Your digestive system is home to about 100 trillion bacteria - beneficial and harmful - known collectively as the gut microbiota.

"An imbalance between good and bad gut bacteria has been associated with inflammatory and metabolic disorders including inflammatory bowel disease, irritable bowel syndrome and obesity", said Professor William Chen, director of food science and technology programme at the Nanyang Technological University.

"This has led to a large amount of research on understanding what constitutes a health-promoting or disease-promoting microbial group", he said. "The gut microbiota of each individual is unique."

"Factors such as diet, geography, host genetics and physiology and drug use influence gut microbial composition, with diet considered the most prominent factor", said Prof Chen. "Moreover, diet is simplest to modulate and provides the easiest route for therapeutic intervention," he said. "Recent studies have linked diet and microbiome with health." Microbiome refers to all the genes that our microbiota contains.

"Evidence suggests that probiotics benefit those with severe food poisoning, as the repopulation of the gut with probiotic microbes inhibits the growth of harmful ones found in contaminated foods", he said. Dr Alex Soh, an associate consultant at the division of gastroenterology and hepatology at National University Hospital (NUH), said that in general, probiotic drugs have been associated with supporting a healthy digestive tract.

"In patients with acute infectious diarrhoea, probiotics may have beneficial effects in shortening the duration of illness and reducing stool frequency, he added. And patients with irritable bowel syndrome may experience reduced overall symptoms and abdominal pain with probiotics," he added.

NOT ALL PROBIOTIC PRODUCTS ARE THE SAME Probiotics - the most common ones being lactobacilli and bifidobacteria - can be packaged in many formulations containing just one organism or a mixture, said Dr Tan. "Effects can differ considerably between one organism and another."

Many products such as fermented or probiotic-rich foods may carry the "probiotic" label, but they are not probiotic drugs and thus may not have shown suitable evidence for health benefit, for instance, said Dr Soh. They may be classified as supplements and, as they are not regulated like pharmaceutical products, the dosages and contents might vary between products, he said.

Consumers should thus follow the manufacturers' usage recommendations instead of taking them as much as they like, he said. For those with complex medical conditions or who are unsure if the supplement is compatible with their medication, they should seek the advice of a healthcare professional, he added.

Consumers should also be aware that not all fermented foods contain live organisms, experts said. The yeast for making beer and wine is usually filtered out in the bottled product, while some foods are heat-treated for prolonged preservation.

"Probiotic microbes are like other microbes, which tend to be killed by heat treatment, with the exception of bacteria which are found near hot spring areas," said Prof Chen. For instance, in miso, the probiotic fungus Aspergillus oryzae is killed when it is heat-treated above 40 deg C, he said. As fermentation generates a lot of micronutrients, the miso soup that you consume will be nutritious, but you may not be taking in more beneficial bacteria, said Prof Chen.

THREE FERMENTED FOODS AND THEIR POSSIBLE BENEFITS Some fermented foods and drinks have found new fans here, some of whom make them at home for daily consumption. However, experts advise caution if you are attempting to cultivate or make fermented or cultured milk products at home.

"However, in the same way that they can grow good bacteria, if the fermentation process is not properly controlled, contamination with pathogenic bacteria can occur, which could result in food poisoning." As there is no recommended daily allowance for probiotics, it is not clear how much fermented foods we need to eat to optimise our gut health, said Professor William Chen, director of food science and technology programme at Nanyang Technological University.

Fermented foods have similar benefits, though the strains of beneficial bacteria used in the fermentation process of different foods may lead to some being more beneficial for gut health than others, he said. For instance, a common probiotic fermented milk beverage may contain just one species of probiotic bacteria.

A good probiotic source should ideally have multiple strains as multi-strain probiotics have shown greater efficacy than single strains," said Dr Gwee Kok Ann, a gastroenterologist at Gleneagles Hospital.

Source: ST