In a study of 20 pairs of identical twin girls approaching or in puberty, aged 9 to 13, one sister took 800 mg of calcium plus 400 IU of vitamin D3 per day while her twin took a placebo. After six months, doctors measured bone strength and found, compared to placebo, girls in the calcium-vitamin D group had up to 66 percent stronger shin and arm bones depending on the specific area of the bone.
Researchers said that bone size, density and strength increased in the spongy-bone areas inside of bones and in the compact-bone areas along the outer shafts of the arm and shin.