Is salt “vicious” for cognitive function? Study finds high-salt diet impacts dementia risk in mice

24 Oct 2019 --- High levels of dietary salt may negatively impact cognitive function by causing a deficiency of the compound nitric oxide, which is vital for maintaining vascular health in the brain. This is according to a new mice study published in Nature by Weill Cornell Medicine researchers. They established that the impaired cognitive function is not due to a loss in blood flow to the brain as originally thought, but rather to the formation of tau protein “clumps,” which is linked to several forms of dementia in humans. The researchers suggest that avoiding excessive salt intake may help to stave off the vascular and neurodegenerative pathologies that underlie dementia in the elderly.

“There is some epidemiological evidence suggesting that high salt diets are associated with cognitive impairment and that the incidence of dementia is higher in countries with high salt consumption,” Dr. Costantino Iadecola, Director and Chair of the Feil Family Brain and Mind Research Institute at Weill Cornell Medicine, tells NutritionInsight.

Commenting on the study, Professor Graham MacGregor, Professor of Cardiovascular Medicine at Queen Mary University of London and Chair of Action on Salt, says, “This important study illustrates the potential mechanism whereby salt, which significantly increases our blood pressure, can directly increase the risk of dementia and be damaging to the brain. This adds to the ever-growing evidence that we should all reduce our salt intake. With the majority of our salt intakes coming from processed food made by the food industry, it is therefore their duty to remove it for the benefit of our health.”

The detrimental impact salt can have on the human body is increasingly understood on both industry and policy levels. Recently, it has been brought to the discussion table that salt available via retailers and salt shakers in restaurants should both carry a front-of-pack, tobacco-style health warnings. Statement author Dr. Norm Campbell noted that unhealthy diets are a leading cause of death globally and excess salt consumption is the biggest culprit, estimated to cause over three million deaths globally in 2017.

“This result was completely unexpected,” says Dr. Costantino Iadecola, Director and Chair of the Feil Family Brain and Mind Research Institute at Weill Cornell Medicine. “We knew that a high-salt diet produced dementia-like symptoms in mice, and we went in thinking the culprit would be reduced blood flow to the brain. It turned out that wasn’t the case at all.”

The body needs salt; but too much can cause severe health risks, especially regarding vascular diseases.The researchers established a chain-reaction between a high-salt diet and  affected cognitive function. High quantities of dietary salt resulted in a decreased production of nitric oxide in blood vessel cells in the brain. In turn, this restricts blood flow to the brain. The researchers initially assumed that restricted blood flow to the brain caused cognitive impairment in their mice subjects, but digging deeper, they found that there was a different explanation. 

Nitric oxide makes up part of a molecular pathway connected to the tau protein. In the former’s absence, the tau protein begins to clump together and creates aggregates. It is precisely these tau clumps that interfere with the proper function of brain cells, which can lead to cognitive impairment. Tau namely provides structure for the scaffolding of neurons. This scaffolding, also called the cytoskeleton, helps to transport materials and nutrients across neurons to support their function and health.

Studying their mice, the Cornell researchers observed that the mice with increased salt intakes could no longer perform daily tasks such as building their nests and had problems passing memory tests. To further explore the importance of tau in dementia, the researchers gave mice with a high-salt diet and restricted blood flow to the brain an antibody to promote tau stability. Despite restricted blood flow, researchers observed normal cognition in these mice. “This demonstrated that’s what’s causing the dementia was tau and not lack of blood flow,” Dr. Iadecola affirms.

He encourages researchers to conduct more rigorous epidemiological studies investigating the link between salt and dementia, particularly studies exploring the link with Alzheimer’s disease and Frontotemporal dementia. However, the current mouse study is a reminder for people to regulate salt consumption, he warns. 

“The stuff that is bad for us doesn’t come from a saltshaker; it comes from processed food and restaurant food. We’ve got to keep salt in check. It can alter the blood vessels of the brain and do so in a vicious way.” He notes that the Mediterranean diet, low in salt, is considered the most effective diet to prevent cognitive impairment. However, once the dementia sets in, there is little that can be done at the moment.

War on salt
The war on salt wages on. In Scotland, food and drinks high in fat, sugar or salt (HFSS) have become an increasing consideration to the government, which is moving to become one of the first nations to propose restrictions to limit the promotion and marketing of HFSS food and drinks on a national scale. 

Moreover, the publication of the European Food Safety Authority’s dietary reference values has set a safe and adequate sodium intake at 2g per day. The body may need salt to function however, the study warns that sodium can cause a number of health risks when consumed in excess. 

While in salt reduction efforts, critics have said that despite meat-alternatives benefiting from a health halo relating to various claims, including gluten-free, organic and vegan, they are often highly processed and packed with salt. Researchers are therefore on a quest to find salt alternatives without compensating savory taste. A white paper from Kerry Health & Nutrition Institute states that umami could be key in salt reformulation and clean label efforts, as well as in creating more appealing senior nutrition diets. 

By Anni Schleicher