Researchers believe that salt activates a type of cell involved in autoimmune conditions

Could a high-salt diet cause the immune system to malfunction, and trigger conditions such as rheumatoid arthritis and type 1 diabetes? That's the thinking behind a new U.S. trial looking at the effect of salt on immune cells.

Researchers believe that salt activates a type of cell involved in autoimmune conditions. These conditions, which have risen in recent years, include type 1 diabetes, rheumatoid arthritis, multiple sclerosis and Crohn's and ulcerative colitis (which cause inflammation in the gut).

They are all thought to occur when the immune system, which normally defends the body from bugs and viruses, goes haywire and mistakenly attacks the body.

Scientists are still unclear what triggers this malfunction, but some believe an environmental trigger is to blame.

Last year animal studies published in the journal Nature found that salt increased a type of cell in the immune system called type 17 helper T cell (Th17).

Levels of these cells - which trigger inflammation - seem to rise in autoimmune conditions, leading to speculation that they might be involved in triggering the immune system to attack healthy tissue.

The study, which was conducted at Brigham and Women's Hospital in Boston, found that animals fed a high-salt diet for three weeks had a dramatic increase in the number of Th17 cells compared with those fed a normal diet.

Another part of the study showed that animals on a high-salt diet also had more severe symptoms of an autoimmune disorder called autoimmune encephalomyelitis, which is an animal version of multiple sclerosis.

Now scientists are testing this theory on humans.

In a new trial at Brigham and Women's Hospital, researchers are comparing Th17 levels in 20 healthy people - half will follow a high-salt diet for five months, while half will follow a normal diet. The team say the findings could lead to patients in the future being advised to follow low-salt diets.

In a second study, at Yale University, 30 patients with the condition autoimmune hepatitis - which causes liver inflammation - are being asked to follow a ten-day high-salt diet.

Their immune cells will be measured, then after a break of three days when they will follow a normal diet, measured again.

Researchers 'believe excess dietary salt may function as a trigger that favours development of autoimmunity'

After that they will be put on a low-salt diet. This will help to show whether salt plays a role in these conditions, say the researchers: 'We believe excess dietary salt may function as an environmental trigger that favours development of autoimmunity.'

Commenting on the study, Graham MacGregor, consultant physician and professor of cardiovascular medicine at the Wolfson Institute, and chairman of Consensus Action on Salt and Health, said: 'This is intriguing research and suggestive of a role for salt in the development of these diseases.

'It is early days, but we look forward to the result of the first human trials with great interest.

'We already know of the adverse effects of salt in a number of conditions, including high blood pressure, heart disease, osteoporosis, and stomach cancer.'

Meanwhile, a study suggests that being overweight increases the risk of autoimmunecondition rheumatoid arthritis by more than a third.

Based on around 200,000 women, the research from Harvard Medical School found that being overweight or obese increases the risk of the disease by 37 per cent.

The study, published in the journal Annals of Rheumatic Diseases, found that the risk was highest among obese women under the age of 55, where it rose to 65 per cent.