45 volunteers, three weeks, two fibres
In 2015, the journal Psychopharmacology published a trial involving 45 healthy volunteers. Each person took one of two prebiotic fibres, or a placebo, every day for 3 weeks.
Researchers weren't testing a diagnosed condition. They were testing whether feeding gut bacteria a specific fibre could change measurable signals in the body and mind of people who were, by clinical standards, well.
This distinction matters. The trial tells us something about how the gut and brain communicate under normal conditions, not about whether fibre resolves any disorder.
Why cortisol was the number to watch
Cortisol is the hormone that spikes shortly after waking, part of what's called the cortisol awakening response. It's a standard, well-studied marker of how the body's stress system is calibrated.
Gut bacteria produce compounds that can interact with the nervous system, including signals that travel via the vagus nerve and molecules that influence hormone regulation. This is the biological basis of what's called the gut-brain axis.
Measuring waking cortisol gives researchers a concrete, repeatable number, rather than relying on how someone says they feel. That's part of why it was chosen for this trial.
What the galacto-oligosaccharide group showed
The trial found that the galacto-oligosaccharide group woke with a lower salivary cortisol response than the placebo group after the 3-week period.
Lower waking cortisol was associated with a second finding: this same group paid less attention to negative words in a standard attention test, a task designed to measure how the brain prioritises threatening or negative information.
The other prebiotic fibre tested in the trial showed no such effect on either cortisol or attention to negative words. Not all fibres behave the same way in this context, and the trial's design lets researchers see that difference clearly.
What this does and doesn't prove
The word 'associated' matters here. The trial found a correlation between taking a specific fibre and lower waking cortisol, plus a shift in attention patterns. It did not establish that the fibre caused every step of that chain, nor trace the exact mechanism.
The sample size, 45 people, is modest by clinical trial standards. It's enough to detect a real signal worth investigating further, but not enough to generalise confidently to all adults, let alone people with diagnosed anxiety or mood conditions.
All participants were healthy volunteers. The trial says nothing about whether this fibre would produce similar effects in someone with a stress-related disorder, and no claim like that can be drawn from this data.
Where the evidence sits right now
What the 2015 trial demonstrates is that a dietary input, a specific prebiotic fibre, can be linked to a measurable change in a stress hormone and a measurable change in attention, within three weeks, in a controlled setting.
That's a meaningful data point for gut-brain research. It shows the axis is not just a theoretical idea, it's something researchers can put a number on.
Anyone curious about their own gut health can note this as one data point among many, not as a instruction to alter their diet in pursuit of stress relief. The honest reading is: interesting, early, worth more research.