Milk is a rich, natural source of calcium
Scientists at Sheffield University asked half a study group of 86 girls, aged 11 and 12, to take a calcium supplement added to fruit juice and got the rest to take fruit juice without any added calcium.
While the girls were taking the supplemented fruit juice, their bone density improved by 1.2%, but two years after treatment, researchers found the bone gain had reversed.
Researchers believe the gain in bone density among the test-group girls was probably due to a suppression of bone remodelling - the process where old bone tissue is broken down while new bone is built.
Chairman of the NOS Scientific Advisory Group Professor Richard Eastell said: "This underlines the need for children and teenagers to take plenty of milk in their diet if they are to build bone strong enough to last a lifetime.
"Calcium supplements can be used as a substitute if they don't like milk, but it is important they keep taking these, or the effect is lost."
Non dairy diets
Experts think many young girls are not drinking enough milk because they mistakenly believe it leads to weight gain.
They also fear vegan diets and others which eliminate dairy products are behind girls turning away from milk.
A campaign by Animal rights group, People for the Ethical Treatment of Animals (Peta), which claimed milk was bad for children's health, was recently suspended.
Teenage girls, who are more likely to suffer from osteoporosis in later life if they do not have a calcium-rich diet, should consume 800mg of the mineral daily.
There are 600mg of calcium in one pint of milk, but other good sources include tofu, spinach and dried fruit.
The Vegetarian Society believes a diet without meat and dairy products need not be deficient in calcium.
Samantha Calvert from the Vegetarian Society said: "Vegetarians are no more likely to be lacking in calcium than any other sectors of society.
"There is no reason why teenage vegetarian girls will be deficient in calcium as long as they're taking a balanced diet."