It has long been known that too much salt can contribute to high blood pressure. New research shows that too much salt can increase susceptibility to infections, and eating two fast-food meals a day can significantly reduce the antibacterial properties of your immune cells.
The World Health Organization (WHO) recommends a daily intake of 5 grams. However, in the US the average intake of added salt is 9 g/day for men and 7 g/day for women. Numerous studies have shown the harmful effects of this excess salt: salt promotes water retention, high blood pressure, and cardiovascular diseases. It also increases the risk of osteoporosis and cognitive disorders in sedentary people, and a new study by the University of Bonn, published in Science Translational Medicine, points to a new harmful effect of salt: It is thought to weaken the immune system by making it more susceptible to bacterial infections.
Salt protects the skin
However, earlier studies have gone in the opposite direction. By stimulating the production of macrophages and T-lymphocytes, salt is said to help fight parasite-caused skin diseases such as leishmaniasis and atopic eczema. Salt is also cytotoxic for bacteria. However, according to Katarzyna Jobin, the lead author of the new study, these observations cannot be generalized to the entire organism, although they could be valid for the skin. The salt content is kept almost constant in the blood and in every organ. Otherwise, important biological processes would be altered. The only exception is the skin, which serves as the body’s “salt reservoir”.
Otherwise, the excess salt is filtered out by the kidneys and excreted with the urine. And here is the problem: the renal medulla, which acts as a “sensor” to maintain the blood’s salt and water balance, causes a build-up of glucocorticoids when you eat too much salt. These inhibit the work of the neutrophil granulocytes, i.e. the immune cells that attack bacteria. Although the number of the white blood cells remains constant, they seem to become much less effective when exposed to glucocorticoids.
Just eating two large hamburgers a day can weaken your immune cells.
To demonstrate this mechanism, the researchers found that mice infected with Listeria bacteria, when fed a high-salt diet, had a 100 to 1,000 times higher concentration of the pathogenic bacteria than normal mice. They came to the same conclusion for urinary tract infections caused by E. coli, but the mice fed a high-salt diet healed much more slowly, the researchers said. The researchers continued the study in humans and fed 10 volunteers a high-salt diet (6 grams of extra salt per day, enough for two large burgers) for a week. As a result, cortisone levels rose by up to 500%.
“Only through investigations in an entire organism were we able to uncover the complex control circuits that lead from salt intake to this immunodeficiency,” stresses Kurts. “Our work therefore also illustrates the limitations of experiments purely with cell cultures.”