Ahh I misread your first post. So you went on a 900 calorie diet that wasn't necessarily a protein modified sparing fast. Now, at a macronutrient level you just eat higher protein and fat whilst avoiding carbs.

Are you certain that you didn't have pre-diabetes or some form of insulin resistance before starting your diet? It might be the reason why you feel like crap when having carbs. At a healthy weight, reasonable levels of carbs shouldn't cause you to feel like crap unless you have some underlying condition that's affected by them.

UPDATE: Another thing that can be going on here is that when you lose weight, you don't necessarily get a reduction in fat cells. Normally, fat cells contain 90% triglycerides. When you lose weight, some fat cells release their triglycerides. But they can fill up with water instead (e.g., perhaps the body's adaptation to famine conditions... where there's no food there's likely no water). Your carb intake may have simply been causing you to bloat with water, and some of your fat cells may have been storing this water too. How long did this 10-20 lbs weight gain last? It's not abnormal to gain 10-20 lbs after losing such a substantial amount of weight because some of that weight lost will have been water-weight.

The weight gain lasted from when I was 170 pounds until I was 305. I had to stop running when I hit 240 or so.

At 305, I was still 30 pounds under my max...but something tells me it wasn't water weight.

no it wasn't water weight, it was the net result of eating approximately 470,000 calories beyond what you burned

Uh, yeah. That's somewhat obvious. How else would I have gained the weight? Magic?

Now comes the million dollar question: Why was I so hungry that I ate 470,000 extra calories? Could carbohydrate driving insulin driving fat explain it?

Homeostasis in the human body doesn't just fail without a reason.

I was stating the conclusion of the theorem I read you as criticizing. If you set a diet plan at say 3000-3300 calories per day, you can just choose not to deviate from it? There's all manner of things I would like to do that I just choose not to do. You just set out that amount of food, say to yourself OK this has all the nutrients I need and no matter how hungry I get, it's not going to kill me... and then you just don't deviate. The real question is why did you feel your hunger was so severe you could not suppress it? How about a lifetime of bad habits caused a psychological reversion to previous bad habits and it was nothing but eating hundreds of calories extra per day? It is possible that for some people, some kind of a strange mechanism is at work, but occam's razor tells me that it is more likely just a series of small bad choices and that the standard model for human nutrition applies. I don't know you, maybe you are the corner case where the model breaks down because hunger signals are amped up or something? In my experiences of my own life, negative results are much more likely to be a result of a series of my small bad choices than on account of me being a corner case. I just have studied human nutrition, I believe the macronutrient model (fiber+protein+carbs) + micronutrient model works and that proteins and fats are both broken down to glucose at particular rates, with particular byproducts. I believe carbs breakdown to glucose "better" than protein or fats with less side effects due to the structure of the molecules (particularly noting the ammonia [I think it is] byproduct of chemical reactions breaking down protein into glucose. I think fat is just about having excess glucose in the blood and carbs just offer efficient means of getting sugar. Look, I completely believe the average diet is hugely overcarbed - once I started watching nutrition most of my intake is veg.