When I lived in downtown Charlotte, NC, there was this great breakfast and lunch restaurant located directly beneath my apartment. Now it’s a food truck, but that’s neither here nor there.

"No, we don't serve tacos. Why does everyone keep asking that?"
It was one of those deals where you write what kind of sandwich and toppings you want on a brown bag with a sharpie and hand it to the guy behind the counter.
They had quality ingredients, and the owner was a great dude, so I’d pop in there a few times a week and always fire up a couple of breakfast sandwiches (salmon/cream cheese/eggs and pulled pork were my standbys), in a bowl without the bread.
One particular morning, a fairly young and extremely skinny guy, probably enrolled in one of the nearby colleges, looked at my bread-less order and said something along the lines of, “Dude, you know carbs are the body’s main source of energy, right? Why would you get this without bread?”
About 25 potential responses ran laps around my brain, ranging from jovially comedic to aggressively berating. I settled on something neutral, possibly even empathetic, which is super hard for me, by saying “Yeah man, that’s what they taught me in college too. But your body only runs on carbs if that’s what you train it to do”.
Although he was completely out of line by basically telling me how I should eat, he wasn’t entirely incorrect. The human body does run off of carbs as its main energy source…when carbs are continuously supplied. Most people don’t realize that once you restrict this supply, and adequately replace it with fats, the body does an amazing job of using fats and ketones instead.
Why don’t more people get this, why isn’t this concept adequately addressed in undergrad nutrition education, why do people “feel like crap” when they cut carbs? The answers to these questions could all be long articles individually, and I want to keep this as a semi-brief primer for people new to the concept, so I will try to answer these questions and more while embracing brevity to the best of my ability. Once we get to parts II & III, I'll be including links to some other websites that have already done a beautiful job of describing fat-adaption and keto-adaption.