New U.S. research on vitamin D supplementation in children suggests that those living in northern latitudes are at a higher risk of vitamin D deficiency.

Carried out by researchers from the Friedman School of Nutrition Science and Policy at Tufts in Massachusetts and colleagues, the new study set out to assess whether the current recommended daily allowance (RDA) of vitamin D was enough to raise blood levels to nutrient sufficiency in children in northern latitudes, especially during the winter months.

As living in northern latitudes means shorter days and less sunlight during the winter, it can also mean far less vitamin D, known as the "sunshine" vitamin.

Moreover, achieving an adequate vitamin D intake, which is crucial to bone health, can be particularly difficult for children to achieve through food alone.

The team looked at the effects of three supplemental doses of vitamin D on 685 schoolchildren age 8 to 15 in Boston.

The children were given supplemental doses of vitamin D3 over a 6-month period, receiving either the current RDA of 600 IU per day, as well as higher amounts of 1000 or 2000.

At the start of the study, 40 percent of the children were either vitamin D deficient or severely deficient with blood levels of <20 ng/mL or <12 ng/mL, respectively.

Only 11 per cent were considered vitamin D sufficient with a blood level of >30 ng/mL.

The team found that although all three doses raised serum vitamin D levels and virtually eliminated any severe deficiency by the end of the six months, it took the highest dose of 2000 IU per day to move the majority of children out of a deficient level and to see the largest number of children attain sufficiency.

This amount is more than three times the recommended daily allowance for vitamin D for children, with the vitamin D upper limit for children nine years and older 4000 IU per day.

As well as more common in northern parts of the world, vitamin D deficiency is also reported to be more common among those with obesity and darker skin pigmentation.

The team found that obese children and Asian children responded less to supplementation, however African-American children, who started at the lowest levels, responded the most.

"Food is the best source for valuable nutrients, but obtaining even just the RDA of vitamin D from food alone would take the equivalent of six eight-ounce glasses of fortified milk," said lead author Jennifer Sacheck. "Improvements to diet and increased awareness are considerations for closing the vitamin D gap when the sun can't do the job."

For children fortified foods such as milk and yogurt are good options for topping up vitamin D levels. Also try making eggs a part of breakfast and swapping meat for vitamin-D fatty fish, such as salmon, trout, tuna and halibut for dinner.

The study can be found published online in the Journal of Clinical Endocrinology & Metabolism.