OK If we line up 1000 humans and ask them what they can physically do, you will get a massive difference. some will have the capacity for 100 pushups, some will be able do none, some will be able to do sub 4 minute kilometers, some won't be able to run for more than a kilometer. Some will look like Mr Burns, others will look like Adonis.
So if we line up 1000 humans and ask them what they can metabolically do. Nearly everybody assumes that they will be all the same. My belief is that their metabolic differences will be alot like peoples physical differences. Some will be able to mobilise stored fat with ease, others won't. Some will be able to convert all the glucose their body needs, others won't. Some will be able to regulate their blood levels of various energy sources with ease, some won't.
I could go on, basically our metabolism is made up of "tools" that help our bodies deal with what ever we might find convenient to put in our mouths at a particular time, as grok did for over a million years. Just like our bicep muscle is a "tool" our bodies use to pull its head/body above a bar or branch.
Ok so lets look at CICO with that in mind. The CICO equation stands unmovable especially in regards to the first law of thermodynamics. So lets ask someone who's "tool" of mobilizing fat is fairly poor (this would typically appear in an obese person) to eat with a calorie deficit for a week. In that week the persons body will automatically adjust and burn less energy, as it can't mobilize its energy stores, it must do this to balance the irrefutable thermodynamic equation of CICO. The result of this will typically be minimal to none fat lost and a noticeable level of lethargy and laziness, at this stage the dieter gives up and comes on the MDA forums to whine.
If we gave this task to chocotaco, we would probably find his body kick in with its well used, functioning, fat mobilizing metabolic tool and he would lose the fat easily. hence we also find him on the forums pushing the CICO barrow.
So there it is my belief that our bodies will use what ever metabolic "tool" or strategy it finds easiest to balance the CICO equation. Our bodies mightn't necessarily use the tool we want it to use, or balance the side of the equation we wish. This is where CICO fails as a dieting strategy.
What is a good strategy then? I hear you ask. Well one that trains our metabolic tools, like we train our physical tools, see the thread in my signature if your interested.