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 f