Giving young children vitamin D supplements may reduce their risk of developing type 1 diabetes later in life, research suggests.
Children who took supplements were around 30% less likely to develop the condition than those who did not.
Type 1 diabetes results from the immune system destruction of pancreatic cells which produce the hormone insulin.
The study, by St Mary's Hospital for Women and Children, Manchester, appears in Archives of Disease in Childhood.
Type 1 diabetes is most common among people of European descent, with around two million Europeans and North Americans affected.
It is becoming increasingly common, and it is estimated that the number of new cases will rise by 40% between 2000 and 2010.
The Manchester team pooled data from five studies examining the effect of vitamin D supplementation.
Not only did the use of supplements appear to reduce the risk, the effect was dose dependent - the higher and more regular the dose, the lower the likelihood of developing the disease.
Previous research has found that people newly diagnosed with type 1 diabetes have lower concentrations of vitamin D than those without the condition.
Studies have also found that type 1 diabetes is more common in countries where exposure to sunlight - which enables the body to manufacture vitamin D - is lower.
For instance, a child in Finland was 400 times more likely to develop the disease than a child in Venezuela.
Separate research has linked low levels of vitamin D and sunlight to other autoimmune disorders, including multiple sclerosis and rheumatoid arthritis.
Further evidence of vitamin D's role comes from the fact that pancreatic beta cells and immune cells carry receptors or docking bays for the active forms of the vitamin.
It is thought that vitamin D helps to keep the immune system healthy, and may protect cells from damage caused by chemicals which control inflammation.
Dr Victoria King, of the charity Diabetes UK, said: "Much more research, in particular controlled trials which compares the results when one group of people are given vitamin D supplements and one group is not, are needed before we can confirm a concrete association between vitamin D and type 1 diabetes."
Governnment experts recommend vitamin D supplementation for at least the first two years of a child's life, although the Chief Medical Officer for England has suggested supplements for the first five years is a good idea.