In my last column, I challenged the practice of giving infants and kids vitamin A — containing vitamins with fluoride. Today, I'll advocate for something else most doctors aren't yet advising: Start extra vitamin D at age 9, and even more above age 40.

Previous columns explain why I'd also avoid extra vitamin A and E in adults, even in multivitamins, and why vitamin D is not only important for strong bones, but may help immunity, reduce cancers, reduce falls in adults and prolong life.

Bone growth is fastest in girls from age 12 to 15, and in boys ages 14 to 17. Despite the need for adequate vitamin D and calcium during this time, teens and adults get only half the calcium they need. As for vitamin D, a 2007 study of University of Hawai'i students and skateboarders who spent at least three hours in sunlight at least five days a week found 51 percent still had low vitamin D levels.

You need 400 IU a day of dietary vitamin D from infancy to age 40. Some juices, milk substitutes and yogurts have 40 to 100 IU of vitamin D per serving, but the best source is milk, which has 100 IU of vitamin D and 300 mg calcium per glass. But how many 9-year-olds drink four glasses (32 ounces) of milk to get their 400 IU of vitamin D? Have them try 1 percent or nonfat flavored milk. At age 9, the requirement for calcium jumps from 800 to 1,300 mg a day, drops to 1,000 mg from age 19 to 50, then rises again to 1,200 mg a day above age 50.

Overweight children have lower vitamin D levels, lower bone mass and more fractures. Exercise like gymnastics or tennis started before puberty can double bone mineral content. The most common childhood fracture (the forearm) has been increasing, peaking between age 10 and 15, so 9- to 40-year-olds who don't drink milk or substitutes should take pills that have both 400 IU of vitamin D and calcium. Those who do drink milk should take vitamin D only one to six days a week so their weekly total works out to 400 IU a day.

Loss of bone mass starts after age 30, doubling to 2 percent a year at menopause, leading to hunchback and hip fractures. Those older than 40 should double their vitamin D to 800 IU a day in pills that also have calcium citrate (better than calcium carbonate for adults and perhaps kids, too). Vitamin D is extremely safe. Toxicity in adults only happens above 40,000 IU a day. As with kids, you may not need all 800 IU if you drink milk.