Doctors say many kids and teens aren't.
14-year-old Benjamin Smolen was a freshman in high school when he began suffering constant pain.
He says his whole body ached.
He was getting headaches and feeling exhausted all the time.
Doctors tested him for everything from arthritis to muscular dystrophy to Lyme Disease.
They discovered he had a severe Vitamin D deficiency.
Doctors say most kids probably aren't getting enough Vitamin D, a compound that aids in the absorption of calcium, which builds strong bones.
When adults don't get enough D, they often suffer from osteoporosis.
When kids don't get it, they never actually develop strong bones.
Dr. Laura Tosi is a pediatric orthopedist at Children's National Medical Center.
"Teenagers will stop taking vitamins and particularly our girls will stop drinking milk," she explains.
Tosi says she's seeing more and more kids coming to her bone health clinic with low Vitamin D levels.
One study out of New York found that 70 percent of American kids aren't getting enough D.
Researchers believe it's because kids aren't playing outside like they used to and they're not getting enough sunlight, which helps the body produce Vitamin D on it's own.
"Now more and more kids are couch potatoes, so they're not getting D from the sun," Dr. Tosi says.
According to the American Academy of Pediatrics, kids under 18 should be getting at least 400 international units of Vitamin D each day.
Considering one glass of milk only has 50 to 100 units, Tosi says kids need to be taking supplements.
That's what Benjamin is now doing.
Immediately after his diagnosis, doctors give him 50,000 units of Vitamin D once a week for eight weeks.
Now that his levels are back up, he's taking a smaller dose daily to get what he needs.
Doctors say they don't regularly test for Vitamin D levels in kids because it's expensive and there isn't one reliable test for it, but there are changes to recommendations coming down the pike in the future, so testing may become more prevalent.