The nutrient 90% of Australians don't eat enough of - and what happens when your gut bacteria go hungry.
We tend to think of fibre as a digestive thing. Eat more, go more. Problem solved.
But that's selling fibre - and your gut - seriously short.
Your gut is home to trillions of microbes, and they don't just passively sit there. They communicate with your brain, train your immune system, regulate your blood sugar, and influence your energy levels. When they're well-fed - specifically with the diverse fibres they thrive on - the ripple effects reach almost every corner of your health.
The problem? Australians average just 20g of fibre per day, well below the recommended 25–38g (and research even shows that 40grams is superior to 25 grams). And it's not just about quantity. Most of us eat the same narrow range of foods week after week, which means our microbes are eating the same narrow diet too.
Here are seven signs that your gut bacteria might be asking for more fibre - and they have nothing to do with your digestion.
1. Your Energy Crashes in the Afternoon
That 3pm slump isn't just about bad sleep or too much coffee. When gut microbes ferment fibre, they produce short-chain fatty acids (SCFAs) - particularly butyrate, acetate, and propionate. These compounds act as fuel for your gut lining and help regulate how steadily energy is released from food.
Without enough fibre to ferment, SCFA production drops. Your gut lining becomes less efficient, blood sugar tends to spike and dip more dramatically, and energy becomes harder to sustain. If you're reaching for sugar or caffeine by mid-afternoon, your microbes may be running on empty.
2. You Keep Getting Sick
Around 70% of your immune system lives in and around your gut. Your gut microbes are actively involved in its calibration - educating immune cells, producing antimicrobial compounds, and maintaining the gut barrier that stops pathogens from getting into the bloodstream.
Fibre feeds the bacteria that produce butyrate, which in turn nourishes the cells lining your gut and keeps that barrier strong. When fibre is low, barrier integrity can weaken, beneficial bacteria decline, and the immune system loses some of its most important allies. If you're catching every cold that goes around, your microbiome might have something to do with it.
3. Your Mood Feels Flat
Your gut produces around 90% of the body's serotonin - the neurotransmitter most associated with mood and emotional regulation. This production is heavily influenced by your gut microbes, particularly certain strains that are fed by specific types of fibre.
The gut-brain axis is a two-way communication system, and when the microbial end of that conversation goes quiet, the effects can be felt in mood, motivation, and mental clarity. Research increasingly links low microbial diversity - often a consequence of low fibre intake - with higher rates of low mood and anxiety. Feeding your gut may be one of the most underrated mood strategies available.
4. Your Blood Sugar Feels Unpredictable
Fibre slows digestion, which slows the rate at which glucose enters the bloodstream. But the story doesn't end there. Certain gut bacteria produce SCFAs that directly signal to the gut and pancreas, improving insulin sensitivity and helping the body manage blood sugar more effectively.
If you notice significant energy swings after meals, strong cravings for sweet foods, or difficulty feeling satisfied after eating, an underfed microbiome may be undermining your blood sugar balance.
5. You Feel Bloated Even When You Haven't Eaten Much
This one feels counterintuitive - if fibre causes bloating, how can needing more of it cause bloating too?
When the microbiome lacks diversity, opportunistic bacteria can overgrow and produce gas from the foods you are eating, even in moderate amounts. A well-diversified gut, fed a range of prebiotic fibres over time, ferments food more efficiently and with less discomfort. Persistent low-level bloating that isn't explained by a large meal may mean your gut mirobes need some training (and building up fibre intake slowly is ideal)
6. Your Sleep Quality Has Declined
Gut microbes follow a circadian rhythm just like you do, and the relationship between gut health and sleep is becoming one of the more interesting areas of microbiome research. The SCFAs produced by fibre fermentation influence the production of sleep-related hormones and neurotransmitters, including serotonin - which is also a precursor to melatonin.
Studies have found that people with greater dietary fibre intake spend more time in slow-wave, restorative sleep. If you're spending enough time in bed but waking unrefreshed, it may be worth looking at what your gut microbes are producing - or not producing - overnight.
7. Your Hunger Signals Feel Off
Fibre doesn't just slow digestion. It actively interacts with hunger hormones, particularly GLP-1 and PYY, which signal to the brain that you're full. This process is partly driven by the microbial fermentation of fibre in the large intestine - meaning the bacteria themselves are participating in appetite regulation.
If you find yourself hungry shortly after meals, never quite satisfied, or eating past the point of comfort without meaning to, your microbiome's fibre-fermentation capacity may not be doing its part to send the right signals.
The Science
How Different Fibres Feed Different Microbes
Not all fibre is the same, and not all gut bacteria eat the same things. Think of your microbiome like an ecosystem - the greater the diversity of fibres you eat, the greater the diversity of species that can thrive. Here's a look at key fibre types.
The key takeaway: variety matters as much as volume. Eating the same high-fibre foods every day sustains the same narrow subset of bacteria. Rotating across fibre types, and including clinically studied prebiotic fibres, feeds a wider, more resilient community.
Your Action Plan
How to Get More Fibre Into Your Day (Without the Fallout)
The goal isn't a dramatic overnight overhaul. It's a steady, consistent increase that your microbiome can actually adapt to. Think of it as training your gut bacteria - build the population of fibre-degraders first, and the rest follows.
