So far, I could find no quantitative studies on homeostasis failure, that is, the lowest intake amount before the homeostatic mechanism fails. In other words, what's the minimum safe daily intake.
The FDA RDA is 8 mg/day (adult males). 18 for adult females, 27 during pregnancy (I wonder if it si possible to eat all tha t iron without supplements).
Some say for vegetarians it's 14.4 (prevalence of non-heme iron); some do not agree upon that though. The 1.8 factor woudl mean almost 50 mg/day for pregnant women and that sounds huge.
The following site has good qualitative suggestions. Surprisingly enough, using a cast iron skillet provides iron fit fo rhuman absorption.
http://www.nomeatathlete.com/iron-for-vegetarians/
