This is interesting as it suggests that there may be some kind of lag between the availability or otherwise of different fuels and what the body uses.
Many if not most people when starting 5:2 find the first few fasts difficult, which I have thought is probably due to the body being 'reluctant' to start fat burning. This study suggests that after a prolonged period of fasting when carbohydrate is available again, the body may be 'reluctant' to change away from fat burning.
Of course the men in this study were fasting for a lot longer than we do. Nonetheless, it might be a potential mechanism that might prevent weight going on during feed days, if we can get a good fat burn going on fast days! It may also add a bit of weight to my theory and that of other IF gurus that it is important to avoid carbohydrate when fasting.