In the alphabet soup of vitamins, making sure your kids get their A-B-C's isn't all it takes.
They also need some "D" to grow strong bones.
But a recent study found a large number of infants and toddlers were not getting what they needed.
When six-year-old Elizabeth Dilauro was a toddler, she tripped and broke her foot.
Two weeks after the cast came off, she broke another foot bone.
"And we've had 10 to 12 fractures since then and um, plus multiple micro-what they call - micro-fractures.", according to Elizabeth's mom Sharyn.
Turns out her fragile bones were the result of vitamin D deficiency.
"I couldn't believe something so simple would cause such a problem. It's changed our lives all these fractures."
At Children's Hospital in Boston, Dr. Catherine Gordon heads the bone health program.
Her research shows a hidden epidemic.
She's found more than 40 percent of healthy adolescents have vitamin D levels that are too low.
"We followed that up with a study of infants and toddlers completely healthy and found that 40% of them have suboptimal levels of vitamin D.", says Dr. Gordon.
Breast-fed babies, she says are at highest risk.
"I'm whole heartedly supportive of breast feeding. However, many mothers are vitamin D deficient, therefore their breast milk is deficient and it puts babies at risk for vitamin D deficiency."
Vitamin D deficiency is treatable, but unlike Elizabeth, most kids have no symptoms and doctors don't routinely check for healthy levels.
"People, they ignore it. They think it just comes naturally but it doesn't and it's really important.", says Sharyn.
Kids who eat a well balanced diet, drink their milk, and get moderate sun exposure shouldn't have a problem with vitamin D.
But kids with low levels are more at risk for osteoporosis, and perhaps other health problems later in life.