Tweetyour internal organs are protected by fat, if it is too low for too long it can cause problems stemming from that. Other than that you will prob be pretty dam tired all the time and have no energy in the gym and struggle just to try to maintain your muscle which prob will not happen anyway. Basically you need some fat IMO to build muscle. You can still stay relatively lean, just not 3-5%, unless you are a Kenyan olympic steeple chase runner.