Excessive salt consumption can be detrimental to health – as is well known, the risk of heart attack and stroke increases. But researchers have now discovered another harmful effect of sodium chloride. Their experiments show: Apparently too much salt has a negative impact on our immune system. A salt-rich diet therefore weakens the ability of certain immune cells to fight bacterial pathogens. As a result, infections are more severe and heal much more slowly, the team reports.
Salt is a vital mineral for our body. But we shouldn’t eat too much of it either. The World Health Organization (WHO) recommends a maximum daily intake of five grams for adults – roughly equivalent to one level teaspoon. In fact, many people clearly exceed this limit. With potential health consequences: A high-salt diet can raise blood pressure in some people and thereby promote cardiovascular diseases such as heart attack and stroke. In addition, sodium chloride appears to affect our immune system. Studies suggest that a salt-rich diet may increase the risk of allergies and autoimmune diseases. In the case of infections with certain skin parasites, however, salt can even accelerate the healing process.
Many times more germs
“The effect of salt on the body’s immune system is controversial and it is still unclear how high levels of sodium chloride affect the different populations of immune cells,” explains Katarzyna Jobin from the University Hospital Bonn and her colleagues. To find out more, the researchers have now investigated the influence of saline foods on the course of bacterial infections. For their study, they first fed mice with excessively salty food for a week. Then they watched how the rodents coped with a urinary tract infection caused by Escherichia coli bacteria. The results revealed that the infection in these animals was significantly worse compared to mice that had been fed normal or low salt.
A similar picture emerged with a systemic infection with Listeria germs, which occur, for example, in contaminated food and can trigger fever, vomiting and blood poisoning. As the scientists found, the immune system of the salt-fed mice was measurably worse against these pathogens. This was particularly evident in the bacterial load in the body: “In the spleen and liver of these animals, we counted 100 to 1000 times the amount of the pathogenic germs,” reports Jobin. As a result, the infection healed much more slowly.
Weakened neutrophils
But how can this effect be explained? The research team found that the kidneys’ response to the salt plays a role. Except for the skin, which acts as a salt reservoir, excess salt is not accumulated in the body. Instead, the kidneys filter out the excess salt and excrete it in the urine. This excretion function is activated by a special sodium chloride sensor in the kidneys. Apparently there is an undesirable side effect: The increased salt excretion leads to deficits in the production of certain enzymes that process precursors of the so-called glucocorticoids. As a result, biomolecules are released that have a similar effect to the stress hormone cortisone. This is traditionally used in medicine to suppress immune reactions such as inflammation. The accumulation of these substances could explain the observed weakening of the immune system.
A second effect is related to the fact that the kidneys produce more urea for salt excretion. However, this inhibits the antibacterial activity of the so-called neutrophil granulocytes, as experiments with mice have shown. These immune cells are part of the innate immune system and represent the most common type of immune cell in the blood. Among other things, they act as phagocytes and primarily attack bacteria. “If they don’t do this to a sufficient extent, infections are much more severe,” Jobin’s team said in a message.
Also detectable in humans
In a next step, the researchers wanted to clarify whether the relationship between salt and neutrophil function also applies to humans. To do this, ten volunteers tested so that they consumed an additional six grams of salt a day. “This corresponds roughly to the amount that is included in two fast food meals – two burgers and two portions of french fries,” says Jobins colleague Christian Kurts. After a week, the team took blood from the study participants and analyzed the granulocytes from it. The result: Compared to before, the immune cells were now significantly less able to cope with bacteria. “These results speak against high salt consumption, especially during bacterial infections,” the scientists state.
Further investigations are now to confirm whether a high-salt diet is also associated with more severe infection courses in human patients – and for which type of infection this effect is particularly important. If it becomes clear that too much salt is actually having a measurably negative effect on the healing of infections, this could spark the debate about the right amount of salt consumption, the researchers conclude.
Source: Katarzyna Jobin (University Hospital Bonn) et al., Science Translational Medicine, doi: 10.1126 / scitranslmed.aay3850