Fibre supplements did nothing for mood in the trials that tested them

Interest in the gut as a lever on mental health has grown fast, and dietary fibre is the most obvious dietary handle on it, since fibre feeds the gut microbes thought to communicate with the brain. Surveys repeatedly report that people who eat more fibre also report better mood, which has fuelled a market in fibre supplements sold on wellness grounds. But eating patterns travel with dozens of other habits, so an association in a survey cannot tell you whether the fibre itself is doing anything. The only way to separate the two is to give fibre deliberately and see what happens.

The authors searched five research databases and pulled together both types of evidence in one review. On the observational side they pooled 23 studies covering 181 405 participants, comparing fibre intake against depressive and anxiety outcomes both at a single point in time and over follow-up. On the trial side they pooled 10 randomised controlled trials involving 740 participants, all of which used fibre supplements rather than food, and compared them with placebo. Screening and risk-of-bias assessment were done independently by two reviewers, and the pooling used random-effects models.

In the studies measuring people at one point in time, higher fibre intake was linked with lower depressive scores and lower anxiety scores, with the anxiety association the larger of the two. In studies that followed people over time, higher fibre intake was again linked with lower depressive outcomes, though the association was smaller still. The trials told a different story: fibre supplementation did not differ from placebo for depressive outcomes or for anxiety outcomes. Notably, only one of the 10 trials enrolled people with a clinical diagnosis of depression, so the trial evidence is almost entirely from people without a diagnosis.

This is a familiar pattern in nutrition and mental health: an observed link in populations that does not survive being tested as a single isolated nutrient. It complicates the simple story that fibre lifts mood via the gut, without ruling it out, because supplements deliver a narrow range of fibre types over short periods while a high-fibre diet is a broader and longer-running exposure. The authors themselves call for more trials in clinical populations and across a wider range of fibres. The open question is whether whole-diet changes would perform differently from capsules.

If you eat plenty of fibre-rich food, this is not a reason to stop, since fibre intake has plenty of other established reasons behind it. It is, however, a reason to be sceptical of fibre supplements marketed for anxiety or low mood, because the trials that tested exactly that found nothing beyond placebo. The associations that do exist are small and come from studies that cannot separate fibre from everything else a higher-fibre eater tends to do. Anyone dealing with depression or anxiety is looking at a question this evidence does not yet answer.