Having plenty of fibre in our diets as part of a healthy lifestyle has been recognised for many years. Fibre was thought to help absorb water in the gut, making stools soft and easy to pass. More recently, the role of fibre in maintaining gut health through promoting the growth of beneficial gut bacteria has been revealed.
What is Fibre?
Dietary fibre (DF) is plant derived. It is indigestible by our own digestion and so passes through the small intestine into the large intestine. Here, it is metabolised by certain gut bacteria by fermentation, promoting their growth while producing short-chain fatty acids (SCFAs) that have numerous beneficial health effects for us. There are different types of DF with different structures. Each can be metabolised by different bacteria, producing different SCFAs that have their own particular effects. The manipulation of the gut microbiota by diet has centred on the use of DF, and many prebiotics are based on it. However, how different types of DFs affect the microbiota, how this might vary among individuals is unclear.
Maize & Tapioca Increase SCFAs
However, in a new study different DFs were found to cause different effects on the gut microbiota. Maize-, tapioca- and potato-derived DF were given to adult volunteers, over a period of four weeks, with the daily dose increasing each week. A group of volunteers were given a digestible starch (corn starch) to demonstrate effects were due to DF. Each DF has a different structure and so might be expected to be metabolised by different gut bacteria. Only the maize and tapioca derived DFs caused changes in the gut microbiota, and they each caused different changes. Importantly, maize DF led to an increase in production of the SCFA butyrate and this was closely associated with an increase in the abundance of the bacterium Eubacterium rectale, a known butyrate-producing gut bacterium. The tapioca DF led to an increase in propionate, a different SCFA, associated with an increase in the abundance of the propionate-producing Parabacteroides distasonis. These increases were off-set by a decrease in branched chain fatty acids, indicative of an increase in metabolism of the DF, with a corresponding drop in metabolism of amino acids.
Many of the usual cautions that apply to numerous scientific studies (e.g. small group sizes, short term study) are appropriate here. However, although there was variability between individuals in their response to the DFs, there was also a very clear highly conserved response among those receiving each of the maize and tapioca DFs. This suggests that there are parts of the response to DFs that is shared widely between people. That the responses to the different DFs raises were different raises the prospect of using of specific DFs to achieve targeted manipulation of the microbiota. For example, Eubacterium rectale, that is associated with better glucose metabolism, is often in low abundance in the guts of type 2 diabetics, and increasing the production of butyrate is viewed as a possible treatment in these cases. On the other hand, Parabacteroides distasonis is often reduced in those suffering from obesity, and propionate can signal satiety (feeling fully fed).
Could different combinations of DFs be used to target specific conditions through manipulation of the microbiota? That is the hope of many working in this area and while there is still a long way to go, this study suggests there is promise in this idea.
Picture: maize, called corn in some countries, is Zea mays, a member of the grass family Poaceae.