I’ve been there (69kgs. 8% bf) as the result of sheer willpower. Almost no sugar of any kind (that’s right, including fruit), around 150g of protein per day, lots of green vegetables (no corn), and no days off for the better part of a year. I did P90X almost every day, ran a few times per week, rode bikes for fun.
I maintained that level of lean-ness until I turned 40. It was an interesting experiment, but I cannot recommend doing this on purpose. At low levels of bf, I was susceptible to illnesses. I probably had a cold every three months. I was not fun to be around because I didn’t eat like anybody else, was always hungry, probably grumpy, and too ‘focused’.
If you want specifics of my diet, hit me up.