The gut microbiota are responsible for the production of numerous metabolites from food; these metabolites then enter the circulation, where they can affect cellular processes by known and unknown mechanisms. Here, Lesley Hoyles, PhD, CBiol, MRSB, of Imperial College London, London, UK, discusses the role of gut-derived trimethylamine (TMA) and its detoxification product trimethylamine N-oxide (TMAO) on the blood-brain barrier. This video was recorded at the Alzheimer’s Research UK Conference 2018, held in London, UK.