What do you mean by "function" normally for the long term ?
like not being hungry and irritable (possibly for not ideal hormonal functioning) and not being prone to nagging injuries
it's rather well known that females need to not stay under 12% in order for their proper hormonal functioning
from what i've read essential fat for men is 3% but who the fuck even ever hits 3%
when i was @ sub-7 for extended periods of time i was very active running 2-3 miles per day 5-6 days per week and lifting 5-6 days per week as well, and at that time i ate junk ie. fries, cheesburgers, pizzas, etc
but i didn't eat ALOT of that junk, maybe a big mac and medium fry and medium coke and then 2 tacos later on in the day