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, 43 years old. Started Primal October 31, 2011, at a skinny fat 111.5 lbs. Low weight: 99.5 lb on a fast. Current weight: skinny-fat 106 lbs. Resetting with the 21-day challenge.
MY PRIMAL: I (try to) follow by-the-book primal as advocated by Mark Sisson, except for whey powder and a bit of cream. I aim for 80-90 g carb/day and advocate a two-month strict adjustment for newbies. But everybody is different and other need to tweak Primal to their own needs.