There is a significant gap between the recommended dietary intake of fibre and the actual intake today, which ultimately means we are not consuming enough of the foods that contain prebiotics, delivered by fermentable fibre.
This poses a significant challenge to public health, given the current and emerging research surrounding the gut microbiome and its link to many areas of health. Arguably, this is why there is a need for a more focused public health message about fibre that includes the important prebiotic role that some fibre provides.
The International Scientific Association for Probiotics and Prebiotics (ISAPP) defines a prebiotic as ‘a substrate that is selectively utilised by host microorganisms conferring a health benefit’.
Essentially, prebiotics feed the beneficial gut bacteria, which in turn elicit a physiological effect on the host, leading to health benefits1.
Why are they important for health?
Prebiotics provide substrate for probiotics, which are live or active cultures in the gut and when provided in sufficient numbers can exert an effect as part of the gut microflora. Certain probiotic strains have been associated with providing beneficial effects to people with certain conditions2.
Probiotics can be taken directly via food or supplements but must be available in large enough numbers to survive the journey to the colon. In contrast, prebiotics are the food for the ‘good’ gut bacteria which produce short-chain fatty acids (SCFA) as the metabolites of prebiotic fermentation. SCFA can reduce inflammation and contribute towards host metabolism. The proliferation of beneficial gut bacteria and the production of SCFAs is thought to be a reason for the health benefits of prebiotics1.
Usually, the carbohydrates, inulin-type fructans and galactooligosaccharides (GOS), fit the criteria to act as a substrate for gut bacteria, and therefore are classified as two major prebiotics. Inulin-type fructans are polymers of fructose and can be sourced from plants, produced from inulin or synthesised enzymatically from sucrose. There is a form of GOS which is enzymatically produced from lactose, in contrast with GOS that can be found naturally in plants, such as beans and pulses, in small amounts3. The rest of this article will refer to GOS in terms of the enzymatically produced ingredient which has been extensively studied.
Despite the health benefits of consuming short-chain fermentable carbohydrates, ingestion of fructans can trigger undesirable gastrointestinal (GI) symptoms such as abdominal pain, flatulence and diarrhoea. Symptoms of Irritable Bowel Syndrome (IBS) may be exacerbated by consumption of fructans, which is why the low-FODMAP approach has been developed. A low-FODMAP diet is a dietary intervention over 4-8 weeks, led by a trained practitioner, where foods high in fermentable carbohydrates are excluded from the diet. This is followed by a graded reintroduction of foods into the diet to investigate tolerance to particular foods.
Although the low-FODMAP diet can reduce adverse GI symptoms, the lack of dietary diversity can affect the composition of the gut microbiota4. GOS has a greater specificity than inulin or fructans for bifidobacteria, therefore inducing fewer cases of GI symptoms and is suitable for people with IBS3. The benefit of a substrate specifically for bifidobacteria, is that these bacteria are a known probiotic that can cause beneficial physiological effects to systems of the human body. A comparison between the prebiotic fibre types is outlined in Table 1.
Table 1. Summary of Prebiotic comparison between Inulin-type fructans and Bimuno®3.
Increasing prebiotic intake
A food-first approach is always recommended and there are different categories of inulin prebiotic sources in the diet. Beta-glucans are soluble fibre compounds with main sources being oats and barley. Resistant starches are formed under a variety of conditions and an example source is green bananas.
Fructan compounds are found in a wide range and structure of foods such as Inulin. These can be found in onions, garlic, asparagus, artichokes and chicory. Other sources of prebiotics are guar gum (used as a food additive as it forms a gel) and xylooligosaccharides (XOS). The XOS are used as commonly in Japan as an ingredient for ‘Food for Specified Health use’5.
Alternatively, to increase daily prebiotic intake, one can include high fibre prebiotic supplements* or foods that have prebiotics added during their manufacturing. Caution should be taken when recommending a prebiotic supplement as a higher dose may also cause GI discomfort, so more is not necessarily better.
Inulin based supplements require a higher dose (10-15g) to achieve a prebiotic effect. Bimuno® contains GOS, and studies show that a lower dose (3.6g) stimulates a prebiotic effect. A higher dose can be associated with GI effects and this is one reason why inulin-based prebiotics are not recommended for those with IBS type symptoms. Unlike Bimuno®, when they are fermented in the colon, they can generate gas and can cause flatulence, bloating and discomfort in some people3.
A decrease in bifidobacteria has been associated with higher levels of inflammation and IBS, as well as obesity and weight gain. This suggests the benefit of initiating the bifidogenic effect with the consumption of Bimuno®5. Individuals with IBS that are sensitive to FODMAPs are often placed on a short-term low-FODMAP diet to reduce their symptoms.
By restricting the intake of FODMAPs the individual is essentially starving the probiotics in the gut, thereby reducing fermentation. This can result in a depletion of the bifidobacteria, alongside the reductions in the symptom-causing bacteria. A low-FODMAP diet has been reported to reduce symptoms of flatulence, abdominal pain and bloating, while bifidobacteria reduced in abundance. This was in contrast to a group using Bimuno® and maintaining FODMAP foods in their usual diet, who saw a fall in GI symptoms and a rise in bifidobacteria6.
Foods that contain dietary inulin-based prebiotics are largely classified as ‘fibre’. With the public in the UK struggling to meet the 30g/day recommended fibre intake, it is no surprise that the population does not consume the recommended daily intake of 5g of prebiotic fibre.
There are different sources of prebiotics that we can get from our food. However, some population groups may struggle to consume enough inulin, perhaps because of a bloating effect they experience, such as groups with functional gut disorders. In these cases, GOS would be preferable.