Sunlight is the single biggest source of vitamin D. But in the depths of winter, folks living in the northern reaches of the United States often don't get enough sunshine on their skin to make much vitamin D. It's essential for maintaining healthy bones and kidneys, and may have other benefits.
For people with darker skin in those higher latitudes, a deficiency of the vitamin is even likelier because pigmentation reduces the skin's production of the vitamin.
To help people with especially low levels catch up, doctors usually recommend extra fish, or dairy or other foods fortified with vitamin D, and sometimes a supplement.
But recent research has found these routine guidelines barely move the needle in some groups at highest risk, such as black teens — especially those who are obese and living in Northern states. But a heftier weekly dose of the vitamin for just two months may do the trick for them, a study published this month in the Journal of Pediatrics suggests.
"This study came about because we were treating our patients, a largely African-American and Hispanic obese population in the Bronx, and found the current regimen wasn't effective," says the study's lead author, Dr. Hina J. Talib, a pediatrician at Children's Hospital at Montefiore and assistant professor of adolescent medicine at Albert Einstein College of Medicine in New York.
Heavier people need more of the vitamin, Talib says, just because of their bigger size.
The minimum level for vitamin D for good bone and kidney function ranges from 20 nanograms per milliliter of blood to 30 nanograms per milliliter among different doctors' groups. Medical specialists with the Endocrine Society and Talib's study define 30 ng/mL as "sufficient" and levels between 20 ng/mL and 30 ng/mL as "insufficient." People with levels below 20 ng/mL are considered deficient by all the recent guidelines.
The Endocrine Society recommends a dose of 600 international units of vitamin D per day for adolescents up to 18 years old who have insufficient levels. Raising those levels may require at least 1,000 IU/day of vitamin D, according to the society's guidelines. But the group currently caps its recommended regimen at an upper limit of 4,000 IU/day.
To test whether larger doses, given less frequently, might be better, Talib's team divided a group of 183 teens whose average levels of the vitamin were 13.7 ng/mL, into three subgroups. Most of the teens identified as black and/or Hispanic. For eight weeks, one group received the recommended 1,000 IU per day, and the second group received 5,000 IU per day. The third group received a whopping 50,000 IU per week, which works out to a little over 7,000 IU per day.
At the end of the two months, only 2 percent of those receiving the low dose had reached sufficient levels of vitamin D; more than half were still deficient. Meanwhile, 56 percent of those getting 5,000 IU a day and 72 percent of those getting 50,000 IU per week had sufficient levels, according to Endocrine Society guidelines.
Once the study ended, all the teens who still had low blood levels of vitamin D were referred back to their primary care providers, who got recommendations on how to bring the teens' blood levels up to "sufficient"; all the study participants were advised to take smaller, maintenance doses of the vitamin for another six months and have their blood levels checked again then.
Vitamin D is essential for youth in particular, says Muna Sunni, a pediatric endocrinologist at the University of Minnesota Masonic Children's Hospital in Minneapolis. "Vitamin D's role is to help our body suck in as much calcium as it can," she explains, and young people need that calcium for bone growth until about age 30.
"Once they're 30, whatever bone strength they have, that's what they're stuck with," Sunni says. And if the body's vitamin D levels are too low to grab the calcium it needs for cell regulation from food, she says, that calcium is leached from the bones.
Taking too much vitamin D can be toxic to the kidney and heart, she warns. But no toxicity or side effects were reported from any of the doses in Talib's study, where all the teens' blood levels were "well below the toxic range of 88-100 ng/mL," according to the study. In general, toxicity from vitamin D is very rare, Sunni says.
Tara Haelle is a freelance health and science writer based in Peoria, Ill., and is on Twitter: @tarahaelle.