Over the past decade, interest in the gut microbiome has grown as efforts to measure its composition and function have developed. We now know that we have more bugs within and on us than we do human cells, and some consider us more bacterial than Homo sapien.

What is the gut microbiome and what factors influence it?

The gut microbiome can be thought of as an ecosystem much like a rainforest, where bacteria, fungi and viruses thrive – some good, some bad. It is recognised that although the gut microbiome alters throughout the lifespan, its major composition typically stabilises in the first few years of life.Many factors affect our gut microbiome, including (but not limited to) our environment, genetics, and antibiotic interventions. From hereafter, we will consider the gut microbiome in terms of the bacteriathat live within this gastrointestinal ecosystem – and these bacteria are termed the gut microbiota.The above factors influencing our gut microbiota are either difficult to manage or simply cannot be changed. However, the foods we consume can also significantly impact gut microbiota and are, importantly, modifiable. Our food choices occur regularly – on a day-to-day (sometimes minute-by-minute) basis and this is empowering. Although our gut microbiota are relatively stable, research shows that we can change their composition, for better or worse, within a matter of 24 hours. To change the composition of our gut microbiota refers to either increasing or decreasing the overall amount (i.e., abundance) and/or types (i.e., diversity) of bacteria in our gut.

What role do gut microbiota play in diet?

We know that food choices are modifiable and that food can rapidly change the composition of gut microbiota, but another interesting fact about our microbiota is that they break down certain foodstuffs that would otherwise go unused. That is, there are some foods we are unable to digest and we need our gut microbiota to do the job for us. Simple carbohydrates such as table sugar move through the stomach and into the small intestine where they are easily metabolised, digested and absorbed for energy by our bodies. However, when we eat fibre, it makes its way all the way through the small intestine and into the large intestine. The large intestine houses most of our gut microbiota and it is here that these microscopic bugs munch and digest the fibre we were unable to access ourselves.

What foods best support a healthy gut microbiota composition?

Although we are yet to characterise exactly which bugs are needed for the optimal mix of gut bacteria, we do know that greater gut microbiota diversity is associated with better health outcomes. And what promotes a greater variety of gut bugs? The simple answer is – fibre-rich whole-foods. Examples include plant foods such as vegetables, fruit, whole-grains, legumes, nuts and seeds. We can also promote a healthy gut by consuming bacteria itself in the form of fermented food such as kimchi, kefir, unpasteurised sauerkraut etc. Be mindful, though, that the benefits associated with consuming fibre-rich food or fermented food are transient and will only last for as long as you are consuming them (or shortly thereafter). This means that if we want to reap their health-benefiting rewards, we need to maintain their consumption as often as every day and ideally with every meal. The National Health and Medical Research Council or NHRMC recommends that Australians’ consume between 25 to 30 grams of fibre per day. Most of us are not reaching this target and are instead consuming less than 20 to 25 grams per day. A few simple steps to increase dietary fibre intake may include swapping white bread or white rice for whole-grain bread or brown rice, respectively. Moreover, by adding just two pieces of fruit to our snack pack or 1 cup of mixed vegetables with lunch or dinner may increase our fibre intake by 5 to 9 grams, respectively.

When these foods are consumed regularly, the gut microbiota produce certain fibre-related metabolites referred to as short-chain-fatty-acids or SCFAs. SCFAs are defined as the byproducts of fermentation of food by the gut microbiota. A study by David et al. (2014) showed that consuming a plant-based diet significantly and rapidly (within five days!) increased levels of fibre-related SCFAs associated with anti-inflammatory modulation. The prescribed plant-based diet consisted of oats for breakfast as well as a mix of vegetables, rice and lentils for lunch and dinner. For snacks, fresh and dried fruit were prescribed. Additionally, feeding our gut microbiota essential complex carbohydrates promotes their ability to regulate our immune system; fight off invading pathogens; produce important nutrients; and, provide us with the extra energy we may need to pick ourselves up in the morning and make that much needed cup of coffee (among other important life things). Alternatively, a day-to-day diet lacking in fibre starves the gut microbiota and can lead to them eating the only food in sight – mucus that lines and protects our gut wallEvidence suggests that if the gut microbiota creep too close or munch down too deeply on this gut wall, our immune system as well as inflammatory processes may kick in.

In summary, the cards are in our hands in terms of the food choices we make. The great thing about food choices is that we have a number of them to make each day, including breakfast, lunch and dinner as well as desserts and snacks. So even if we forget to include fibre-rich foods in our lunch, we can aim to include them as part of our 3pm “slump snack”. The very powerful decision to include more fibre-rich foods in our diet can increase gut microbiota diversity and subsequently promote beneficial physiological outcomes. So, let us eat like our Grannies used to eat and include gut-friendly fibre as often as possible!

Melissa Lane is a PhD candidate within Deakin University’s Food & Mood Centre. She holds a Bachelor of Nursing (Pre-registration), Graduate Diploma in Psychology and Bachelor of Psychological Science (Honours). Melissa has a particular interest in the link between the gut microbiota, diet and physical as well as psychological outcomes.