A new gut-brain connection has been revealed in a study from scientists at Weill Cornell Medicine. The research found that mice fed a high-salt diet directly led to cognitive impairment, dementia, and reduced blood flow in regions of the brain commonly associated with learning and memory.

The study examined mice administered diets with food containing either four or eight percent salt, which equated to between eight and 16 times more salt than a normal healthy diet. These are extremely high levels of salt and only comparable to an extraordinarily high level of human consumption.

The negative results were dramatic, with a 28 percent drop in blood flow in the cortex, and a 25 percent drop in the hippocampus after just eight weeks. The mice on the high-salt diet also performed significantly worse on several behavioral tests, including a maze test, nest building and an object recognition test.

The impaired blood flow in the brain was found to be related to a reduction in the production of nitric oxide, a gas generated by endothelial cells. This reaction to the high-salt diet was shown to be reversible, with cerebral blood flow normalizing four weeks after returning to a regular diet.

Further study homed in on the mechanism that could be causing this response, and it was revealed that it all began with an adaptive immune reaction in the gut. In response to the high salt intake, white blood cells overproduced interleukin 17 (IL-17), a protein known to reduce nitric oxide in endothelial cells.

Perhaps the most intriguing part of the study came when the researchers administered a drug called ROCK inhibitor Y27632 that reduced levels of IL-17 and prevented the reduction in nitric oxide production. The mice responded positively to the treatment, improving in both cognitive tests and behavioral observations.

"The IL-17-ROCK pathway is an exciting target for future research in the causes of cognitive impairment," says Giuseppe Faraco, first author of the study. "It appears to counteract the cerebrovascular and cognitive effects of a high-salt diet, and it also may benefit people with diseases and conditions associated with elevated IL-17 levels, such as multiple sclerosis, rheumatoid arthritis, inflammatory bowel disease and other autoimmune diseases."

Of course, it is worth noting that this study was just in mice, and the levels of salt the animals were fed was extremely high. In humans the only current cognitive impairment known to be caused by salt intake is related to high-blood pressure and hypertension resulting in what is called vascular dementia.

What was notable in this mouse study is that the dementia developed in the animals was irrespective of blood pressure. If these effects do translate to humans then it means that high salt intake could be having a damaging effect on our cognitive abilities that builds over long periods of time.

The study was published in the journal Nature Neuroscience.

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