Eating more than the recommended amount of salt disrupts the antibacterial function of a type of immune cell, research in mice and humans has found.
According to the American Heart Association (AHA), 9 out of 10 people in the United States consume too much salt (sodium chloride).
The Dietary Guidelines for Americans, which the Department of Health and Human Services publish, recommend that people consume no more than 2.3 grams (g) of sodium per day. This amount is roughly equivalent to 5.8 g of salt, which would fit into a level teaspoon.
The reason for the recommendation is that there is good evidence that excess dietary salt raises blood pressure, which is a risk factor for heart disease and stroke.
However, a new study featuring in Science Translational Medicine suggests for the first time that such a diet could also make it harder for the immune system to destroy bacteria in some human organs.
Researchers at the University Hospital of Bonn in Germany were surprised to discover that a high salt diet in mice exacerbated a common bacterial infection of the kidneys — Escherichia coli.
To test whether the deleterious result of a high salt diet was purely a local effect on the kidneys, the researchers infected the mice with Listeria and found that this body-wide, systemic infection was also worse on a high salt diet.
These findings were unexpected because previous research has found that excess dietary salt promotes healing in animals infected with skin parasites.
Skin acts as a reservoir for excess salt, and immune cells in the skin called macrophages are known to become more active in these salty conditions.
In contrast, it seems that a different type of immune response cell, the neutrophil, which is key to the body fighting bacterial kidney infections, becomes less effective in the face of a high salt diet. The researchers were intrigued to find out why.
In the rest of the body, however, the kidneys help maintain the concentration of salt at optimum levels for metabolism by excreting excess sodium.
The new study suggests that in the process of the kidneys regulating high blood sodium levels, they inhibit one arm of the immune system, impairing its ability to fight off bacterial infections.
The researchers fed one group of mice a high salt diet and gave a normal diet to a “control” group of mice for comparison.
When infected with Listeria, the liver and spleen of mice on the high salt diet contained 10–100 times more bacteria than those of the controls.
Similarly, excess dietary salt made kidney infections with E. coli worse.
The researchers traced this impaired ability to fight off bacterial infections to immune cells called neutrophils, which ingest bacteria.
They believe that the kidneys’ response to high dietary salt may indirectly affect the neutrophils.
The kidneys use a molecular mechanism for detecting excess sodium in the bloodstream and excreting it in the urine.
But in the process, this mechanism raises levels of steroid hormones called glucocorticoids, as well as a waste product called urea.
Both glucocorticoids and urea inhibit the ability of neutrophils to kill bacteria.
To confirm these findings in humans, the researchers put volunteers on a high salt diet.
One of the scientists, Prof. Christian Kurts from the Institute of Experimental Immunology, University of Bonn, explains:
“We examined volunteers who consumed 6 g of salt in addition to their daily intake. This is roughly the amount contained in two fast food meals, i.e., two burgers and two portions of French fries.”
After a week on the high salt diet, the volunteers had higher levels of glucocorticoids in their bodies.
Glucocorticoids are well-known for their immunosuppressant properties. Doctors use one of the most familiar, cortisone, clinically to reduce inflammation.
In addition, the researchers discovered that neutrophils that they extracted from the blood of volunteers on the high salt diet were less effective at killing bacteria in a laboratory dish.
In their paper, the scientists speculate that reducing dietary salt intake might help combat bacterial infections in the kidney, in contrast to its effect on skin and gut infections.
Their findings are preliminary, however, and need larger clinical studies to confirm them.