Your gut bacteria, also referred to as your gut microbiota, are one of the most densely populated microbial communities on earth. Within your gut right now, there are literally trillions of bacteria, and amongst them, hundreds of different groups (bacterial species). These gut microbiota provide immune, protective and metabolic functions that are crucial for good health.


The diversity and amounts of bacteria living within your gut are influenced by genetics, age, disease, stress, exercise, living conditions, medications, and most importantly diet; particularly, fibre and prebiotics.


The composition of your diet, your habitual dietary intake and any acute dietary changes you make, all impact on the microbial communities within your gut. For example, a significant change in your macronutrient (carbs, fats and protein) and fibre intake can rapidly induce changes in your gut microbial communities. This has been demonstrated in as little as 24 hours in humans switching between a plant-based diet rich in fibre (>30g/day) to a meat-based diet that contained no fibre.

Unfortunately, typical Western diets contain high amounts of animal protein, fat, sugar and starch, and low amounts of fibre. The average person in Australia consumes less than 50% of their fibre requirements on any given day. This is a big burden on health because low fibre intake is associated with a reduction in gut microbial diversity, a depletion in the number of gut bacteria and a subsequent increased risk for numerous chronic diseases, including obesity, cardiovascular disease, type 2 diabetes and colon cancer.

Fortunately however, various studies have demonstrated that an increase in both dietary fibre and whole grain intake results in a subsequent increase in gut microbiota diversity and gut bacterial numbers, as well as a reduced risk for chronic diseases and mortality. These improvements in gut health can be seen very soon after making improvements in the diet.


Dietary fibres are solely found in plant foods and when consumed, they are not digested nor are they absorbed, but in turn end up in the large intestine (aka colon). It is here in the large intestine where majority of gut microbiota live, and when specific fibres arrive, they are subject to bacterial fermentation. These specific fermentable fibre are known as “Prebiotics”.


Dietary fibres are found in grains, legumes, fruits, vegetables, nuts and seeds.


It is important to note that not all fibres are the same, due to variable chemical compositions, as well as physiochemical properties. For example, bananas contain fructans and resistant starch, and apples contain fructose and sorbitol, all different types of fibres, with different effects within the gut. These fibres have different lengths and structures, causing them to act and be acted on in varying ways.


Physiochemical characteristics of fibres include solubility, viscosity and fermentability.

Solubility refers to how easily the fibre dissolves in water. Soluble fibres, found in oats and fruits for example, dissolve well in water and break down into gel-like substances; this assists smooth bowel motions. In contrast, insoluble fibres, found in the skins and husks of vegetables and grains, do not dissolve easily in water, but they do bulk stools and hasten gut transit time. This means they increase the speed by which your food is converted to stools and then passed.

Viscosity refers to fibres that thicken when mixed with fluids. This is a common attribute of soluble fibres. Fermentability refers to fibres that are digested by gut bacteria, causing by-products such as gases and short-chain-fatty-acids (SCFAs) to be released.

Different foods contain different amounts of fibre, as well as specific types of fibres. It is important to consume a variety of fibre types, as well as adequate to high amounts of fibre daily.

Now without getting too specific and worrisome on knowing the exact right amount of each different type of fibre to consume, it is much easier and more practical to approach eating for good gut health in the following manner…


A plant-based diet that contains a variety of wholefoods will provide many different types of dietary fibres, thereby supporting greater diversity in gut microbiota composition, in turn improving overall gut health.


In a nut shell, eat a variety of plant foods.

Part 2 of this blog will outline prebiotics, including their endless benefits on gut health, as well as where and how to eat them.