Dietary Salt Causes Cognitive Impairment

For several years, researchers have been studying "the gut-brain axis."  Essentially, there is a form of communication between gut microbes and the brain, and when things go wrong in the gut they can go wrong in the brain.  Also, it's easy to make the case that excess salt intake will have a negative effect on cognition because it raises blood pressure.  High blood pressure is bad for the heart and is a risk factor for Alzheimer's disease.  What's good for the heart is good for the brain.

But a December 2018 article in Scientific American (reproduce here by Pocket) discussed a separate gut-brain axis involving salt but that does not involve blood pressure.  The article's author cited research published in Nature Neuroscience about work with mice.  It found "excess dietary salt suppresses resting cerebral blood flow and endothelial function, leading to cognitive impairment. The effect depends on [certain cells] in the small intestine."  "Endothelial function" refers to the work of endothelial cells lining the blood vessels that control contraction and relaxation of the blood vessels, among other things.  Its failure contributes to a number of bad consequences for both the heart and the brain.

The research with mice points to a pathway between the gut and the brain via the immune system.  Then excessive intake of salt can impact the immune function in the small intestine with a negative effect reflected back to the brain.  A very important consequence is that blood flow may be restricted to the hippocampus and the cortex -- ground zero for Alzheimer's disease.

Prior the research on salt, there was research studying other contributions that issues in the gut can make to Alzheimer's disease.  One idea is that microbes may leak from the gut when the spectrum of gut microbes is disturbed.  The leaked microbes can promote inflammation throughout the body, including in the brain.  Inflammation in the brain plays a role in Alzheimer's disease.

The inflammation can cause impairment of the blood-brain barrier.  The blood-brain barrier protects the brain from pathogens by only allowing small molecules and ions to pass in and out of the blood vessels in the brain.  This is another work of the endothelial cells lining the blood vessels in the brain.  (Special proteins regulate the passage of certain specific molecule that must pass in and out.)  Some studies suggest that impairment of the blood-brain barrier allows beta amyloid molecules to enter the bloodstream and then be more widely distributed in the brain, thus propagating Alzheimer's disease.

Speaking for myself, I have always had fairly low blood pressure.  During periods when it seems too low my cardiologist suggested raising my intake of salt.  Maybe that's good advice, but I'm wondering what it means for neurodegeneration in light of this research.