The efficacy of metabolism is in my opinion very much the big question mark here, and why I think relying on one-size-fits-all concepts like "a carbohydrate translates to 4 kilocalories" is simplistic and in all likelihood flawed.
I do not doubt this, but is there any evidence that these variances are significant? Treating human bodies as 100% efficient food to energy converters is useful for practical reasoning about weight loss; unless these factors are significant, focusing on macronutrients, genetics, or the "hormonal situation" is just getting lost in the weeds.
It just has to do with the way the body deals with sugar. Sugar produced from the digestive process goes straight to your bloodstream as glucose if your body needs it, but if you already have enough ... it is sent to your liver where it is converted to glycogen. The rub is, your liver can only store so much glycogen (about 100mg of it I think), after that ... any sugar that hits your liver is converted to straight fat.
First, we do know that simple sugars lead to insulin spikes, that's measurable and repeatable. All carbohydrates (save cellulose) are processed into the monosaccharide glucose at some point.
And the finite amount of glycogen the liver can store probably also could use some attribution or citation. I haven't heard of that and I'm always very leery about these hard-and-fast #s in terms of measurements that kick off some process that we should expect in all humans.
If you were thinking of substituting fructose for glucose in your diet, to get a lower insulin response, reconsider.
> Even protein spikes insulin
Can you explain this mechanism? From my research, this is a demand-driven process via gluconeogenesis. The effect on blood sugar is so minimal here as to suggest that insulin secretion is nominal at best.
Dietary fat does not have any effect on insulin, btw.
This is why nutritional ketosis is driven primarily by hepatic glyocgen depletion.
Many cyclical ketogenic diets use this hack to replete glycogen in their muscle tissue (in the form of a once per week carb refeed) in order to fuel glycolytic workouts during the week. While it's hard to completely avoid filling your hepatic glycogen stores during the refeed, they can be burned through pretty quickly by the brain in the course of a day or two provided you restrict dietary carbohydrate to < 20g/day. It's also worth noting that glycogen repletion is generally preferential to muscle tissue first, then the liver second.
Not to get into a pissing match, but I think "well proven" needs some citation here.
The role of insulin does not bypass thermodynamics.