Hey guys, sorry I didn't get a chance to weigh in sooner,...
As far as an 'adaptation phase',.. well, it's not like I maintain a very lean physique year round. If you look at my usual 'walking around' weight vs my contest weight, there's usually a 25-30 lb difference. Of course I will say that I had to really re-learn how to view carbohydrates when I started competing. Before that point, while I didn't avoid them by any means, I was always aware of what I was eating.
I think Poliquin was the one who said that while you have essential amino acids and essential fatty acids, you have to EARN your carbs. So I had to (and still do!) actually increase the amount of carbs I was eating when I did my first contest prep. Each successive prep, I've actually increased the amounts I eat. Could this be my body better adpating, or is it just me getting a little more fearless in terms of pushing back against the mainstream media's views.
The difficult part in trying to figure out if my physiology has indeed improved in how it handles macros is that each year you learn from the last, and make little changes. I keep playing around with my numbers, my cardio regimen, especially my supplements, so it's not like anything is being held constant. Still, what I routinely hear when talking to other pros is that each year you compete, your body becomes more capable of achieving a leaner state.
This has always made sense to me, as even some of the top conditioned athletes (I always use Brian Whitacre as an example) weren't sporting the level of near-fatfree-ness that they currently do back when they were amateurs. Again though, the only way you will ever know if the body itself adapting is the sole reason for the seemingly better handling of nutrients, is if someone followed the exact same contest prep year after year (and even then, if they start out at different weights from unrestricted offseason diets, it would skew it as well).
My final answer? I'd say that it's a combination of the body improving how it handles everything (not just better insulin sensitivity), as well as getting smarter about your diet and how your individual body handles macros.
(Hope the answer you want is in there somewhere, I just get rolling sometimes -lol)