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How improving your gut health can help support your immune system

Dr Joanna McMillan

70 per cent of our immune function is centred in the gut

A young woman cuts up avocado in the kitchen while smiling
A young woman cuts up avocado in the kitchen while smiling

With the outbreak of COVID-19 in the community, and flu season upon us, many of us are keen to do all we can to remain healthy.

That’s where your immune system comes in.

Your immune system is your body’s first line of defence against illnesses and it’s made up of a complex network of cells, tissues and organs found in a variety of locations including your skin, spleen, lymphatic system and, most significantly, your intestines.

“About 70 per cent of our immune function is centred in the gut,” explains accredited practising dietitian Dr Joanna McMillan. “Immune function needs certain nutrients to work well and it needs a healthy gut microbiome because the microbiome is intimately connected to immune function.”

So what is the gut microbiome? It’s a collection of around 100 trillion microorganisms such as bacteria, fungi and viruses living in your gut – ideally, in harmony. A healthy gut microbiome is one with a high diversity of microbes, and more of the ‘good’ type of bacteria than the ‘bad’. But when that balance is disturbed by factors such as a poor diet, illness or prolonged use of antibiotics, the resulting disharmony – known as dysbiosis – could make the body more susceptible to disease.

Limiting stress and not smoking are two ways to keep your gut microbiome healthy, and making good food choices is another way to keep it in tip-top shape.

A young man chops an onion in the kitchen with a glass of red wine

Loading up on fibre

According to Joanna, the best way to feed your microbiome is to eat a diet rich in fibre, which is a type of carbohydrate that doesn’t get broken down like most other foods we eat. Instead some fibres travel to the gut where they are fermented, and the by-products of that process communicate with our immune system, Joanna explains.

“The most important thing is a diversity of different fibres,” she says. “Just taking fibre supplements is not the same as eating a plant-rich diet, which gives you lots of different types of fibres. So include wholegrains (not refined grains) legumes such as beans, lentils and chickpeas. Include fruits and vegetables, and don't forget nuts and seeds. So those are the key plant food groups to really think about.”

One key type of fibre is a prebiotic, which passes through the gastrointestinal tract undigested, where it stimulates the growth of good bacteria in the large intestine. Food sources of prebiotics include beetroot, asparagus, chickpeas, red kidney beans, nectarines, watermelon, dried dates, cashews, pasta, oats and wheat bread.

You need to be careful not to overdo it, however, as an excess of prebiotic fibre can cause gas and bloating. And if you’re increasing your fibre intake, you need to drink plenty of water – something fibre needs to work.

Related: The sources and benefits of fibre

If you’re looking to support your gut to be as healthy as it can be, eating probiotic foods is also a good idea.

Other gut-health friends

If you’re looking to support your gut to be as healthy as it can be, eating probiotic foods – not to be confused with prebiotics – is also a good idea. Probiotics are a group of live microorganisms that help harmonise the balance of good and bad bacteria in the gut. They also secrete protective substances that help activate the immune system and block pathogens from creating major disease. Probiotics are found in miso, kimchi, yoghurt, kefir (a dairy drink) and pickled vegetables such as pickles or sauerkraut.

Another key food component of a gut-healthy diet is foods high in polyphenols - chemical compounds with antioxidant properties.

“Sources of polyphenols include tea, coffee, cocoa, red wine, berries, pomegranates, leafy greens, herbs and spices,” Joanna says.

Related: The difference between prebiotics and probiotics

Gut-health foes

Of course, not all foods are good for your gut – some are actually detrimental to intestinal function. That includes foods high in artificial sweeteners (such as soft drinks), emulsifiers (found in processed foods such as ice-cream), excessive amounts of meat and (for some people) dairy.

Dr Joanna McMillan

Dr Joanna McMillan is a PhD qualified Nutrition Scientist and Accredited Practising Dietitian, plus one of Australia's favourite and most trusted health and wellbeing experts. She is the founder of Get Lean – an online lifestyle change program – and has authored several books including Brain Food and The Feel-Good Family Food Plan, as well as her Audible original audiobook, Gutfull. Jo's favourite song to dance to is Flashdance... forgive her - she was an aerobics instructor for 15 years!