On this thread, you said you were a 15-year-old male.
good god, dude, get a grip.
You should be stuffing your face with protein and good fats and veggies and probably some white rice.
NOT fasting for "longevity" or any other reason for that matter.
If you deprive your body of nutrition while you're still growing, you'll probably cut your longevity short for some other reason.
Enjoy your youth,
And don't think of fasting until you're 30 or 30 pounds overweight (not likely), whichever comes later.
5'0" female, 42 years old.
Started Primal October 31, 2011, at a skinny fat 111.5 lbs.
Current weight: 101.5. lbs and holding steady. Spring yardwork here we come!
Co-worker 1: Needs to lose ~50. Now he wants to start Mayo Clinic Diet. Yeesh. Give it up, man.
Co-worker 2: Needed to lose ~55. Lost 20 from stress. Started Primal in Sept, lost 20 more, but gained 10 back on a carb spree. We're working on it.