A study conducted by researchers at the University Hospital of Bonn has shown that a high salt intake inhibits the immune system’s ability to combat bacteria.
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The researchers found that when they fed mice a diet that was rich in salt, the animals developed significantly more severe bacterial infections than when the diet was not rich in salt.
Furthermore, when people added six grams more salt per day to their usual daily intake, their immune cells were significantly less effective at fighting bacteria. This additional salt intake would be approximately the amount consumed if a person ate two fast-food meals.
No more than five grams of salt per day
The World Health Organization (WHO) advises that the maximum intake of salt per day should be no more than five grams, which is the equivalent of about one teaspoon full. However, data has shown that in Germany, people often consume far more salt than this recommended amount.
Data collected at the Robert Koch Institute indicate that German men consume an average of ten grams per day and women an average of more than eight grams per day. This has adverse health effects since too much sodium chloride is known to increase blood pressure and the risk of hypertension, heart attack, and stroke.
However, the current study, which was recently published in the journal Science Translational Medicine, now suggests that impaired immunity can be added to the list of adverse health effects.
Professor Christian Kurts from the university’s Institute of Experimental Immunology says: "We have now been able to prove for the first time that excessive salt intake also significantly weakens an important arm of the immune system."
The researchers were surprised by the findings since previous research has suggested that the opposite is the case. For instance, some research has shown that animals with certain parasitic skin infections clear the infections much more quickly once they are put on a diet rich in salt. Macrophages – the large phagocytic cells that target and ingest parasites – become more active in a salty environment and several researchers have therefore previously assumed that salt generally has a positive effect on the immune system.
However, lead author of the current study, Katarzyna Jobin says "Our results show that this generalization is not accurate.”
Firstly, the concentration of salt in blood and in the organs is regulated and generally sustained at a constant level, since key bodily processes would be disrupted if it was not. The only organ where this does not apply is the skin, which serves as a reservoir for salt. This explains why consuming additional salt helps to combat certain skin diseases. However, other organs do not get exposed to the extra sodium chloride found in a high-salt diet. Rather, the additional salt is processed in the kidneys and passed into the urine for excretion.
Furthermore, a level sensor in the kidney detects the additional salt and induces its excretion. However, this has an adverse side effect because the sensor also triggers the accumulation of a group of corticosteroids called glucocorticoids and these inhibit immune cells called granulocytes.
What happens when granulocytes are inhibited?
Similar to macrophages, the main role of granulocytes is to ingest foreign cells, particularly bacteria, and if they are prevented from doing this, infections can progress and become significantly more severe.
Jobin says the team was able to demonstrate this in mice infected with listeria - bacteria that are present in contaminated food, for example, and which trigger fever and sepsis.
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."
Katarzyna Jobin, lead author
The researchers found that urinary tract infections in these mice also took much longer to resolve when the animals were fed a diet high in salt.
Excess salt also seemed to impair the immune response in people. Kurts and team assessed participants put on a diet that included six grams more salt per day than their usual intake.
"This is roughly the amount contained in two fast-food meals, i.e. two burgers and two portions of French fries," says Kurts.
Increased glucocorticoids and decreased granulocyte function
Blood samples taken from the participants after they had been on the diet for one week, showed that the glucocorticoid level had increased and that granulocytes had become much less effective at combatting bacteria. This effect is not particularly surprising since the most well-known glucocorticoid – cortisone – has previously been used as an inflammation suppressant.
Kurts points out that “only through investigations in an entire organism were we able to uncover the complex control circuits that lead from salt intake to this immunodeficiency."
"Our work therefore also illustrates the limitations of experiments purely with cell cultures," he concludes.