Unexplained bone fractures in babies are usually due to abuse, but researchers have suspected that sometimes they could be due to low vitamin D levels. A new study now casts doubt on that idea.

Vitamin D is needed for healthy bones, and an overt deficiency can cause rickets, a softening in children’s bones that may lead to pain and deformities like severely “bowed” legs and abnormal curves in the spine.

X-rays of kids with rickets show some of the same abnormalities commonly found in abused children. And some researchers have speculated that low vitamin D levels - even those not low enough to cause rickets - could be the true cause of some bone fractures blamed on abuse.

To look into that question, researchers at Children’s Hospital of Philadelphia measured vitamin D levels in 108 babies and toddlers treated for broken bones at their center. Out of every 10 fractures, seven were due to accidents and three were blamed on child abuse.

The researchers found that relatively low vitamin D levels were common - but no more common among the children thought to be victims of abuse.

Nor was low vitamin D more common among children with multiple fractures (a potential sign of abuse) versus those with a single broken bone.

“Our study indicates that a low vitamin D level should not discourage consideration of abuse when a child presents with unexplained fractures,” lead researcher Dr. Samantha Schilling told Reuters Health in an email.

She added that “it is possible, and likely, that at some threshold, very low vitamin D levels contribute to fracture susceptibility in children.” But this study was not designed to show what that threshold is.

In fact, Schilling said, no studies have been done to pinpoint a vitamin D “threshold” below which children’s fracture risk goes up. Even when it comes to rickets, she noted, the correlation between vitamin D levels and rickets severity is unclear.

The findings, published in the journal Pediatrics, are based on 108 children younger than 2 who were treated for bone fractures over 1 year. Schilling’s team found that eight children had a vitamin D deficiency, while 34 were deemed to have insufficient levels (not low enough to be considered a deficiency) and 66 had sufficient levels.

The researchers found that children with fractures thought to be caused by abuse were no more likely to have a vitamin D deficiency or insufficiency.

In addition, low vitamin D was no more common among these abused children than it was in earlier studies of healthy children, the researchers say.

On the other hand, the findings do not necessarily mean that unexplained bone fractures in babies are always due to abuse, or that low vitamin D has no role in such cases, according to Dr. Colin R. Paterson, who wrote an editorial published with the study.

“I believe that not just vitamin D deficiency, but a number of other bone disorders, can cause fractures that can readily be misinterpreted as child abuse,” Paterson, a retired staff physician at the University of Dundee in Scotland, told Reuters Health in an email.

“It has often been assumed,” he said, “that if parents are unable to provide an explanation for fractures they must be lying about assaults inflicted by themselves or others.”

But, Paterson said, it is possible for rickets or certain other bone disorders - like the genetic brittle-bone condition osteogenesis imperfecta - to make young children vulnerable to fractures even with “normal handling.”

Infants and children with bone fractures do not routinely have their vitamin D levels measured, according to Schilling, because there is no evidence that it’s useful to do so.

She added, though, that it might be helpful in cases where an exclusively breastfed infant has a fracture. That’s because breast milk is low in vitamin D, and unless breastfed infants are given vitamin D supplements, they are at risk of having “suboptimal” levels.

To help prevent that, the American Academy of Pediatrics recommends that breastfed infants be given daily vitamin D drops. (Infant formulas are already fortified with the vitamin.)

Young children can get their vitamin D from fortified cereals, milk and orange juice, or from the few foods that contain the vitamin naturally - like fatty fish and egg yolks. Experts recommend that infants get 400 IU of vitamin D per day, while children ages 1 and up should get 600 IU.

SOURCE: Pediatrics, online April 11, 2011.