If your parents ever told you to go play outside for the sake of a vitamin or insisted you drank a glass of milk to grow strong bones, they were talking about vitamin D. Though humans have only known of its existence for about a century, the nutrient’s ability to stave off physical development problems has made it a big part of public health conversation and explains why cereal boxes advertise that their product delivers a dose.
“Fortification has been successful in preventing vitamin D deficiency in most people in the U.S.,” says Daniel Roth, a pediatrician at the Hospital for Sick Children in Toronto. Though our bodies make the nutrient after ultraviolet B exposure, we don’t always make enough, and adding the vitamin to food staples has helped a majority of the residents in the U.S. and Canada.
Whether ingested or made in our skin, vitamin D makes its way to our liver, where it breaks down further into a crucial tool that helps our guts absorb the calcium and phosphorus we eat. Both nutrients get funneled to our bones, though calcium also has another responsibility: helping muscles contract. A regular heartbeat, for example, hinges on calcium moving into cardiac cells.
If we don’t have enough vitamin D around to recruit calcium from our diet, our bodies find new ways to get the latter nutrient at the right levels in our bloodstream. The most convenient option? Diverting calcium away from growing bones. Rerouting a mineral meant for those long, white calcium reserves can keep other, crucial bodily systems functioning, even if it means weakening a kid's developing physical support structure. “Short term, it’s less of a problem than muscle contractility being disrupted,” Roth says. For very young babies and adolescents, these phases of life mean growing fast and potentially dealing with a calcium-control system that’s struggling to recalibrate.