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The myelin coating on nerves is damaged in people with MS.

Vitamin D seems to help control a gene known to increase the risk of multiple sclerosis — a finding that suggests taking vitamin D supplements during pregnancy and early in life may help prevent the disease.

"I think it's the first time that the environment and genetics are brought together, this is something that links the two," Dr. George Ebers, a Canadian who is based at the University of Oxford in London, told CBC News. 

"It's saying that the environmental factor, which is everybody's favourite at the moment, which is vitamin D, directly interacts with the gene region … that has the biggest effect on your MS risk."

MS is the most common disabling neurological condition affecting young adults and 2.5 million people worldwide. Canada is known to have one of the highest rates of MS in the world, with an estimated 55,000 to 75,000 people affected by the disease.

The disease results in the loss of nerve fibres and their protective myelin sheath in the brain and spinal cord, causing neurological damage.

'If too little of the vitamin is available, the gene may not function properly.' — Dr. Julian Knight

It is not clear what causes MS but other research has suggested that vitamin D, produced in the body through exposure to sunlight, plays a part.

Proteins activated by vitamin D in the body can alter a common genetic variant that is connected with the disease, Ebers and his colleagues at Oxford and the University of British Columbia report in Friday's issue of the PloS Genetics.

Immune system involved?

About one in 1,000 people in the U.K. are likely to develop MS. The number rises to around one in 300 among those carrying a single copy of the variant, called DRB1*1501, and one in 100 of those carrying two copies inherited from each parent.

"In people with the DRB1 variant associated with MS, it seems that vitamin D may play a critical role," said study co-author Dr. Julian Knight. "If too little of the vitamin is available, the gene may not function properly."

The researchers think that this gene-environment interaction may affect the ability of the thymus to do its job of producing an army of T cells that identify invading pathogens, such as bacteria and viruses, and attack and destroy them.

Normally, the thymus deletes T cells that pose the greatest risk of attacking the body's own cells and proteins.

Vitamin D supplements

But in people who carry the mutation, the researchers think a lack of vitamin D early in life may impair the ability of the thymus to carry out the deletion. The T cells then go on to attack the body, leading to a loss of myelin on nerve fibres.

"Our study implies that taking vitamin D supplements during pregnancy and the early years may reduce the risk of a child developing MS in later life," said the study's lead author, Dr. Sreeram Ramagopalan.

"Vitamin D is a safe and relatively cheap supplement with substantial potential health benefits. There is accumulating evidence that it can reduce the risk of developing cancer and offer protection from other autoimmune diseases."

In the U.K., experts now advise that pregnant women and children under five years old should take vitamin D supplements. In Canada, the MS Society said only that people should talk to their doctors.

The best way to judge the effectiveness of the supplements would be to treat an entire population and see what happens, something Ebers said wouldn't be harmful and might prevent many cases of the degenerative disease.

More research is needed to show a cause-and-effect relationship, agreed Dr. Paul O'Connor, director of the MS program at the University of Toronto.

"That theory of taking vitamin D in that preventive way is an interesting theory, and it may well be true. And there's nothing in these findings that contradicts that theory, but there's nothing in these findings that supports that theory either."

Still, for those shivering under parkas, starved of sunlight, supplementing the diet with vitamin D is safe, inexpensive and appears to have many health benefits, O'Connor added.

The study was funded by the U.K.'s MS Society, the MS Society of Canada, the Wellcome Trust and the Medical Research Council.

With files from the Canadian Press