I have been forced to go on a low-carb/high fat diet after getting diagnosed with Type 2 diabetes. My maternal grandfather and uncle had it, so I hit the genetic lottery :-( Actually I could have just taken the pills the doc gave me, but I wanted to use it to also take the opportunity to shed some of the excess pounds I had accumulated last few years of not riding (injury and other health crap). I was 266 lbs at Labor Day, and am now 224. Goal is sub 200 (I'm 6'2"). A1C was 6.8 before, and at end of Dec was down to 5.4 , and the doc said she may taper me off the metformin that she originally said I would need for the rest of my life.
Still feeling my way out on how to mix this diet with biking, but so far it has been interesting figuring that I don't constantly need to shovel in calories like I used to to keep the bonk away. I did a 4 hr 45 mile/4500 feet of climbing ride (and getting caught in rain and hail!) over the weekend on just Nuun, and never felt like I was bonking. So yes I guess I am fat-adapted now. I still am very careful with my pacing, but mainly because as I said I am still trying to figure out how to make this all work together. Yes I could probably put out a few more peak watts with more carbs in my diet (I feel more like a diesel than a sports car), but my trade-off is staying healthy over peak performance.
That said, diet is a very personal thing - what works for me may not work for you. I just seem to have a genetic disposition to not handle carbs well, so I have to work with what i was given. Your mileage may vary....