Nearly 10 per cent of people live with chronic kidney disease – in other words, their kidneys can't adequately filter waste from their blood. Now a Canadian study links that malfunction to impaired cognitive performance in reasoning, working memory and reaction time.

The study was conducted by Shady Rahayel, a neuropsychologist and professor in the Department of Medicine at Université de Montréal specialising in brain imaging and computational neuroscience.

In their lab, Rahayel and his team studied the correlation between the glomerular filtration rate – the kidneys' blood filtration capacity – and the cognitive function in approximately 16,000 healthy people.

Participants with weaker kidney function had a lower cognitive performance, regardless of age, sex, education, body mass index or cardio-vascular health. They also demonstrated brain atrophy – that is, shrinkage of the cerebral cortex in the frontal lobes.

Bridging the kidneys and the brain

The mechanisms by which kidney disease is associated with cognitive impairment and neurodegeneration aren't yet fully understood. But when the kidneys are damaged and cannot properly filter blood, waste products do have an impact on the brain.

But that's not all, said Rahayel.

The main cause of kidney disease is diabetes: it causes vascular problems that also damage the brain, he noted. Hence there must be other phenomena at work, since the correlation between kidney and cognitive functions remains, even when the vascular factor is taken into account.

This is also what brain atrophy – as revealed by brain imaging – appears to suggest.

"We measured the thickness of the cerebral cortex in 50,000 locations," said Rahayel. "Although the individuals did not yet present kidney disease, we could already see that kidney function was related to brain atrophy. We also know that atrophy is related to neurodegeneration, dementia and Alzheimer's disease, in particular.

"That's what kidney-brain research is seeking to clarify: if we can better understand the brain regions that are most affected by kidney function, we may ultimately be able to suggest more targeted forms of intervention."