Diet rich in salt weakens the antibacterial immune defense

A high-salt diet weakens the immune system, according to a new study by the University of Bonn in Germany and published in the journal Science Translational Medicine.

For the study, mice fed a high-salt diet were found to suffer from much more severe bacterial infections. Human volunteers who consumed an additional six grams of salt per day also showed pronounced immune deficiencies. This amount corresponds to the salt content of two fast food meals, the researchers said.

The World Health Organization recommends no more than five grams of salt per day, equivalent to approximately one level teaspoon. Many people exceed this limit, according to the study. Sodium chloride, salt’s chemical name, raises blood pressure and thereby increases the risk of heart attack or stroke.

This finding is unexpected, the researchers said, as some studies point in the opposite direction. For example, infections with certain skin parasites in laboratory animals heal significantly faster if these 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.

The results show this is not accurate, the researchers said. The body keeps the salt concentration in the blood and in the various organs largely constant. Otherwise important biological processes would be impaired. The only major exception is the skin, which functions as a salt reservoir of the body. 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. The kidneys have a sodium chloride sensor that activates the salt excretion function. As an undesirable side effect, this sensor also causes glucocorticoids to accumulate in the body. 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.

In human volunteers, the excessive salt intake also resulted in increased glucocorticoid levels. That this inhibits the immune system is not surprising, the researchers said, because the best-known glucocorticoid cortisone is traditionally used to suppress inflammation. 

"Only through investigations in an entire organism were we able to uncover the complex control circuits that lead from salt intake to this immunodeficiency," said Christian Kurts, PhD, co-author of the study from the Institute of Experimental Immunology at the University of Bonn. "Our work therefore also illustrates the limitations of experiments purely with cell cultures."