Because they are not metabolised, these molecules are referred to as "refractory".

Those with higher levels of para-cresol sulphate, made from para-cresol produced by bacteria in the gut, metabolised paracetamol less effectively than those with lower levels.

The scans, which used a technique called positron emission tomography (PET), were designed to monitor changes in the way the brain metabolised glucose, the fuel it needs to function.