I did the 5:2 diet for a whole year ending this past March. I was enthusiastic about the diet for about 7 months. Weight came off as promised and my blood pressure normalized. I had the best A1c I’ve had in over a decade.
BUT, and it was a huge “but”–over the second half of the year things got ugly. First I started gaining weight despite eating exactly what I had been eating while losing. Then I found myself ravenously hungry on my non-fast days. Crazy, eat everything you see hungry.
Now mind you, I have been watching my weight very closely since 1998 and I had maintained a 17% weight loss for a decade without getting crazy about food, so I am no stranger to dieting. But I had never had the kind of problem with hunger I developed in those latter months of 5:2 except for before my diagnosis when I was getting blood sugars going up to the mid 200s and then back to the 90s after every meal. But with 5:2 diet what was happening was I was getting normal blood sugars but still getting that kind of ravenous hunger, which told me I had done something to really mess up my metabolism.
Moreover, I was participating in a diet support forum and learned that I was far from being alone in having this reaction. There were several other people who had gone through the same thing after a year on the diet. Many had become more stringent in attempting to counteract it, only to find that their weight gain continued. Many had regained all they’d lost.
So I came to the conclusion that the long-term fasting diet had somehow flipped my body over into a state where it was convinced I was facing famine. I stopped dieting, stopped looking at the scale, accepted that I was going to have to regain some weight in order to get back to my usual state. It has been two months now and I am only gradually getting back to where my relationship with food is getting away from the fast/binge cycle that it had gotten into. I have packed on the tummy fat that I had completely gotten rid of on the diet and a bit more.
So what I would advise now is do the diet for a while, but as soon as the weight loss stops, stop fasting immediately and go back to eating a reasonable amount of calories that would be appropriate for your new weight. Do not keep fasting.
And if you are still losing but feel like you are losing control of your eating on non fast days that might also be a good sign it is time to get back to something more balanced.
Some people don’t experience this problem, and for them fasting can be a good long term way of eating. But if you see the signs of issues arising, do not keep pushing because that seems to make the metabolic imbalance much tougher. Our brains are very devoted to keeping us from starving and if they thing that is what is happening, they will make us eat as much as possible to suvive!