The human kidneys filter and act as a clearinghouse for eliminating wastes from the blood. Now it appears that their lessened performance may serve as warning signs for memory loss and slower cognitive processes in the elderly.

Rush University Medical Center researchers reported in a a study published in the September issue of the journal Neurology that decreased kidney function was associated with a downward slope in thinking, reasoning, and memory in older adults.

Dr. Aron Buchman, associate professor of neurological sciences at Rush and the lead researcher of the study, said his group aimed to answer the following question: “If we knew how old [the subjects were] and knew [what] the renal function looked like, could we predict something about their cognition?”

The study’s results showed a correlation between impaired kidneys and a faster rate of cognitive decline in the old.

“One possibility is that both the kidney disease and the decline can have a common cause like inflammation, hypertension, [or] high cholesterol,” said Dr. Daniel Hier, a neurologist at the University of Illinois at Chicago College of Medicine. “It’s [also] possible that the impairment in kidney function itself actually causes the decline.”

In the normal population, there’s a decline in learning and memory in  people as they age, he said. What’s intriguing about this study, he said, is the rate of the decline.

“[Basically], a 25 percent reduction in kidney function was the equivalent of [the subjects] aging three years faster,” said Heir, who is not involved with the study.

Researchers took blood samples from over 850 subjects participating in the Memory and Aging Project at Rush University, which tracks older adults from the Chicago area and studies neurologic conditions that may be at the root of some diseases associated with aging. The blood samples were then analyzed for specific markers that would tell researchers how much urine was being produced. For purposes of this study, kidney decline was defined as the disruption of normal urine production and the decline in the kidney’s ability to remove wastes from the blood. 

Subjects completed a battery of tests that measured different cognitive abilities. Tests were administered annually, ranging from one to five years, due to the rolling admissions aspect of the Memory and Aging Project. 

The reseachers found that declines in kidney function were associated with declines in semantic memory, long-term memory dealing with words and symbols; episodic memory, recall of events in a person's life, and working memory, a type of short-term memory. However, they found kidney decline was not linked with the type of memory that allows you to estimate the distance between your car and the curb, or perceptual speed, the ability to quickly distinguish visual details. 

Dr. Marsel Mesulam, the director of the cognitive neurology and Alzheimer’s disease center at Northwestern University, praised the study: “It’s an established fact in [medical] textbooks that abnormal function in other areas of the body can influence brain function…but this study shows that less dramatic [abnormalities] also influence cognitive function." 

Continuing research in this field may help tell whether scientists need to focus on preserving kidney function or work on impeding a possible common factor in order to preserve memory, Hier said.