The Salty Shake Up
Salt, or Sodium Chloride (NaCl), is a mineral compound that is part of a wider class of compounds called the “Salts”. In the natural world salt is a crystalline mineral known as halite, or “rock salt”. It is also found in huge quantities in sea water where 1 litre of sea water is thought to contain 35 grams of salt (3.5% salinity).
Salt is essential for life, due to it containing sodium, specifically it has a high proportion of Sodium ions. It is necessary for plants in small quantities, but animals use sodium in large amounts in order to help generate nerve impulses and in order to maintain electrolyte and fluid balance, heart activity and wider metabolic functions. Knowing all of this it isn’t surprising that saltiness is one of us humans basic tastes. Salt has been a major part of human diets for thousands of years, and has quickly risen to becoming the major provider of sodium in the western world.
Although salt is a critical part of human homeostasis, it is well known that too much salt has been shown to have a myriad of negative health effects. These range from increased blood pressure, cardiovascular disease, stroke and kidney disease. The current recommended salt intake for healthy adults is 5g a day, which equates to 2000 mg of sodium a day. This is reduced to 1000 mg of sodium (2.5 g salt) in those with cardiovascular risk factors, as a studies have suggested such a decrease in consumption can reduce the risk of cardiovascular disease by 30%.
Salt is known to increase the risk of stroke. It has been hypothesised that the main factor that linking salt to stroke is its ability to increase blood pressure. Now scientists from Weill Cornell Medicine in New York, have revealed that in mice eating too much salt suffer from cognitive impairment in the absence of high blood pressure. Up until recently however there was no evidence of the impact that salt has upon cognitive ability, let alone any understanding how this may occur, but this study has opened up new possibilities for understanding as the scientists believe they may now know the reason why salt can have this profound cognitive effect!
The study involved feeding mice between 8 and 16 times the normal amount of salt they required, then performed cognitive tests, after two months on this diet the mice were unable to recognise new objects compared to control mice, and also struggled to find their way out of a maze.
Scientists originally believed that this cognitive impairment was a direct effect of salt entering the brain and causing damage to brain tissue. However when they took the brain tissue and analysed it using immunoblot techniques they discovered something else was going on! They discovered that the mice on high salt diets (HSD) had a build up of tau protein, the protein associated with Alzheimer's disease.
The researchers didn’t stop here however they began to probe the reason, or indeed the mechanism, why the HSD might have this effect. They found that HSD caused an increase in immune T-cells in the guts of the mice and that these T-cells release chemical messengers that travel through the blood brain barrier and signal to the brains blood vessels to produce less nitric oxide.
These reduced levels of nitric oxide result in lower blood flow and increased CDK5 activity, an enzyme that promotes the build-up of tau. Interestingly, if the scientists restored nitric oxide in the mice on HSD their cognitive ability improved.
This study is the first of its kind to demonstrate a causal link between dietary salt, blood vessel dysfunction and the brain. It also challenges the perceived notion that reduced blood flow is a trigger for dementia in the brain, as when tau is reversed and the blood flow remains low, dementia disappears.
Journal reference: Nature, DOI: 10.1038/s41586-019-1688-z