oh i hated that low carb flu. i dove in both feet, and for the first week i was ok. my girlfriend was doing it with me, so i had her for support. i was also having a hard time with the "eat more fat" thing, and she was good about encouraging me along those lines. i would say it was in my second week that i started to feel really tired- i mean BONE tired. my muscles were so sore and i wasnt recovering from resistance training workouts at all. then, i got a scratchy throat way in back and up, like where strep starts, so thats what i thought i was getting. i took a week off of working out thinking i was getting the flu. i had a TERRIBLE headache that lasted for a few days and that wouldnt respond to NSAIDs ot tylenol. i pretty much spent two days in bed, flanked on either side by two very unproductive and whiny days. i had a ton of mucus, too and was blowing my nose like i had a cold the whole time. oh, and i couldnt think straight. it was like my brain was full of concrete.
then, one morning it all just lifted and was replaced by this wonderful feeling- like energy that you know is clean and natural, not artificial like from caffeine. the first time it happened, i blamed it on some viral infection and couldnt figure out why i was the only one in my family who got sick. those first couple of months though through the holidays, i had a few REALLY non-paleo days, and realized that about two days after a big cheat, i would get that flu again. i eventually put two and two together- it was never as bad as when i first went through it, but i definitely notice than whenever i binge on sugar and carbs, i can plan on that headache, fatigue, sore throat, runny nose, and muscle stiffness a few days later.