As Canadians brace for "vitamin D winter"—months when the sun's angle is too low to produce the vitamin in the skin—a McGill University study explains why vitamin D deficiency early in life is associated with a higher risk of autoimmune diseases.

The study is published in the journal Science Advances.

“An aging thymus leads to a ‘leaky’ immune system,” said lead author John White, a Professor in and Chair of McGill’s Department of Physiology. “This means the thymus becomes less effective at filtering out immune cells that could mistakenly attack healthy tissues, increasing the risk of autoimmune diseases like type 1 diabetes.”

He noted that researchers have known for years that vitamin D helps the body absorb calcium for strong bones, and that more recent research has discovered its crucial role in regulating the immune system.

“Our findings bring new clarity to this connection and could lead to new strategies for preventing autoimmune diseases,” he said.