A current study under leadership at the University Hospital Bonn, has concluded that a high salt diet not only is bad for blood pressure, but also for the immune system. Mice who were fed a high salt diet were found to suffer from more severe bacterial infections. Human volunteers who also consumed high levels of salt also showed pronounced immune deficiencies.
Sodium chloride which is the chemical name for salt, raises blood pressure which leads to the risk of stroke or heart attack. However, not only that – the recent study has proved for the first time that excessive intake of salt also significantly weakens an important part of the immune system.
The research team found their findings unexpected as previous studies pointed in the opposite direction. For example, infections with certain skin parasites in lab animals heal much faster if they consume a high salt diet. The macrophages which are immune cells that attack, eat and digest parasites, are very active in the presence of salt. Several doctors concluded therefore that salt has a generally immune enhancing effect.
The results from the current study show this generalization to be inaccurate. There were two reasons for this. The body keeps the salt concentration in the blood and in a variety of organs largely constant. Otherwise biological processes within the body would be impaired. The only major exception is the skin as it functions as a salt reservoir for the body. This is the reason additional intake of salt works so well for some diseases of the skin.
However, other parts of the body are not exposed to the additional salt that is consumed with food. It is filtered by the kidneys then excreted in the urine. This is where the second mechanism comes into play. The kidneys have a sodium chlorine sensor that will activate the salt excretion function. As a negative side effect however, this sensor also causes so called glucocorticoids to accumulate in the body. These in turn inhibit the function of granulocytes which are the most common type of immune cell contained in blood.
Granulocytes are scavenger cells. However, they do not attack parasites but mainly bacteria. If they are not able to do this to a sufficient degree, infections will proceed much more severely. The team was able to show this in mice that had listeria infections.
The team fed one group of mice a high salt diet and gave a normal diet to a control group of mice for comparison. In the liver and spleen of the mice fed a high salt diet they counted 100 to 1,000 times the number of disease causing pathogens. Listeria are bacteria that can be found in contaminated food for instance and can result in fever, vomiting and sepsis.
They also found infections of the urinary tract were found to heal much more slowly in lab mice that had been fed a high salt diet. The team traced this impaired ability to fight off bacteria infections to immune cells called neutrophils which digest bacteria. They believe the kidneys’ response to the high salt diet may indirectly affect the neutrophils.
Salt also appears to have a negative effect on the human immune system. The team examined participants who consumed six grams of salt in addition to their normal daily intake. This is about the amount contained in two fast food meals – two burgers and two servings of french fries. After one week, the team took blood from the participants and examined the granulocytes. The immune cells coped much worse with bacteria after the participants had started to consume a high salt diet.
In was also noted in the human participants, the excessive salt intake also resulted in increased glucocorticoid levels which inhibits the immune system. The best known glucocorticoid cortisone is typically used to suppress inflammation. Only through the team’s investigations in an entire organism were they able to uncover the complex control circuits that lead from salt intake to this particular immunodeficiency.
The findings are preliminary, and need larger clinical studies to confirm. However, according to the World Health Organization, a person should never consume more than 5 grams a day of salt. This amount corresponds to about one level teaspoon. Data from the Robert Koch Institute show that on average men consume 10 grams of salt per day and women 8 grams per day.
To view the original scientific study click below