When the Northern Lights dance across the arctic sky in Finland each winter, some say it makes up for the 18 hours of darkness during the extra-long nights. But that psychedelic light show doesn't change the fact that kids there often are deprived of exposure to vitamin-D-producing sunlight.

Researchers from Turku, Finland, wanted to see what the health repercussions were for vitamin D-deprived kids 3 to 18 years old by the time they were 30 to 45.

It turns out that those with the lowest vitamin D levels when they were young had the most plaque-clogged carotid arteries as middle-age adults. That raises the risk of stroke.

What does this mean for North Americans, where 75 percent of teens and adults are vitamin D deficient? Well, parents should think about their kids' vitamin D levels, because how youngsters live today determines the quality and quantity of their lives tomorrow.

And when it comes to taking supplements, don't be wimpy! According to the National Institutes of Health, the safe upper limit for vitamin D is 1,000 to 1,500 IU per day for infants; 2,500 to 3,000 IU per day for children 1 to 8; and 4,000 IU per day for children 9 and older.

So get a blood test to check if your child is deficient (and to track the effect of taking a supplement).

The National Institute of Medicine says infants and children shouldn't be below 11 ng/mL. We personally aim for levels around 35 ng/mL. Above 35, even up to 50 to 80, is better than below.