In the winter, pre-teen girls may need more vitamin D—either from diet or supplements—to have healthy bones, a new study says.
The goal of the study was to figure out how much vitamin D girls aged 11 to 12 would need to take every day in order to keep the levels of this nutrient in their blood at healthy levels. Some experts think that since these girls are growing fast, vitamin D might be particularly important.
Vitamin D aids bone growth by helping the body absorb calcium. An extreme lack of vitamin D can lead to rickets, which causes delayed growth, muscle weakness, and teeth problem in kids. But in the U.S. at least, rickets is rare, according to the Centers for Disease Control and Prevention.
Our skin can make vitamin D when it’s exposed to sunlight, or we can get the nutrient in our diet or as supplements.
In the new study, published in the American Journal of Clinical Nutrition, Kevin Cashman, head of the School of Food and Nutritional Sciences at University College Cork in Ireland, and colleagues found that teen girls need a vitamin D intake of about 750 IU per day to have levels in their blood that allow for healthy bone growth.
This amount is slightly higher than the US Institute of Medicine’s (IOM) recommendation of 600 IU per day for everyone between the ages of one and 70.
This discrepancy is not surprising. Most experts disagree over what the right amount of vitamin D really is.
Complicating the matter is that the girls in the study all lived in Finland or Denmark, two northern countries with very little sunlight in the winter. And even when it’s sunny, the angle of the sun makes the light weaker. As a result, some researchers say it’s not clear whether the new finding applies to girls in the US.
How far north you live matters a lot, said Connie Weaver, spokesperson for the American Society for Nutrition who wasn’t involved in the study. She said that since most of the US gets more sunshine, base levels of vitamin D are going to be higher.
Except for babies less than a year old, “there’s not evidence anywhere that says children will benefit from vitamin D supplements,” Weaver said.
On the other hand, “Latitude does matter, but vitamin D levels are often not as simple as the amount of sunshine someone gets,” said Dr. Steven Abrams, professor of pediatrics at the Baylor College of Medicine in Houston. He was not involved in the study either.
Abrams said that wearing sun block and not spending time outside can cut down on the vitamin D we get from the sun.
Even on sunny days in the winter, people in northern countries “still might not have enough to make vitamin D,” said Cashman.
In the new study, Cashman and his colleagues kept track of vitamin D levels in almost 150 girls in Finland and Denmark for six months, about the length of the winters there. Each of the girls, 11 to 12 years old, was given a supplement containing either five or ten micrograms of vitamin D, or a placebo (a dummy pill).
Because the girls were all getting some vitamin D in their diets, they had a range of levels, Cashman said. At the end of the winter, he and his team analyzed the amounts of vitamin D that had been in the girls’ diet and in their blood. “Once you know that relationship, it allows you to predict what the recommendation needs to be,” he said.
According to the IOM, about one in four girls in the US aged nine to 13 has less than the recommended level of vitamin D.
This doesn’t necessarily mean that these girls are going to have problems, but “it means that below that level that there’s a risk that all of the needs for bone health are not being met,” Abrams said. “Above that level, it should be adequate for things that vitamin D does, like absorb calcium.”
The current recommendations aren’t based on solid data, Weaver said.
“We don’t have any real data that says that if you’re below that level, there’s a real problem,” she said.
Until there’s more information available, Weaver says, the IOM guidelines– 600 IU per day—offer good practical guidance.