275 people, three groups, one question
A trial published in the BMJ in 2009 recruited 275 people with irritable bowel syndrome from primary care clinics and split them into three groups.
Each group took a daily fibre supplement for 12 weeks: 10 g of psyllium, 10 g of bran, or a placebo with no active fibre.
The design meant researchers could compare two different fibres against each other and against a control, rather than simply asking whether fibre in general helped.
Why gut researchers look at fibre at all
Irritable bowel syndrome has been linked in various studies to differences in gut bacteria, gut sensitivity, and how quickly waste moves through the bowel.
Fibre is one of the few dietary inputs that changes all three of these at once, because different fibres are fermented at different rates and by different bacterial species.
This is why researchers have wanted head-to-head fibre trials for irritable bowel syndrome specifically, rather than assuming all fibre acts the same way.
The 2009 trial was designed around that exact gap, comparing a soluble, gel-forming fibre against an insoluble, more fermentable one.
What the numbers actually showed
In the first month, the trial found that 57% of the psyllium group reported adequate symptom relief, compared with 35% on placebo.
By three months, average symptom severity scores had fallen by 90 points in the psyllium group, versus 49 points in the placebo group.
Bran, by contrast, was no better than placebo for most of the 12 weeks, and the trial reported that dropouts were highest in the bran group because symptoms got worse.
This pattern was associated with the physical behaviour of each fibre type in the gut, though the trial did not measure the underlying mechanism directly.
What this evidence does and doesn't tell us
A single trial of 275 people, however well designed, cannot settle how fibre works for every person with irritable bowel syndrome.
The improvement with psyllium was measured against placebo, and the association between fibre type and symptom scores does not prove a single biological pathway caused the difference.
Bran's poor performance may reflect how coarse, insoluble fibre interacts with an already sensitive bowel, but the 2009 trial did not test this directly.
Reading a study like this honestly means noting what it measured, over what period, and in whom, rather than extending its findings beyond the original 275 participants.
A practical detail worth remembering
The clearest takeaway from this trial is specific: 10 g of psyllium daily was linked to measurable changes in symptom scores over 12 weeks, while 10 g of bran was not.
Anyone reading fibre research for irritable bowel syndrome should ask which fibre was tested, not just whether fibre was tested at all.