Marie,
The research that has been done on lactation and osteoporosis showed that
though calcium was mobilized from bone during lactation (especially from 6-12
months), it was replaced later.  Women who have breastfed have greater bone
density and fewer fractures in later life.  All amenorrhea is not the same,
amenorrhea from anorexia. for example, is the result of malnutrition and a
deficit of body fat disturbing estrogen production, whereas lactational
amenorrhea is the result of deliberate suppression of several hormones
involved in ovulation.  I think there is a difference in an adaptive process
and a pathologic process even if the symptom is the same.
JMHO.
Catherine Watson Genna, IBCLC  NYC