No one knows how to prevent type 1 diabetes. But there are emerging reasons to wonder if we could have done more.

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A few months ago, our six-year-old daughter, Bisi, was diagnosed with type 1 diabetes. It's a condition she will have to manage carefully for the rest of her life, to avoid the many terrible potential complications of the disease. Since the diagnosis, my husband and I have often said to one another things like: "What if we had...?" "Maybe if we had ..." And the other will respond by (half) jokingly quoting back the text on the box of diabetes information we received at the hospital from the Lilly pharmaceutical company (which makes the insulin Bisi now must take with her meals): "Pause. Take a deep breath. And remember: You couldn't have prevented what's happening with your child, so try not to beat yourself up."

The truth is, there is no known way to prevent type 1 diabetes. It is as a disease where our own immune system attacks our pancreas, eventually shutting down its ability to produce insulin. But there are some emerging theories about what makes type 1 diabetes more likely. The most intriguing of these -- the one that causes my husband and me to take the most deep breaths -- is the theory that links low vitamin D levels to type 1 diabetes.

Several studies have shown this link, including one published in September in the journal Diabetologia. In that study, researchers from the Naval Health Research Center in San Diego and from universities in California and Florida compared the vitamin D levels of military servicemembers diagnosed with type 1 diabetes with the vitamin D levels of a control group of servicemembers without diabetes. They found that those servicemembers with the lowest concentration of vitamin D in their blood were 3.5 times more likely to develop diabetes than their peers with the highest vitamin D levels. In 2008, the Archives of Disease in Childhood published a meta-analysis of observational data from four studies of vitamin D intake by young children in various European countries. Their conclusion: "Children being supplemented had a 29% reduction in risk of developing type 1 diabetes compared with their peers who were not being supplemented."

My husband and I don't know that our daughter was vitamin D deficient -- blood tests for vitamin D levels in children are not standard, even though the American Academy of Pediatrics estimates that more than 75% of infants aren't given the recommended amount of vitamin D, and even though low levels of vitamin D have been implicated in a wide range of conditions, including cardiovascular disease, osteoporosis, multiple sclerosis, and some cancers. But we think it's pretty likely her level was low. We weren't all that great about giving her the vitamin D drops that her pediatrician recommended for breastfed infants (formula is fortified with vitamin D).

We live in New England, where the angle of the sun is such that the summer months are the only ones where you can get sufficient vitamin D from the sun's rays. And I had been obsessively slathering Bisi and her brother, Jamie, with sunscreen ever since my father was diagnosed with melanoma, a disease that killed him when the kids were 3 and 5. (My husband, and it seems in retrospect that he was right, always argued for the kids to get their 15 minutes in the sun with no sunscreen, so they could get their vitamin D.) In any case, many of the health benefits of vitamin D are thought to begin at much higher levels than the current RDA of 400 IUs for infants and children (the recommendation was upped from 200 in 2008).

In the hospital, Bisi's endocrinologist told us that if low vitamin D levels contributed to Bisi's diagnosis, it was one of many causes--a kind of layering of effects. Bisi had a genetic susceptibility -- we know now that T1D is linked to other autoimmune diseases that run in our families. A virus like coxsackie, which many children get, may have increased her susceptibility. There may also be another, as yet undiscovered, environmental trigger. T1D increased 23% in the U.S. from 2001-2009, and worldwide it's increasing at a rate of about 3% a year. No one knows exactly why. Still, at least in Bisi's doctor's view, vitamin D is an important part of the puzzle. "Do you have another child?" he asked us in the hospital. We told him about Jamie, who is 8. "Give him vitamin D supplements."

Now, both of our children are taking 2,000 IU of vitamin D every day -- Jamie, because as the sibling of someone with T1D, he is at higher risk, and Bisi because there is some evidence that taking vitamin D can help extend the life of the few insulin-producing cells she still has.