- To much salt causes an immune response in the gut that can be damaging to the lining of blood vessels, a study of live mice and human cells showed
- When this happens, the brain gets less oxygen and is deprived of a chemical key to the formation of memories
- But, when the researchers from Weill Cornell University switched the older mice back to regular diets from high salt ones, their memories improved
Eating too much salt could increase the risk of Alzheimer’s disease, according to new research.
While the US Department of Agriculture recommends we consume about three-quarters of a teaspoon of salt each day, – equivalent to about eight individual-sized bags of chips – most Americans eat nearly 50 percent more than that on a daily basis.
Experiments on mice and human cells suggest that salty foods trigger an inflammatory immune response that deprives the brain of oxygen and harms neurons, triggering behavioral and mental problems.
Importantly, these effects were reversed by returning to a normal diet, providing evidence a change in lifestyle really does work.
An inflammatory response to a high salt diet may contribute to the development of dementia by partially blocking the flow of oxygen-rich blood to the brain, a study found
There are about 5.5 million Americans living with dementia and Alzheimer’s in the US.
Scientists are unsure what causes the degenerative disease and its associated cognitive and memory impairments, but research suggests that both genetic and lifestyle factors contribute to its development.
Plaques or tangles of certain harmful proteins, called beta amyloids, in the brain are the best known markers of Alzheimer’s, but study author Dr Constantino Iadecola said that the brains of people with dementia also have ‘problems with the blood vessels in the brain, they do not look normal.’
Dr Iadecola, a leading expert on stroke and dementia at Weill Cornell University, said the salty diet led to inflammation – which is linked to all major diseases.
His team fed mice a high salt diet – comparable to the excessive proportion of more than one teaspoons a day, found in some human diets.
The study, published in Nature Neuroscience, found that within a few weeks the salty diet led to dysfunction in the endothelial cells that line blood vessels and a reduction in blood flow to the brain.
The gut has an immune response to salt that involves an increase in the number of immune cells known as TH17 – boosting levels of a pro-inflammatory chemical they release called IL-17.
The chemical is damaging to the the endothelial cells that line blood vessels, and in turn ‘the chemical reaction suppresses nitric oxide, which is kind of a potent multi-mediator,’ said Dr Iadecola.
Nitric oxide ‘does all sorts of helpful things [like] help blood vessels relax. It is also needed to make the hippocampus form new memories, and is important to cognitive function,’ he added.
Without nitric oxide, neurons ‘struggle to function,’ he said.
The rise of IL-17 in the blood’s plasma caused the salty diet’s damage to the cerebrovascular system that supplies blood to the brain – leading to behavioral difficulties.
In some types of dementia – including the the most common form Alzheimer’s – the flow of blood to the brain is believed to be reduced.
Dr Iadecola said: ‘Normal cognition function requires an adequate, well-regulated delivery of blood flow.
‘Neurons are very finicky, like little children, they want only one kind of food: only glucose and oxygen.’
Without adequate supplies of those two foods, neurons do not function as well.
The study found mice fed a high-salt diet struggled to identify new objects in recognition tests – showing their non-spatial memory was worse.
Dr Iadecola said: ‘In aged mice fed a high salt diet, performance at the novel object task was impaired earlier, eight weeks, than in young mice, 12 weeks.’
Mice fed a high salt diet struggled in a maze that tested their ability to find an escape hole, which requires spatial memory to learn and remember a location.
They also forgot how to build a nest, using fewer materials in their efforts.
Dr Iadecola said: ‘Nest building and burrowing are spontaneous rodent behaviors and akin to activities of daily living typically altered in patients with cognitive impairment.’
Although the results were obtained in mice, Dr Iadecola also showed that IL-17 similarly affects human cerebral endothelial cells.
He said: ‘This extended lifetime of intake of salt could potentially be one of those factors that works with others to create the dementia.’
But the effects of the salty diet were reversed after the mice were returned to a normal chow diet.
After four weeks, brain scans showed blood flow and endothelial function was healthy again.
He said the findings reveal a new ‘gut-brain axis’ linking dietary habits to mental ill health.
This shows a change in lifestyle or new prescription drugs could help reverse or prevent these effects, said Dr Iadecola.
‘This research not only highlights the importance of the immune system for brain health but also suggests that changes in the gut can play a role,’ said Dr Sara Imarisio, head of research at Alzheimer’s Research UK.
‘The findings highlight the importance of cutting out excess salt in our diets, as well as identifying possible new avenues in the search for treatments to help those with memory problems or dementia,’ she said.
That does not mean that there is a one-size-fits all prescription for salt-intake, however, according to Dr Iadecola.
‘Probably, as in hypertension, salt is going to emerge as a personalized approach, where the [recommended amount] is different depending on what is going on with your person physiologically,’ he said.
He also suggested that drugs that block the effects of the harmful IL17 chemical could help to protect brains from damage that can lead to dementia, beginning long before symptoms show up and it’s ‘too little too late.’