Here's a pretty good diagram of the Krebs cycle...
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The human body is truly an amazing thing, in the way that it can process carbs, protein (amino acids) or fat to create ATP (energy used by the muscles, as well as to repair damage to said muscles). It is the body's multi-path, multi-source insurance that it can continue to function, regardless of what diet may be available (think pre-historic times, when the menu was quite varied, and the body had to adapt to what was available). Essentially, the body has an "easy" path for energy (carbs), and not so easy path (protein), and a really energy intensive (hard) path (the break down of stored body fat) to achieve what it needs.
Being insulin sensitive (your heat furnace feeling) is a (good) sign that (1.) your body is kicking out enough IGF's (insulin Growth Factor) to support recovery and (2.) you're efficiently processing food sources into glucose. You probably also notice a slight visible vascular swelling (similar to a "pump" you get when lifting weights) in your arms when you're insulin sensitive (say, after you eat a really carb heavy meal, like mashed potatoes or some type of pasta). This is a good thing, as it means you system is working properly, primed to break down food sources, and that you are getting a good balance of the three key elements in your diet.
I notice the veins in my arms get very pronounced after carb loading when I'm insulin sensitive (usually when I'm cutting, and limiting fat and carb intake). You may also notice a minor sense of euphoria. This is evolution talking, and is the result of the release of serotonin as a result of eating carbs in an insulin sensitive state. When caveman had a excess amount of energy ingested (especially after being limited/starved), the brain evolved to give that sense of satiety a reinforcement of "happiness" as a reward for doing well in the food gathering department. Hence why after a good meal, 1.) we feel "good" and 2.) why we want to sleep after eating (though when insulin sensitive, drowsiness is usually replaced with alertness and euphoria (or a "sense of well being"). If you're consistently getting drowsy after a carb heavy meal, you're probably eating slightly too many carbs. The drowsiness is an evolutionary reaction, to sleep and store the excess energy as bodyfat.