Eating an overly salty diet weakens your immune system and makes it harder for your body to fight off bacterial infections, a study discovers.
Researchers from the University Hospital of Bonn fed mice a high-salt diet and found they suffered more severe bacterial infections as a result
They then fed human volunteers an extra six grams of salt each day – about the same as two fast food meals – and found they developed immune deficiencies.
The World Health Organisation says humans should consume no more than 0.17 ounces of salt per day, which corresponds to roughly one teaspoon.
Studies in Germany found that many people regularly exceed the WHO recommended teaspoon of salt in their diet, putting their health at risk.
Figures from the Robert Koch Institute suggest that, on average, men consume 0.4 ounces of salt per day, and women eat more than 0.3 ounces.
Christian Kurts, from the University of Bonn, said their new study proves that excessive salt consumption weakens an important part of the immune system.
The findings are surprising, as previous research seemed to show the opposite, according to Kurts.
For example, infections with certain skin parasites in lab animals heal significantly faster if they consume a high-salt diet.
Macrophages – immune cells that attack, eat and digest parasites – are particularly active in the presence of salt.
Several physicians concluded from the macrophages observation that sodium chloride has a generally immune-enhancing effect.
The new study’s lead author Dr Katarzyna Jobin, of the University of Würzburg, said: ‘Our results show that this generalisation is not accurate.’
She said the body keeps the salt concentration in the blood and in the various organs largely constant otherwise important biological process wouldn’t work.
The only major exception is the skin, which functions as the body’s salt reservoir.
This is why the additional intake of sodium chloride works so well for some skin diseases – but not for the body generally.
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.
This sensor also causes so-called glucocorticoids to accumulate in the body which inhibit the function of common types of immune cells in the blood.
Granulocytes, like macrophages, are scavenger cell, but they attack mainly bacteria rather than parasites. If they do not do this, infections proceed much more severely.
‘We were able to show this in mice with a listeria infection. We had previously put some of them on a high-salt diet,’ said Jobin.
‘In the spleen and liver of these animals we counted 100 to 1,000 times the number of disease-causing pathogens.’
Listeria are bacteria that are found for instance in contaminated food and can cause fever, vomiting and sepsis.
‘We examined human volunteers who consumed six grams of salt in addition to their daily intake,’ said Kurts.
‘This is roughly the amount contained in two fast food meals, for example two burgers and two portions of French fries.’
After one week, the scientists took blood from their subjects and examined the granulocytes, finding that immune cells coped much worse with bacteria after the they had started to eat a high-salt diet.
Professor Kurts added: ‘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.’
The research was published in the journal Science Translational Medicine.