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"Many environmental factors affect MS, such as vitamin D, smoking and Epstein Barr virus infection. Our study shows that high salt intake may be another environmental factor affecting MS patients," said lead researcher Dr. Mauricio Farez, of the Raul Carrea Institute for Neurological Research in Buenos Aires.
Multiple sclerosis is a disease of the nervous system that causes weakness, visual disturbances, trouble with balance, numbness and thinking and memory problems. The most common form is called relapsing-remitting MS, meaning symptoms subside and then become worsen.
Earlier research found that salt may alter autoimmune response, which is involved in the development of MS.
Farez cautioned that this study does not show that salt causes MS to worsen, but there does seem to be an association.
"This is a small observational study showing a relationship between salt intake and MS disease activity, and these data need to be further validated in larger studies, including different populations," he said.
For the study, Farez's team measured the levels of sodium (the main component of salt), creatinine and vitamin D in the blood and urine of 70 patients with the relapsing-remitting form of MS. Creatinine is a marker of inflammation, and low levels of vitamin D have been associated with MS.
Sodium intake was divided into three levels: less than 2 grams daily, between 2 and 4.8 grams a day, and more than 4.8 grams daily. Current guidelines for heart disease prevention recommend a maximum sodium intake of 1.5 grams to 2.4 grams per day. At the upper end, that's just under half a teaspoon of table salt a day.
Farez's group found that people with daily sodium intake of between 2 and 4.8 grams and those who consume more than 4.8 grams -- a little less than a teaspoon of salt -- were up to four times more likely to have more episodes of worsening MS symptoms as those who consumed the least salt.
To check the progression of the disease in patients' brains, the researchers analyzed X-rays and scans. They found that patients who had the highest salt intake were about 3.4 times more likely to have their disease worsen, compared with those with the lowest salt intake.
Similar results were found in a second group of 52 MS patients, the researchers added.
"It is too soon to say that MS patients should cut their salt intake," Farez said. "Our findings could serve as a basis for clinical trials with salt restriction in MS patients," he said.
The report was published Aug. 28 in the Journal of Neurology, Neurosurgery and Psychiatry.
Salt's influence on MS is a subject of increasing interest, said Nicholas LaRocca, vice president of health care delivery and policy research at the National Multiple Sclerosis Society.
"At this stage you really can't assign cause and effect, but it's beginning to look like there is a significant role of salt in MS disease activity and progression," he said.
The mechanism for this association isn't known, LaRocca said. Salt may make the immune system more prone to the disease, he suggested.
He agreed that there isn't enough evidence to recommend that MS patients reduce salt in their diet.
"However, in a more general sense, we should all be watching our salt intake. We should all be careful about consuming excessive amounts of salt," he said.
For the study, salt intake was estimated from sodium excreted in urine samples the participants provided three times over nine months. In addition, the researchers tracked the course of the patients' MS from 2010 to 2012.
After accounting for factors such as smoking, age, gender, length of time after diagnosis, weight, treatment and vitamin D, the link between more salt and worsening MS remained, the researchers said.
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SOURCES: Mauricio Farez, M.D., department of neurology, Raul Carrea Institute for Neurological Research, Buenos Aires, Argentina; Nicholas LaRocca, Ph.D., vice president, health care delivery and policy research, National Multiple Sclerosis Society, New York City; Aug. 28, 2014, Journal of Neurology, Neurosurgery and Psychiatry