A high-salt diet can weaken the immune system and leave the body vulnerable to certain types of infection, according to immunologists writing in the journal Science Translational Medicine.

They investigated the effect a diet high in salt had on the immune system, first in mice and then in humans. The mice on saltier diets suffered more severe bacterial infections, while the humans displayed pronounced immune deficiencies.

"Our findings suggest that the daily recommended intake of 5 to 6 grams of salt should not be exceeded by much," corresponding author Christian Kurts, from the Institute of Experimental Immunology at the University of Bonn, told Newsweek.

"Lowering salt actually does not invigorate the immune system according to our work," he said. "The recommended salt amount seems to be good for the immune system."

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Kurts and his team compared the immune response of two groups of miceone fed a high-salt diet and the other a normal-salt dietto a urinary tract infection (UTI) caused by uropathogenic E.coli (UPEC).

The researchers found UTIs were aggravated by the high-salt diet, with the mice on these diets showing four to six times more UPEC than the control group. Recovery times also took longer; results that the author's attribute to hormones released to help the body excrete excess salt (glucocorticoids), which have the added effect of suppressing the immune system.

High-salt diets can also cause accumulation of urea in the kidney, which may suppress the antibacterial capabilities of white blood cells, the researchers suggest.

When the mice were infected with listeria, those on the saltier diet experienced more severe reactions to the bacteria. The researchers counted between 100 and 1,000 times the number of pathogens in the spleens and livers of mice in the high-salt category.

To investigate further, the researchers placed 10 volunteers on a high-salt diet for a week. This involved taking three tablets a day that added up to an extra 6 grams of salt to their dietequivalent to what you might find in 1.5 to two large fast food meals.

By the end of the week, the volunteers were showing signs of immunodeficiency. While numbers of neutrophils, a type of white blood cell, remained stable, their ability to fend of bacteria was lower than at the start of the study.

Concluding, the team say that because Western diets are now high in salt, "these findings reveal that people might be making themselves more vulnerable to bacterial infections."

Kurts said the best way people can boost the immune systems generally is to ensure they have had all appropriate vaccinations. "Apart from that, measures that are healthy in general are good also for our immune system: vitamins, moderate exercise, avoid junk food."

Below are four ways people can boost their immune system through diet and lifestyle.

There is a growing body of scientific evidence that suggests a healthy lifestyle revolving around good diet, exercise, sleep and reducing stress can help the immune system to run more efficiently.

In contrast to a high-salt diet, eating a healthy balanced diet consisting of plant-based fiber can support the body's "good" bacteria, which in turn may help the immune system, explained Dr. Louisa James, Lecturer in Immunology at Queen Mary University of London, U.K.

"The trillions of bacteria in our gut work in exquisite symbiosis with our immune system," James told Newsweek. "They stimulate our immune cells to make antibodies, they secrete by-products which maintain specialised immune cells that regulate our immune system and they even produce antibacterial and antiviral chemicals that can fight pathogens on our behalf."

Contrary to popular belief, there is limited evidence to show taking vitamin C supplements helps to fend off the flu, unless you are deficient in the first place. However, an analysis of 29 studies did suggest taking vitamin C supplements could reduce the amount of time a person infected with the common cold displayed symptoms, particularly if they were active.

James instead emphasizes the importance of vitamin D, which we mostly recieve through sunlight, in regulating the immune system.

"Vitamin D is essential for bone health but is also important for regulating our immune system," said James, pointing to research published in 2017 that found vitamin D supplements were a safe way to help protect people deficient in the vitamin from respiratory infections.

Vitamin D may help the body's macrophagesa type of white blood cellfunction better, while there is some evidence that vitamin E helps the T cells, "the other side of the immune system," Professor Janet Lord, Director of the Institute of Inflammation and Ageing at the University of Birmingham, U.K., told Newsweek.

Daniel Davis, Professor of Immunology at Manchester University, U.K., is skeptical about the role of nutrition on the immune system, telling Newsweek: "Obviously nutrition is important, certainly at some level, but it also seems to be very personal.

"The way that we respond to this or that food seems to vary hugely from one person to the next, and understanding why that is and being able to come up with clear health advice about nutrition is only just beginning to be done."

Davis, whose book The Beautiful Cure explores the immune system and the body's ability to fend off disease, points to the sheer number of books on nutrition and different dietary approaches, "which tells you something about the fact that we don't really know what's going on."

Another factor that research suggests could influence the body's response to infection is sleep.

Immune cells have adhesion receptors on their surface, explained James. These receptors (called integrins) enable them to move around the body and interact with other cells. However, levels can fluctuate depending on the presence of certain stress-related hormonessuch as adrenaline and noradrenaline.

In a paper published in the Journal of Experimental Medicine in 2019, scientists in Germany revealed levels of these hormones change throughout the day, dipping when we sleep. According to the study's authors, sleep loss can reduce levels of receptors on immune cells, which in turn may impair their ability to operate as effectively as they would under normal circumstances.

In contrast, regularly having a good night's sleep can enhance the immune system by improving immune cells' ability to attach to the targets.

Twin studies and laboratory tests have also emphasized the role of sleep in the proper functioning of the immune cells; showing shorter sleep durations are associated with a depressed immune system, while getting rest offers the immune system a chance to regroup when infection risk is low.

However, Davis says it can be hard to disentangle the effects of a lack of sleep from other factors, such as different eating times and stress.

"One thing that does seem to really affect the immune system is chronic stress," said Davis, and it relates to the hormone cortisol.

Levels of cortisol naturally peak and trough throughout the day, for example, rising as the body gets ready to wake up. The hormone plays an important role in the body's flight or fight response and remains at elevated levels when we are confronted by a source of stress.

"When you're in a state of fight or flight, your body is dealing with this current emergency and a lot of other bodily processes are dampened down," said Davis. These bodily processes include the immune system.

"This is fine in the short-termstress is not something to be entirely avoided. But if you have chronic stress and cortisol levels stay very high, it becomes a problem and makes you more susceptible to the flu and other viruses."

The obvious answer to this is to create an environment and lifestyle that minimizes sources of chronic stress, but Davis warns there is very little solid evidence that practices known to reduce stress can directly boost the immune system. The research is not there yet, he said.

"One thing that's becoming clear is that physical activityexerciseis a really powerful influencer of the immune system," said Lord, who co-authored a study investigating the immune system of cyclists aged 55 to 79. The researchers found their immune system more closely resembled that of a twenty- or thirty-something than someone their own age.

Movement gets the blood pumping and gives immune cells the chance to circulate the body. "Good circulation is essential to allow our immune cells to migrate throughout our body and perform their essential roles in immune surveillance," said James.

A key part of the immune response is inflammation and people who exercise regularly appear to have a more efficient immune system that is better at turning on and turning off inflammation, Lord explained, but warned that extended periods of sitting could undo the benefits.

"Sitting is the new smoking," she said. "We're hunter gatherers in a modern society. We haven't evolved to be sitting around all day."

Her advice is to not sit down for longer than an hour, even if it means walking up and down the stairs or investing in a standing desk. When it comes to physical activity, it doesn't matter what it isjust as long as you do it.

"Don't punish yourself by doing some form of physical activity you don't like. You like gardening, get out and do some gardening. If you like a brisk walk, do that," said Lord. "Just do something."

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Too Much Salt May Lower Your Ability to Fight BacteriaHere Are Four Ways to Help Boost the Immune System - Newsweek