i assume your a male, so you need to weigh about 160+.
fasting is out of the question for you, unless you have a serious problem and was in the hands of a professional fasting expert.
if you believe you have mercury, then certainly you lost allot of it with your fasting and more and more will come out as time passes on.
i would start on the daily intake of plant calcium, iron, etc. along with the hormone balance 6 per day and plenty of plant fats daily and see what happens.
unweight is something i normally see in the elderly who are very sickly and with those that have or are chronic smokers/drinkers. with them, i assume they have damaged their livers and kidneys to the point of none function. otherwise for the rest of us, we all would prefer to drop a few pounds.
good weight comes from strong muscles, while grains just fatten us up for market.