I have done long term (48+ hour) fasts in the past, but recently have become so comfortable with them after a lot of mistakes/experimentation that I thought I would share. Why? Long term fasting is an effective way to cut bloat after a refeed - I use them after cheat meals or holidays to get rid of water weight, then resume standard 18/6 IF (a la Lean Gains) for general maintenance. First off, most people can fast for at least 2 months straight without adverse effects - and obese people can fast for 6+ months. The limit of how long you can fast is your body fat reserves - your body will happily burn fat (not muscle/bones) for energy as long as it's there, then (and ONLY THEN) it starts to cannibalize itself. Here's one clinical case, of many, of a 382 day fast without adverse conditions: http://www.ncbi.nlm.nih.gov/pmc/articles/PMC2495396/ The point of these extreme examples is that fasting for a week, or even two, will have no negative effect IF you do it correctly. That being said: fasts longer than 24 hours require SERIOUS thought, supplementation and planning - fasting without these things is potentially dangerous. first of all, you need complete vitamin supplementation. A complete multi is essential, as well as calcium, magnesium and vitamin D supplementation. I also take, although they are probably not essential, B12 and fish oil. Can you lift while fasted? Yes. And for some people it's even more effective - as with me. Lifting is also important for maintaining lean muscle mass while fasted ('eat stop eat' has a large section on this). This is where BCAA (branched chain amino acids) come into play. BCAA are essential for recovery and building/sustaining lean muscle. That being said, BCAA are worthless for people who eat normally; large amounts of BCAA are contained in regular dietary protein sources such as meat. BCAA become important when training fasted, especially when it comes to weight training - otherwise you will only maintain, and not build, muscle, in a fasted state. One big mistake i made in the past was fasting without sodium or electrolyte supplementation. This resulted in me "conking" around the 48 hour mark and becoming dizzy/getting headaches. Why? Sodium levels in the blood become so low through urination that your blood pressure actually becomes problematically low. The easy solution to this is just consume a broth daily of at least 2g sodium. Bullion is an excellent source, and very easy to make. Those who aren't lazy can make broth from animal carcasses - google that. Broth also reduces soreness from lifting, so it's a win win. Lastly, fasts can become more bearable with sugar-free gums and coffee. Make the coffee black or use very small amounts of low calorie creamer with 1 g or less sugar. As you do fasting more often hunger will no longer even be noticeable. If you plan carefully you should feel perfectly fine and have no energy problems while fasted. I would argue that lack of broth supplementation is the most common mistake in this regard.