels of "good" bacteria found in the bowel that may have been lowered by a stressful lifestyle and an unpredictable diet. The result is a well balanced intestinal flora, which helps maintain a healthy immune system.²

What health effects do live yoghurt drinks have?
The suspicion that consumption of live fermented milk drinks may have positive health effects has been around for nearly 100 years, since Metchnikof postulated that the longevity of rural Bulgarians was due to their large consumption of these drinks.³

For the bacteria in the drink to exert any health effects, they would first have to survive their journey through the gastrointestinal tract. This journey exposes them to the acidic conditions of the stomach, and then to the hydrolytic enzymes and bile salts of the small intestine.⁴ A small number of bacteria do survive the journey (approximately 0.01-2%)⁴ and are found in the faeces.⁵ The minimum amount of live yoghurt drink (containing Bifidobacterium species) that had measurable therapeutic effects in humans was 400-500g a week, at a concentration of 1.0 x 106 CFU/g.⁶ A significant proportion of live yoghurt foods tested for these levels of bacteria in Australia, the United States, and Britain were found not to contain these minimum levels.¹

The most documented health effects of live yoghurt drinks are specific protection from diarrhoeal diseases and the advertised general enhancement of the immune system. Studies looking at the effect on diarrhoeal diseases have shown that live dairy supplements prevent diarrhoeal diseases in Japanese schoolchildren,⁷ and reduce the frequency of both antibiotic associated diarrhoea⁸ and traveller's diarrhoea.⁹ Many studies show positive effects of live yoghurt on many in vitro and in vivo immune indices,⁴ including enhanced humoral¹⁰ and cellular¹¹ immune responses.

It has also been suggested that live yoghurts can suppress the growth of the bacterium H. Pylori, and so have a role to play in the prophylaxis and management of conditions associated with H. Pylori, such as peptic ulcer and possibly gastric cancer.¹²

How are these health effects achieved?
Several compatible and complementary theories exist as to how the ingestion of live yoghurt can have these health effects. Bacteria from the live yoghurt that survive their passage through the gastrointestinal tract have been shown to bind to epithelial cells¹³ and the luminal surface of M-cells¹⁴ in the wall of the gut. By attaching to epithelial cells, it is postulated that the yoghurt derived bacteria inhibit the binding of enteric pathogens to the epithelial cells. This process is referred to as "competitive exclusion."¹ Binding by the bacteria to gut associated lymphoid tissue(GALT) has been shown in animal models to result in enhanced production of cytokines and IgA, as well as increased mitogenic activity of Peyer's patch cells and splenocytes.⁴

The bacterial genera cultured in live yoghurt drinks have also been shown to produce lactic acid and antimicrobial peptides referred to as bacterocins, which may inhibit the growth of enteric pathogens. This could be the explanation for the finding that increased amounts of lactobacilli in the human gut inhibit the growth of pathogenic bacteria.¹⁵

The possible anticancer effects of live yoghurt drinks may be due to several mechanisms. Inhibition of bacteria that convert procarcinogens to carcinogens and enhanced activation of the host's immune system by the mechanisms described above may play a part. There is also evidence that some live yoghurt bacteria can themselves inhibit the formation and breakdown of carcinogens. For instance, some of the bacterial genera have been shown to have nitrate reductase activity,¹⁶ an enzyme that breaks down nitrates into nitrites and then further into nitrogen or ammonia. Nitrites are an important intermediary in the formation of highly carcinogenic N-nitroso compounds,¹⁷ and so by reducing the concentration of nitrites in the bowel, these bacteria may protect in part against bowel cancer.

A recent paper in Science shows that B.Thetaiotaomicron, a normal component of the human intestinal microflora, modulates expression of genes involved in several important intestinal junctions, including absorption, mucosal barrier protection, and xenobiotic metabolism.¹⁸ This implies that the makeup of intestinal microflora may also influence gut physiology at a fundamental level.

There is also evidence that non-bacterial components of yoghurt have immunostimulatory effects that may contribute to the health effects. Peptides from the fermentation of milk, whey protein, conjugated linoleic acid, and calcium have all been shown to have immunoenhancing effects in vitro and often in vivo.⁴ Other components within live yoghurt could also be confounding factors for some studies.

Conclusions
Far from being a con, live yoghurt drinks have been shown to have several possible health effects. Further research is needed to elucidate further the mechanisms of the reported benefits and see how enhancement of the immune system translates into clinical effects. Randomised controlled trials in humans are also needed to properly ascertain the health benefits from live yoghurt drinks. Techniques to enhance survival of yoghurt bacteria in the gastrointestinal tract may be another area of future research. Live yoghurt drinks may eventually be a complementary medicine for many gastrointestinal conditions, immunosuppression, and even certain types of cancer. They may be of particular relevance to medicine in the developing world, too, given their relative cheapness and the high incidence of diarrhoeal diseases.