I think much of regain comes from attempting to go back to eating foods that simply aren't good for our health, from a nutrition perspective.
I have started looking up the Glycemic Index values for ALL the foods that I like to see which are very high and which are low. When I get to the maintenance phase I will be trying to build a balanced diet of vitamins, minerals and calorie/carb/healthy fats/protein ratios so I don't NEED to take supplements.
I know that I am predisposed to gain weight. I've only to look at all the women in my family to see that, but I can CHOOSE not to make it worse and that is the path I'm determined to take.
I don't see the point of spending the time and money on IP, only to throw it away by trying to revert to the former eating patterns. I feel too good now, much less when I get to target weight, to allow myself to slide back into that self-loathing, embarrassed and constantly feeling like crap pit that I crawled out of.
Of course metabolism changes on IP. If you think about it, your liver is going from processing predominantly carb-based foods and providing energy to burning fat to provide ketones for energy. This is a massive metabolic change and it logically has to have a long term effect on how the liver functions. I'm sure that over time, when we revert to using carbs/protein as our primary fuel source it will correct itself, but I doubt it'll happen quickly. Your metabolic rate is also linked to your activity level and your new weight, so of course your caloric intake will be much less for maintenance than your starting weight level.
I'm glad you asked this question. I see so many other weight loss forums where people are decrying IP as a "I gained it all back and more when I stopped", but they aren't honest enough to admit that they didn't phase out properly or they reverted to their previous lifestyle habits and that annoys me. If you take responsibility for what you put into your mouth, then you stand a greater chance of keeping the weight off. I love to see our returnees, because it's a sign that the IP mentality DOES work when they go off the rails or have gains outside tolerance levels that they set.