The latest study to show a link between a high-salt diet and memory and thinking problems was done in mice. But earlier research on humans suggests a link between high-salt diets and brain health as well.
In the newest study, researchers fed mice either 8 or 16 times the normal amount of salt in their diet. Within 8 to 12 weeks, the mice showed signs of memory and thinking problems. They had trouble telling new and familiar objects apart. It got harder for them to get through mazes. And they couldn't build a nest as well. These behaviors are all central to a mouse’s interaction with the world.
“We translate that in humans to activities of daily living, and that’s what we would call severe cognitive impairment or dementia,” says Costantino Iadecola, MD, a professor of neurology and neuroscience at Weill Cornell Medical College in New York City and the study’s lead researcher.
For humans, the impairment would be like having memory problems, disorientation, and not being able to dress, cook, pay bills, or do other everyday things.
Iadecola says the amount of salt he and his colleagues fed the mice was extreme. It likely would be 5 to 6 times the salt in a typical American diet.
But he says “no one knows exactly how much salt Americans eat, because most of it is hidden in processed foods or restaurant foods. I’m sure it’s grossly underestimated.”
Also, mice showed the same level of impairment no matter which of the two high-salt diets they ate. That suggests to Iadecola that less salt could cause the same problems: “I suspect that you can get cognitive impairment from even lower levels than we used.”
The study could change the conversation about diet with her patients, says Pinky Agarwal, MD, a neurologist who specializes in movement disorders like Parkinson’s disease at EvergreenHealth in Kirkland, WA, and a fellow of the American Academy of Neurology.
When doctors think about salt and patient health, they focus mostly on high blood pressure, or hypertension, cardiovascular disease, and stroke, she says.
““If a patient does not have hypertension, we don’t talk about salt at all,” says Agarwal, who was not involved in the study. “This study provides a good reason for us to ask everyone about salt intake.”
The effects that Iadecola found follow an indirect path. When the mice ate their doctored food, the excess salt triggered a response in their small intestine. It started to produce large amounts of Th17, a type of white blood cell involved with the immune system. This led to a rise in levels of another component of the immune system, a protein called IL-17.
That was a problem for the mice, and it likely would be for humans as well. When the body produces too much IL-17, it can no longer supply enough of a chemical called nitric oxide, which is made by cells lining blood vessels. When nitric oxide levels drop, blood vessels cannot relax, which leads to less blood flow to the brain.
“You need nitric oxide for the brain to work correctly,” says Iadecola.
This discovery that salt can lead to memory and thinking problems via reactions in the gut is exciting, says neurologist David Hafler, MD, who was not involved in the study.
“This shows that dietary influences may be a major influence in vascular dementia,” says Hafler, professor and chairman of neurology and professor of immunobiology at the Yale School of Medicine.
Hafler’s own research supports the significance of this new study. In 2013, he and his colleagues published the first study demonstrating that excess salt boosts levels of Th17 and influences the development of multiple sclerosis. That discovery may help explain the rise in MS and other autoimmune diseases, like psoriasis, inflammatory bowel disease, and rheumatoid arthritis.
“The effect of salt is just absolutely dramatic,” says Hafler.
Iadecola says that some studies have shown higher rates of strokes and dementia in countries where people eat a lot of salt.
“Nobody has figured out how and why, and it’s been very controversial,” he says. “Some people argue that you may do harm by limiting salt intake too much, while others encourage people to curb it to a very small amount.”
A study published in March 2017, for example, found a link between low-salt diets and worsening thinking skills among adults over 50, especially those older than 80.
Two previous studies, however, reached different conclusions.
Canadian researchers reported in 2011 that older adults who got little exercise were more likely to have memory and thinking problems if they ate a high-salt diet.
In 2013, a Turkish study showed that lower levels of salt were linked to better brain ability in adults with high blood pressure.
Agarwal says there's probably a need for tests and questionnaires that will let doctors find out how much salt their patients eat. Most dietary salt, after all, does not come from the saltshaker on your kitchen table. It’s in processed foods and restaurant meals, and that makes it hard to calculate.
Another complication: Different people will have different salt requirements, just like mice. In Iadecola’s study, older mice showed signs of impairment earlier than the younger mice. Other things, such as genetics, also will play a part in how much salt is too much.
“It’s becoming very clear that it very much depends on the genetic variation within the individual as to what salt will do,” says Hafler.
He says that evolution shaped humans to need about 500 milligrams of salt per day. Many Americans eat more than 6 times that amount every day, the American Heart Association says.
“We’re consuming much more salt than nature intended,” Hafler says.
If you love salty food, there could be some good news. Once the mice returned to their normal diet, their brain function returned to normal as well. That suggests that problems caused by eating too much salt may be reversed once you cut back. And, of course, these problems may be prevented by avoiding a high-salt diet in the first place, says Agarwal: “It’s something that can be controlled.”
The mice in the study remained on their high-salt diet for only 3 months. Is there a point at which the brain will begin to suffer permanent damage?
“We don’t know what will happen if you do this for 10 years,” says Iadecola. “My suggestion is that because the blood flow is reduced in the brain and because then the blood vessels in the brain don’t work correctly, something major is going to happen if you keep this up. … Watching salt intake may be an important step to preventing dementia.”