This is the Billion Dollar Question.
We have zero data from which to draw conclusions. I have seen a study that found that found that a short stint of alternate day fasting at 500 calories dropped leptin levels significantly.
http://onlinelibrary.wiley.com/doi/10.1038/oby.2010.54/fullIf you are hungrier after losing any significant amount of weight on a fasting diet, it suggests that your leptin level has dropped. I am seeing a certain amount of this. It is to be expected with any significant weight loss. The question is whether the drop in leptin is less than what you'd see with other diets.
I'm at the point, weight-wise, now where in the past I could NOT maintain, but ate my way back up about 5 lbs, at which point I could.
So I will be very interested to see how I fare over the next few months.
But until there are 10s of thousands of people who have eaten 5:2 diets for 5 years or more, it is premature to draw conclusions.
My experience with low carb diets was that it took a good 5 years to see if they really worked. All the data you see cited by those who profit from selling diet-related books, TV shows, etc, is always drawn from 1 and 2 year studies but the big drop off of previously committed dieters happens in that 3-5 year window.
I'm very eager to learn more about this, but would caution us all to be very careful not to draw any metabolic conclusions from studies lasting anywhere from 2 weeks to 6 months. Nor should we even get excited by those lasting a year or two. When we have a population of people who have eaten the diet successfully for 5+ years, that's when I'll start getting really excited.