We learn early on about the calories and exercise equation. Calories in, energy out, but if calories > energy out, we gain weight. Simple. Too simple.
If one takes up diet as a conscious topic, versus simply rooting around, eating whatever's available, then it pays to mentally construct a more elaborate picture of what's going on. Why? Because monitoring diet is a way of self monitoring health, which in turn is one's golden door into medical knowledge. Why pass that up? It'd be like a form of self starvation: too little protein, in the form of mentally challenging content.
It won't be news to many readers that the model evolved to be more like a Prius, meaning the popular hybrid gasoline + battery powered car. Of course a battery is not ultimately a source of power; it all comes from gas (food, fuel). However gas and electricity may be used in combination. Actually, braking (deceleration) may power a battery also, not just fuel.
The two "power chains" we're talking about are the carbs 'n glucose cycle, versus the ketone cycle. In both cases, the goal is to make ATP to keep cells healthy and active, in muscles, in the brain. In tissues generally. The usual fuel (more like gasoline) is glucose. Glucose powers the Krebs Cycle which generates a lot of essential byproducts, including ATP.
But if glucose is less readily available, the body has a secondary fuel system, more like the battery system. Actually, it has two secondary systems: one based on burning fat directly, and one based on creating ketone bodies, molecules that may be bussed from the liver through the bloodstream, to target tissues, which have their own ways to reverse the chemistry to get a Krebs cycle.
What about this Krebs Cycle? That's where we find bioengineering (the naturally occurring kind) at its finest. We've even learned it may run in reverse, especially when oxygen is more iffy.
Learning about ketones and ketosis has inspired a lot of folks to take up intermittent fasting, including me. I talk about the "ketone briquettes" that get thrown on my "brain fire" to make a slightly different color of flame. Metabolizing ketones is at some level the opposite of being "fat and happy" i.e. secure and comfortable with one's metabolic surroundings. The brain gets that nutrition might be an issue and is stimulated to think harder, about where a next meal might be coming from.
Optimally, no enforced starvation is going on and one is intellectually satisfied that finding a next meal is not the issue. Nevertheless, the heightened mental acuity may be used to address other less literal forms of hunger. Fasting begets flashes of genius is the theory. I've been trying it out occasionally, whereas I'm mostly content to stay at the subgenius level.
