This is because most Americans have an acidic diet, due to high intake of acid-promoting protein from meat. The body draws bicarbonate, an anti-acid or alkaline chemical out of bone to neutralize this acid, in the process dissolving a bit of bone. Low blood sugar, caffeine and alcohol promote calcium loss. But if a bone fracture has just occurred, a high protein diet encourages healing. Milk, although high in protein, does not promote acid.
Fat cells produce estrogen, which lowers the risk of osteoporosis after menopause.
This is one of the very few health advantages of being overweight. However, there have not been any studies to evaluate the effect of boron supplements for osteoporosis. Although most research shows calcium increasing bone mass and reducing fractures, a few studies show little or no benefit. Probably calcium deficiency is an important factor in some people's osteoporosis but not in everyone's. There's much more we can look at for osteoporosis prevention than just calcium and estrogen.