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The month you are born will determine your risk of developing multiple sclerosis, numerous studies have found.
For people living in northern-latitude countries like Canada, the risk of MS peaks in individuals born in May and drops in those delivered in November.
Medical researchers have speculated that low levels of vitamin D could be playing a role. The vitamin is made naturally in skin exposed to sunlight. And that could mean women who are pregnant during winter may not have enough vitamin D to pass on to the fetus.
Now a new British study has taken this research a big step further by showing how low levels of vitamin D during pregnancy could disrupt the normal development of the infant's immune system.
For the the study, the researchers collected cord blood – or blood extracted from a newborn's umbilical cord – from 50 babies born in May and 50 born in November between 2009 and 2010 in London.
The blood was analyzed to measure vitamin D levels and so-called "auto-reactive" T-cells – potentially dangerous immune cells that can attack the body's own tissues.
The findings, published in the journal JAMA Neurology, revealed that babies born in May had significantly lower levels of vitamin D and higher levels of the auto-reactive T-cells. The presence of these "bad" T-cells could set the stage for MS, an auto-immune disorder in which aberrant immune cells attack myelin, the insulating layer that forms around nerves, including those in the brain and spinal cord.
"This is actually the first study to show a possible mechanism" that could lead to MS, said the lead researcher, Sreeram Ramagopalan of the University of London.
He noted that a child's immune system must learn to distinguish between foreign invaders, such as harmful microbes, and the body's own tissues. That education starts while the infant is still in the womb with the production of a steady stream of T-cells.
The thymus, an organ in the chest, plays a dual role in this learning process. It produces the T-cells and also acts as a quality-control centre. "One of the jobs of cells in the thymus is to present every protein in the body to developing T-cells," Ramagopalan explained in an e-mail. "Any T-cells that recognize a body's own proteins are 'auto-reactive' and have the potential to cause damage if released into the rest of the body. They are normally destroyed if they react to the body's own proteins."
However, "vitamin D appears to be needed for the thymus to appropriately present all proteins in the body and low levels [of vitamin D] allow auto-reactive T-cells to escape" into the bloodstream," he said.
The study suggests low levels of vitamin D during this period could get the immune system off to a bad start and increase the risks of MS developing later in life.
Ramagopalan believes pregnant women should be urged to take vitamin D supplements to safeguard the fetus.
"In the U.K. and in Canada, people are severely deficient in vitamin D in the winter. And I think that is a worrying problem, especially for pregnant women. They need to be taking thousands of International Units, not the governments' recommended dose of a few hundred which will not really budge their levels that much."
Many scientists who specialize in vitamin D research believe adults should be taking up to 5,000 IUs a day – far higher than what many public health organization suggest. For instance, Health Canada's daily recommended amount for pregnant and lactating women is 600 IUs and the upper tolerable limit is set at 4,000 IU.
"We are caught in a bit of a Catch-22," said Ramagopalan. "Governments want us to prove if you give vitamin D you will prevent people from getting MS. So you need a randomized-controlled trial which they are not willing to fund."
He noted that the average age of onset of MS is 30. So a clinical trial, which starts with pregnant mothers and then follows their offspring, would need to last at least three decades.