Milk and other dairy products have long been recommended for building children's bones. Yet the long-term benefits of childhood dairy intake have not been fully clear.

In the new study, researchers at Boston University School of Medicine followed 106 children for the time they were 3 to 5 years old until they were between the ages of 15 and 17. Throughout the study, parents periodically kept food diaries to record what their child was eating each day. A cup of milk or yogurt, 1.5 ounces of natural cheese and 2 ounces of processed cheese were all considered equivalent to a serving of dairy.

At the end of the study period, when the children were high school age, they underwent bone scans.

In general, the researchers found, children who ate at least two daily servings of dairy had greater bone mineral content and greater bone density as teens compared with their peers who ate less dairy throughout childhood.

The findings held true when the researchers accounted for other factors in bone density and health, like body weight, height and exercise levels.

Dr. Lynn L Moore and her colleagues report the findings in the Journal of Pediatrics. The study was partially funded by the National Dairy Council.

Peak bone mass is acquired during adolescence, Moore's team notes. Although genetics accounts for a large share of a young person's ultimate bone mass, diet and exercise are "crucial modifiable contributors," the researchers stress.

Dairy products contain bone-building calcium, and fortified milk is a prime source of dietary vitamin D, which is essential to the body's absorption of calcium, Moore and her colleagues point out. There is also evidence, they add, that dairy proteins -- whey, in particular -- enhance bone-building while inhibiting bone breakdown.

"The findings of this study confirm the importance of a diet rich in dairy and other protein sources on adolescent bone mass," the researchers conclude.