Well, my situation is a bit different from many of yours as I lost a good amount of weight ten years ago and did manage to keep it all off until this last year. Even then I only regained 7 lbs. But what changed this year was two things. One was I had to stop taking metformin, which I had taken for the past 9 years, and which really helps me control my blood sugar and appetite, as I developed a stomach infection (diagnosed by biopsy) that took almost 9 months to clear and the other was that the technique I had used for the past decade to keep maintaining, which was to go back on my old diet for a month any time the weight crept up 3 or 4 real pounds, became too hard to keep using because my TDEE has dropped to where if I cut calories "gradually" I have to eat 1100 or 1200 calories a day for months to lose any significant weight. That is, as most of you undoubtedly know, pure hell. I could diet for a month or two just fine eating 1450 calories, but that extra couple calories less made me feel starving and crazy.
So that was why I turned to 5:2 which has worked very well for me. It let me create a deficit that led to gradual weight loss (2 lbs a month) but it let me eat MORE than I was used to eating on feed days. But one side effect of eating this way has been that I have been eating a lot more on feed days than I have been for the past 10 years, when I have been eating to maintenance calories, and I have become used to eating more, and really liking it. Obviously, if I keep eating that way without those 2,000 calories of deficit every week, bad things will happen.
Hence the anxiety, because I would like to know that I can go back to eating at my TDEE the way I used to.
I appreciate the suggestion to try the eating window. I'm just not sure I can do it every day. That feels perilously like dieting all the time to me, and that is not going to work for me at all.
I am trying to debug the stomach problem. I had started taking metformin again a month ago, which I had had to stop because the stomach infection made it too painful to take it. But I have taken that drug for many years in the past and at higher doses than I'm taking now without any stomach problems, so I hesitate to blame metformin for the many bouts of diarrhea.
I also have a history in the past of getting stomach problems like this at this time of year, which I think has to do with the fact that our water comes from a well and is not treated, just filtered for sediment. I am now drinking only boiled or bottled water.
Beyond that, I have also had problems eating guacamole at a certain restaurant in the past, and that also was a factor recently, though my husband ate it too, and suffered no ill effects.
I have read reports here on the forum that some people get diarrhea after fasting. So that too might be a factor, though I didn't have any problem with it the first 3 months I fasted. However, I am wondering if the combination of metformin and fasting, and perhaps eating a high fiber raw vegetable slaw on fast days, might have tipped things over the edge, digestively.
I've stopped taking the metformin and taken this week off fasting, and I'll see if the digestive tract calms down. If it does, I'll fast without the drug (and without the high fiber slaw) and if that works, reintroduce a lower dose of metformin. I really do need to take the metformin, (long boring story, and no, low carb alone won't solve the problem. I've tried that for long periods of time.)
I'll be very happy to be able to get back to fasting, and if the elimination sequence doesn't provide an answer, it will be off to the doctor (a rather grumpy and unhelpful one, alas) for some tests.