The researchers caution that their results are not proof of cause and effect, but rather evidence of a complex interplay between low vitamin D levels and hemoglobin, the oxygen-binding protein in red blood cells. The investigators say several mechanisms could account for the link between vitamin D and anemia, including vitamin D's effects on red blood cell production in the bone marrow, as well as its ability to regulate immune inflammation, a known catalyst of anemia.
To capture the interaction between the two conditions, researchers studied blood samples from more than 10,400 children, tracking levels of vitamin D and hemoglobin. Vitamin D levels were consistently lower in children with low hemoglobin levels compared with their non-anemic counterparts, the researchers found. The sharpest spike in anemia risk occurred with mild vitamin D deficiency, defined as vitamin D levels below 30 nanograms per milliliter (ng/ml). Children with levels below 30 ng/ml had nearly twice the anemia risk of those with normal vitamin D levels. Severe vitamin D deficiency is defined as vitamin D levels at or below 20 ng/ml. Both mild and severe deficiency requires treatment with supplements.
When investigators looked at anemia and vitamin D by race, an interesting difference emerged. Black children had higher rates of anemia compared with white children (14 percent vs. 2 percent) and considerably lower vitamin D levels overall, but their anemia risk didn't rise until their vitamin D levels dropped far lower than those of white children. The racial difference in vitamin D levels and anemia suggests that current therapeutic targets for preventing or treating these conditions may warrant a further look, the researchers say.
"The clear racial variance we saw in our study should serve as a reminder that what we may consider a pathologically low level in some may be perfectly adequate in others, which raises some interesting questions about our current one-size-fits-all approach to treatment and supplementation," says lead investigator Meredith Atkinson, M.D., M.H.S., a pediatric kidney specialist at the Johns Hopkins Children's Center.
Untreated, chronic anemia and vitamin D deficiency can have wide-ranging health consequences, including organ damage, skeletal deformities and frequent fractures, and lead to premature osteoporosis in later life.
Long known for its role in bone development, vitamin D has recently been implicated in a wide range of disorders. Emerging evidence suggests that low vitamin D levels may play a role in the development of certain cancers and heart disease and lead to suppressed immunity, the researchers note.
Anemia, which occurs when the body doesn't have enough oxygen-carrying red blood cells, is believed to affect one in five children at some point in their lives, experts say. Several large-scale studies have found severe vitamin D deficiency in about a tenth of U.S. children, while nearly 70 percent have suboptimal levels.
"If our findings are confirmed through further research, low vitamin D levels may turn out to be a readily modifiable risk factor for anemia that we can easily tackle with supplements," says senior study investigator Jeffrey Fadrowski, M.D., M.H.S., also a pediatric kidney specialist at Johns Hopkins.