Estrogen is a primary inhibitor of bone resorption in both men and women
Mauras et al, Growth hormone, insulin-like growth factor I and sex hormones: effects on protein and calcium metabolism 1999.
Estrogens and androgens increase intestinal absorption and the retention of calcium
Michaelsson et al., Dietary calcium and vitamin D intake in relation to osteoporotic fracture risk, 2003
Estrogen, Testosterone, and other androgens have roles in facilitating bone growth
Cannel, John,Vit D: When, Why, Where and How Much 2005 lecture
A decline in sex steroids may aggravate the effect of vitamin D deficiency - bringing the total vitamin D-like activity to a low enough level that a higher intake of vitamin A begins to become harmful.
Sex steroid deficiency appears to "turn on" the otherwise dormant association between vitamin A and fracture risk. (Maybe this same correlation can be similarly "turned on" by vitamin D deficiency and "turned off" by D sufficiency).
