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Bugs for your brain? The effect of prebiotics, probiotics and fermented foods on cognition

MEGHAN HOCKEY, Wolf Marx and MELISSA LANE

When it comes to health, we’re all after a quick fix, but new research shows neither popping a prebiotic or probiotic pill, or consuming fermented foods, has an effect on boosting our brain power. So, does this mean you should throw out those pills and kefir for good? Not quite.

Cognition – the scientific term for brain power – refers to the mental processes of gaining knowledge and understanding, including thinking, knowing, remembering, judging and problem solving. Research into the role of diet on cognition is rapidly growing with studies showing that certain diets (i.e. the Mediterranean diet) could have positive benefits for cognition, in particular for the elderly. Although it’s unclear how foods may benefit cognition, researchers have a gut feeling the answer may lie in the microbiota-gut-brain axis – the communication pathway between the bacteria (or bugs) that live within our gut and the brain.

For this reason, it’s thought that supplements and foods that have the potential to change the composition of the gut microbiota, may in-turn have benefits for cognition. This includes probiotics, which deliver beneficial strains of bacteria to influence the gut microbiota composition and activity, and prebiotics, which can be thought of as food or substrates used by the microbiota to grow. Both prebiotics and probiotics can be consumed in supplement form or via foods such as fermented foods (i.e. probiotic milks, kefir and kimchi). As suggested, the consumption of these products can alter the composition and function of the gut microbiota. In turn, this can lead to changes in how the gut interacts with the brain – via the immune system, vagus nerve and neurotransmitters – which could lead to potential changes in how we think and feel.

But findings from our new meta-analysis, suggest there was no significant effect for prebiotic, probiotic or fermented food interventions for global cognition. The meta-analysis published in Neuroscience and Biobehavioural Reviews included 22 randomised controlled trials that investigated probiotic (11 studies), prebiotic (5 studies) and fermented foods (6 studies) interventions. Fourteen (64%) of the included studies within this meta-analysis reported that a prebiotic, probiotic or fermented food intervention improved at least one cognitive measure. But when the results of the included studies were pooled (or combined), these findings were no longer significant which meant there was no apparent evidence of an effect of prebiotics, probiotics or fermented foods on overall cognition.

So why were the overall results no longer significant? Many of the included studies were in healthy populations where cognitive changes may be unlikely to be seen: Put simply, if your petrol tank is already full, adding in more petrol is not going to make your car travel any further.  For probiotic interventions, studies also varied in the strain and dose of bacteria used (known as colony forming units). Different strains have different properties and we don’t yet know what the optimal strain for cognition is (if indeed these probiotics are able to colonise the gut in the first place – but that’s for another time!). Studies were also relatively small in sample size, which means there may not have been enough participants to detect a statistically significant difference, if one existed. And lastly, adding to the complexity, studies were also relatively short in duration which begs the question: Does it just take time for prebiotics, probiotics or fermented foods to work their magic?

In short, these results may not reflect a lack of an effect, but rather a lack of robustly designed studies investigating the effects of prebiotics, probiotics or fermented foods on cognition. Before we write them off as a health fad, there are many more areas of research we need to explore including what strains are best, what dose may be optimal and who (or which populations) may benefit from these interventions. Interestingly, other meta-analyses do show promise for other brain outcomes like depression. But from what we currently know, feeding your gut bugs through prebiotics, probiotics and fermented foods, is unlikely to be a magic fix to boost your cognition.