It's really only a sliver of time when humans build the bulk of their skeleton. At age 9, the bones start a big growth spurt. And by the time puberty ends, around 14 or 15 years old, the adult-sized skeleton is all but done, about 90 percent complete.
But doctors say a lot of children aren't getting what they need to do that. Calcium and vitamin D are essential, sure, but so is lots of time jumping and running.
"It's the magic window of time when bone is built," says Dr. Laura Tosi, an orthopedic surgeon who directs the pediatric bone health program at Children's National Health system in Washington, D.C. And when it comes to bones, "bigger is definitely better," she says, "The wider and thicker the bone, the harder it is to break or tear."
Just about everybody knows that calcium and vitamin D are essential to build strong bones. But children and teenagers are all too often shunning the foods that would help them get enough calcium and vitamin D to build those bones.
Federal health officials recommend that children between the ages of 9 and 18 get 1,300 milligrams of calcium every day. That translates into four to five glasses of milk or the equivalent. According to Dr. Neville Golden, an adolescent medicine specialist at the Stanford University School of Medicine, most teens are not drinking anywhere near that amount.
Only about 15 percent of high school students are drinking milk, according to a 2011 federal survey. And only about 9 percent of girls do. In part that's because milk drinking is viewed as "uncool," says Golden, and teens, particularly girls, often think of milk as fattening.
But that's not true, he says. One glass of skim milk contains no fat and approximately 80 calories, about the same caloric content as an apple.
Now of course there are other sources of calcium — yogurt and cheese, for example. But lots of kids think those are fattening, too, Golden says. And while vegetables can be a source of calcium, you'd have to eat an awful lot of them to reach the recommended levels – one cup of broccoli has just 42 mg of calcium. Fortunately, there are other food sources, including fortified orange juice, breakfast cereals, and tofu.
Calcium supplements are often recommended for adults to meet their recommended daily requirement. But for kids, Golden says, it's far better to eat calcium-rich foods. That's because the body absorbs calcium better from food than it does from supplements.
Even if boys and girls get the recommended amount of calcium, if they're not also getting Vitamin D it won't make a difference. "You can drink as much calcium as you like, but if you don't have enough vitamin D, you're not going to absorb it," Golden says. The vitamin, which is actually a hormone, interacts with the lining of the stomach and intestines to help calcium get absorbed.
In 2010, the Institute of Medicine said that children and most adults need 600 international units of vitamin D a day. They found no evidence of widespread Vitamin D deficiency, though people these days spend much less time out in the sun, which stimulates the body's production of vitamin D.
Vitamin D is found in foods like fatty fish such as salmon and sardines, but teenage girls were least likely of all age groups to get enough vitamin D in food, according to a 2010 study. One interesting point, says Golden: farmed salmon is not as vitamin D rich as wild-caught salmon. And like calcium, Vitamin D is added to foods like milk and orange juice.
If a teenager still isn't getting enough vitamin D from fish, milk and other healthy foods, Golden says a reasonable workaround is taking supplements of about 400 international units per day.
Perhaps the secret ingredient to building strong bones for a lifetime is exercise, says orthopedic surgeon Tosi. One study found that when children jumped up and down between classes, totaling about 15 minutes a day, they added mass to their leg bones.
The bounce is what's important, Tosi says. "We think the bounce probably sends an electrical or other signal right up the skeleton, saying 'OK, time to give more, time to build more bone'."
So, jumping, jogging, tennis and basketball are all good options. Federal health officials suggest kids get at least one hour of exercise every day.