So many answers! I really do enjoy this forum, people here are kinda awesome.
I just quickly wanted to pass on a link about a possible side effect of extreme caloric deficiency:
I will absolutely agree that there is no such thing as 'Starvation Mode'; however, our bodies do have hormonal/metabolic reactions to long-term weight loss and calorie restriction. 'Adaptive reduction of thermogenesis' just means that your body produces slightly less heat, in order to conserve energy; it tends to become a concern when consuming <80% of your TDEE (Total Daily Energy Expenditure). Stated differently, the 'literature' says you should be aiming for no more than a 20-25% deficit (I'll keep looking for the source I read, for that number...). When losing fat there are plenty of positive changes in your body, but there is also the potential for long-term downregulation of your body's energy consumption (ie. you could have to eat fewer calories than an individual of the same weight, whom has not been overweight). As always, be sure to do your own research, ask questions, and check with medical professionals with any concerns when changing your diet/lifestyle/fitness choices.