Well after reading @leokitten’s excellent post about the ketogenic diet and how well he did (which post told me everything I needed to get started), as well as an enthusiastic endorsement from one of my sons who has done very well on it, with great reluctance I gave it a try a few weeks ago. I used the cronometer app which I recommend highly (this app can be used with any type of diet). I had a question about the app, emailed them and got a response within a day or 2 (!) Plus it's pretty powerful. My son liked it better than My Fitness Pal. I was pretty well-prepared and cut my carbs drastically almost immediately. By the evening of the third day I began to think I could not stick with it, my fatigue was so profound (not PEM, had not overdone anything) plus my brain was quite fuzzy, I thought it was the “keto flu”. Then the 4th day I woke up and though still fatigued and fuzzy, felt a fair amount better and thought I could stick it out. I felt well enough to make a light trip to the grocery store, only getting 10 or 12 things, and then I think a few other minor errands - e.g., maybe paying some bills, it’s foggy . . . And by that night severe fatigue hit and I woke up the next day crashed. I had not done enough to provoke PEM in a normal state. And the PEM lasted twice as long as normal. So I had to stop keto - I could not function and obviously something was wrong. My guess is that my body was having difficulty going into ketosis - burning fat for fuel - despite rather rigorously following the diet, and thus relied even more heavily than usual on amino acids. About 4 years ago I discovered that branched chain amino acids cut my PEM recovery time in half, have taken them religiously ever since, and that recovery time has remained constant, until keto. Suddenly I was back to the bad old days. I am pretty positive that keto depleted my BCAAs, and probably several other aminos. It took me close to 10 days to recover from my 4 days on the keto diet. It just occurred to me that if I had increased my BCAAs while doing keto, I might not have reacted so badly to it, but I don’t have any desire to try an experiment with it again. Those 4 days took too much out of me! I had been primarily hoping to have some of the benefits @leokitten wrote about - better sleep (a biggie!), increased energy, etc., and losing some weight would have been icing on the cake (if one were allowed to have cake with icing on that diet!) However, reading about the ketogenic diet has prompted me to add more fat to my diet and now I am going to try what @Learner1 is doing: roughly 50% calories from fat, 25% from protein and 25% from carbs. But I’m going to be careful, watch out for unexplained fatigue, etc. I haven’t started this yet, have been dealing with other things, but it’s on my to-do list . . .