We do not know what causes osteoporosis.   It sounds from what you say that
it is the estrogen which is being blamed, but this is probably not likely.
But, we do know that normal woman lose calcium during pregnancy and
lactation, but that breastfeeding women bounce back after the baby weans,
perhaps to levels higher than previous.  In a young woman with osteoporosis,
this does not necessarily hold.  Nevertheless, there is no evidence that
breastfeeding will make her situation worse.  Whoever is telling her not to
breastfeed knows no more of the effect of breastfeeding on osteoporosis than
anyone else, the two situations coexisting being rather unusual.  It
certainly would be better for her baby and for her in other respects.
Surely she should be encouraged to breastfeed, and the progress of her
problem can be followed, rather than being told straight off she cannot
breastfeed.

Jack Newman, MD, FRCPC