Urine analysis can monitor gut microbial metabolism, and impaired kidney function can increase blood levels of microbial metabolites that may cross the blood-brain barrier.

“We know that a lot of these metabolites, when they make their way into the bloodstream, eventually are excreted through the kidneys in urine. So actually we can monitor the metabolism that's going on in your gut by actually looking at the metabolites that are present in your urine, because many of those originated in your gut, from your gut microbes. But people with kidney disease, whose kidneys' filtering processes is not functioning properly, actually build up high levels of many of these metabolites into the bloodstream, and that can lead to more of these molecules making it across the blood-brain barrier.”