Researchers found a high salt intake may raise the risk of MS for people genetically susceptible to the condition.
The study suggests that while high intake of salt may be a risk factor for multiple sclerosis (MS), it is likely to be so only in people with a genetic risk. Also, for some genetic risk groups, high salt intake is more likely to be a trigger for the disease in women than in men.
MS is a debilitating disease where the immune system attacks nerve tissue in the brain, spinal cord and optic nerve. Genetics and environment are thought to be risk factors, as is gender - incidence in women has roughly tripled in the last century.
Previous studies have hinted at high salt intake as one of the environmental risk factors for MS, but have not gone so far as to explain how it interacts with other factors and what the underlying disease mechanism might be.
Now, a new study from the University of Vermont in Burlington offers some answers, as Dr. Dimitry N. Krementsov, first author and immunobiology researcher, explains:
"We hope to provide a comprehensive understanding of how and why environmental factors interact with individuals' unique genetic makeup to influence autoimmune diseases such as MS."
For their study, the team used three genetically different groups of mice and fed them either a diet high in salt or a control diet with normal levels of salt. They then induced a disease called autoimmune encephalomyelitis in the mice - the closest mimic to MS in humans.
The results were different in the three genetic groups. In one group, both males and females fed a high-salt diet showed worse symptoms of MS.
In the second genetic group, only the females a high-salt diet showed worse symptoms of MS, while in the third group, high salt intake did not affect MS symptoms.
Salt may affect blood-brain barrier in MS-susceptible groups
When they looked at the biological changes, the researchers concluded that the critical factor was genetics.
They found that in all cases where high salt intake made MS symptoms worse, the mice had a weaker blood-brain barrier, while their immune cells appeared unaffected.
The blood-brain barrier normally protects the central nervous system - the brain, spinal cord and optic nerve - from attack by white blood cells in the immune system.
However, people with MS have a leaky blood-brain barrier, which allows the immune cells to travel into the tissue of the central nervous system and attack the myelin sheath - a protective coating of proteins and fatty substances that insulates the nerve cells and stops electrical signals from leaking away.
When the myelin sheath is destroyed, nerve impulses leak away and peter out, and plaque builds up along the nerve fiber. This results in the classic symptoms of MS: increasing numbness, paralysis, loss of vision and difficulty with balance and walking.
Dr. Gerald Weissmann, editor-in-chief of The FASEB Journal - where the study is published - notes that we need to ingest enough salt so our bodies function, but not too much to cause things to go wrong. He concludes:
"This report helps shed light on what can go wrong in individuals with genes that make one susceptible to autoimmune disease. It also helps us understand how much salt is just right for any given individual."
Earlier this year, Medical News Today learned how researchers from the University of Montreal in Canada discovered a new drug target for halting MS. In the Annals of Neurology, they describe how blocking a molecule called MCAM - which allows white blood cells to pass through the blood-brain barrier - led to a 50% reduction of MS in a mouse model of the disease.