@MaryAnn,
I'm not impressed by any of the research I've seen about either fasting in general or IF. But my previous researches have convinced me that calorie restriction is what makes all diets work. The genius of 5:2 is that you can get the benefits of calorie restriction without that horrible day in and day out deprivation that occurs with other plans.
I was chatting with the friend who turned me on to this diet and we agreed that the beauty of it is that we only feel like were are dieting for 3 hours or so, twice a week (the three hours before dinner on a fast day, for for me). The rest of the time, no diet! This is a friend who I have known for 20 years who has never stuck to a diet longer than three weeks. She is down 25 lbs and still on this one, 7 months later. That has to be worth something!
But beyond that, I think we will only have a better idea what the long term effects are after many people have been on this diet for 3 years. With other diets, that is how long it takes to see a) if reasonable amounts of people can maintain their weight loss and b) if the health benefit persist.
With most diets I've looked at, the health benefits touted by the guys who write the books show up in very short term studies (six weeks to three months) but tend to disappear in longer studies--and that includes weight loss! Some diets that look very good over 3 months end up looking pretty bad at 3 years, not because the diet stinks, but because the diet teaches people bad habits that make maintenance very hard.
Diets that involve eating foods that come in prepared packages or cans fall into that category, as they don't teach people how to eat properly "free range." Low carb diets also fall into that category for too many people, because they learn that eating a lot of fat is healthy--which it is if you are eating at a ketogenic carb intake level. But after a few years, they forget the latter part of that last sentence. They let their carbs creep back up to a non-low carb level, but continue dowsing their salads with blue cheese and pounding down bacon. That does not end well!
I'm only 5 months into this diet, so I don't pretend to know how sustainable it is. But the very fact that I'm still on it, says a lot. I haven't been able to do a weight loss diet longer than 6 weeks for quite a few years as my TDEE is so low that the only diet that takes off pounds is a near starvation diet.
But we'll all have to talk about this issue again in thee years to see what the real, long term health benefits are!
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