Too much salt may weaken the immune system

A high-salt diet is not only bad for one’s blood pressure but may also be bad for the immune system. This is the conclusion of a study published in the journal Science Translational Medicine.

Mice fed a high-salt diet were found to suffer from much more severe bacterial infections. Human volunteers who consumed an additional 6 g of salt per day also showed pronounced immune deficiencies. This amount corresponds to the salt content of two fast-food meals.

This finding is unexpected, as some studies point in the opposite direction. For example, infections with certain skin parasites in laboratory animals heal significantly faster if these animals consume a high-salt diet: The macrophages, which are immune cells that attack, eat, and digest parasites, are particularly active in the presence of salt. Several physicians concluded from this observation that sodium chloride has a generally immune-enhancing effect.

“Our results show that this generalization is not accurate,” said Katarzyna Jobin, lead author of the study, formerly with the University of Bonn, in a university press release. There are two reasons for this: Firstly, the body keeps the salt concentration in the blood and various organs largely constant. Otherwise, important biological processes would be impaired. The only major exception is the skin: It functions as a salt reservoir of the body. Therefore, the additional intake of sodium chloride works so well for some skin diseases.

However, other parts of the body are not exposed to the additional salt consumed with food. Instead, it is filtered out by the kidneys and excreted in the urine. And this is where the second mechanism comes into play: The kidneys have a sodium chloride sensor that activates the salt excretion function. As an undesirable side effect, however, this sensor also causes so-called glucocorticoids to accumulate in the body. And these, in turn, inhibit the function of granulocytes, the most common type of immune cell in the blood.

Granulocytes, like macrophages, are scavenger cells. However, they do not attack parasites, but mainly bacteria. If they do not do this to a sufficient degree, infections proceed much more severely. “We were able to show this in mice with a Listeria infection,” explained Jobin. “We had previously put some of them on a high-salt diet. In the spleen and liver of these animals, we counted 100 to 1,000 times the number of disease-causing pathogens.”

Sodium chloride also appears to have a negative effect on the human immune system. After volunteers consumed 6 g of salt in addition to their daily intake for one week, the scientists took blood from their subjects and examined the granulocytes. The immune cells coped much worse with bacteria after the test subjects had started to eat a high-salt diet.