You’ve added the oats and lentils, so why don’t you feel any better? Alexandra Cortina, working at the intersection of psychology and nutrition, explains the missing link between fibre, fermented foods and how we feel.
It’s a pattern I see often in clinic. Someone has done everything they’ve read about: porridge in the morning, lentils in the week. Their fibre intake has gone up, yet they feel bloated, tired and flat. The problem usually isn’t their effort. It’s that fibre is only half of the story.
The fibre paradox
Fibre feeds your gut bacteria. They ferment it into short chain fatty acids such as butyrate, which help protect the gut lining and keep inflammation in check. UK guidelines recommend 30g a day, and the average adult manages around 20g. Closing that gap is worthwhile. But a 2021 Stanford study suggests fibre alone may not transform your gut. Researchers put healthy adults on either a high fibre diet or a diet rich in fermented foods for ten weeks. The fermented food group saw their gut diversity steadily rise and 19 markers of inflammation fall. In the high fibre group, diversity stayed the same and those markers didn’t move. The researchers found that some people in the fibre group weren’t fully breaking down the extra fibre. How they responded depended on the bacteria they started with. Fibre can only be used by the microbes you already have. If the right ones aren’t there, adding more may just leave you uncomfortable.
Why this matters for your mood
This isn’t only about digestion. Your gut microbes produce compounds that influence inflammation and communicate with the brain, and the evidence linking diet and mental health keeps growing. In the SMILES trial, adults with moderate to severe depression who received twelve weeks of support to eat a Mediterranean style diet, alongside their usual treatment, were far more likely to reach remission than those given social support. Around a third recovered, compared with fewer than one in ten. A separate Belgian study of more than a thousand people found certain gut bacteria were consistently lower in people with depression, even after accounting for medication. Neither study proves that gut bacteria causes low mood. Together, they suggest that what we eat and how we feel are more closely linked than we once thought.
Where psychology meets nutrition
My work sits at the intersection of psychology and nutrition, and I see every day how closely the two are connected. Low mood, anxiety and stress show up in the gut, and what happens in the gut shows up in how we think and feel. When someone is running on erratic meals, little fibre and a gut that’s struggling, they often have less in reserve for everything else. It is harder to concentrate, harder to regulate emotions and harder to engage with the work of feeling better. Food is not a replacement for mental health support. But it shapes the physical foundation that everything else rests on. When that foundation is steadier, people often find they have more capacity to reflect and to remain flexible under stress.
Seeds and soil
The way I explain it to clients is simple. Fibre is the soil. Fermented foods may help supply new seeds. The Stanford researchers suggested that combining the two could be the logical next step, and while that hasn’t yet been tested directly, it’s the approach I take: feed the microbes you have, introduce new ones and give your gut time to adjust.
How to put it into practice
Pair fibre with fermented foods. Overnight oats made with kefir, a lentil salad with a spoonful of kimchi, or wholegrain toast with live yoghurt and berries all do both jobs at once.
Eat a rainbow. Different coloured plants contain different polyphenols, many of which your gut bacteria also feed on. Try to include red, orange, yellow, green and purple foods across the week, such as berries, peppers, squash, leafy greens and red cabbage.
Build fibre slowly. A sudden increase is the most common reason people give up. Add one fibre rich food every few days and drink plenty of water.
Start small with fermented foods. One or two servings a day is a good beginning. Choose products labelled live or unpasteurised, as heat treatment destroys the bacteria.
Track how you feel. Note your energy, mood and digestion for a few weeks. Changes tend to be gradual and easy to miss.
A word of caution
Diet supports mental health, but it doesn’t replace professional care. If you’re struggling with low mood, speak to your GP. Fermented foods may not suit everyone, including people with histamine intolerance or some digestive conditions, so seek personalised advice if you’re unsure.