Probiotics, prebiotics and fermented foods: are they effective?

In the human gut there are between 500-1000 species of bacteria and they are categorised as either harmful or beneficial. Too many harmful bacteria in the gut may cause diarrhoea and gut infections so the role of beneficial bacteria is to stop the growth of the harmful bacteria. Beneficial bacteria, or probiotics, also help to improve digestion and absorption of essential nutrients.

Probiotics are live bacteria which, when consumed, may act as beneficial bacteria in the large intestine

Examples of probiotics are Yakult and Actimel, both of which contain the safest and most researched strain of probiotic, Lactobacillus. Using clinical dosages in controlled studies, both Lactobacillus and Bifidobacteria probiotics have been shown to reduce the severity of diarrhoeal disease such as rotavirus in children and antibiotic associated diarrhoea. To be effective, they should be taken at the first sign of diarrhoea and throughout the period of illness. Current research trials are identifying whether probiotics may have beneficial outcomes for other conditions such as irritable bowel syndrome, ulcerative colitis, eczema, constipation and obesity. Although these trials remain inconclusive, there is no disadvantage in taking probiotics for these particular conditions and they may even help, however they are not suitable for individuals who are immunosuppressed.

For probiotics to be effective they must be alive when consumed and able to survive the harsh conditions of the gut. Many factors affect whether a probiotic reaches the colon, including which particular strain of probiotics is used, the acidity of the food eaten with the probiotic, other nutrients present in the food and whether the food is heated as this will destroy the probiotic. Due to these confounding factors and a lack of regulation over their commercial production, national guidelines promoting daily consumption of a probiotic do not exist. However, the majority of well individuals may safely include a probiotic that contains Lactobacillus or Bifidobacteria as part of a healthy balanced diet.

Prebiotics are foods which are indigestible in the gut and when they reach the colon, promote the growth and activity of beneficial bacteria

There are 2 main types of prebiotic, fructo-oligosaccharides and galacto-oligosaccharides and they can be found in high fibre foods namely, leeks, asparagus, onions, wheat, oats, garlic, chicory, soyabean, peas, lentils, and Jerusalem artichokes. The prebiotic content of a food is unaffected by heat treatment. Wheat has been shown to provide approximately 70% of the prebiotic in the typical western diet, although few individuals manage to consume the recommended quantity of 5g prebiotic daily as 100g wheat (approximately 3 slices of bread) provides just 2.7g prebiotic whilst a whole onion provides 2.6g. However, increasing fibre in the diet and cooking meals from scratch using fresh ingredients will all contribute towards improving the prebiotic content of an individual’s diet. Consuming a prebiotic alongside a probiotic may help to ensure the longevity, stability and beneficial action of Lactobacillus or Bifidobacteria bacteria in the colon.


Fermented foods and beverages are products where the growth of bacteria, yeast or mould has been actively promoted using enzymes to enhance the flavour and texture of the food

Fermentation originated in ancient times to help preserve food but interest in the potential health benefits of fermented foods has recently developed and there is now a large variety of products available. Examples of fermented foods are kefir (fermented milk product), kombucha (black tea), tempeh (soyabean product), unpasteurised cheese, pickles, miso, sourdough bread and sauerkraut.

Controlled trials have demonstrated a beneficial effect of regular consumption of fermented foods on blood pressure, cholesterol and glucose metabolism, however evidence to promote their use in treatment of irritable bowel syndrome and other gastrointestinal conditions has yet to be substantiated. Health benefits are potentially due to the presence of probiotics within the fermented food, altering the proportion of beneficial bacteria in the colon. Due to a lack of safety guidelines in the production of fermented foods, they are not recommended for certain individuals eg pregnant, elderly, immunosuppressed individuals or young children. From a nutritional point of view, fermented foods tend to be high in salt and fat and a lack of research and evidence for their use is the reason why they do not feature in national dietary guidelines. However, the majority of individuals may safely include small amounts of fermented foods regularly as part of a healthy well-balanced diet.

For more information on probiotics and prebiotics, check out the fact sheet from the British Dietetic Association or drop me a line to find out if they are suitable for your child


1.     The Role of Prebiotics and Probiotics in Human Health. Journal of Probiotics and Health O’Bryan et al 2013

2.     Human gut microbiota and prebiotics. Nutriwebinar. Professor Glenn Gibson. 2018

3.     Fermented foods: fad or favourable addition to the diet? The Lancet Gastroenterology and Hepatology. Jan 2019

4.     Feeding the Gut Microbiome. Nutriwebinar. Laura Tilt RD. 2018

Joan GavinDietitian4kids