We have five hormones to raise our blood sugar and only one to lower it. Just this one simple fact tells me that glucose (carbohydrate) was never meant to be our primary source of fuel. We also have very limited storage capacity for glucose. We can only store 100 grams (400 calories) in our livers and 350 grams (1400 calories) in our muscles. Our other fuel source fat however is found in large quantities in our fat cells. Even the leanest amongst us stores at least 40,000 calories worth of fuel in the form of fat. This tells me that we should be burning primarily fat as our fuel source and preserving glucose for the few cells that can only run on glucose. The short answer as to how does one become a fat burner is to make glucose the scarce fuel supply that it is meant to be. Leave glucose for the few cells that can only use glucose as a fuel source. This will force the rest of the body to use fat as its fuel source. This will leave your glucose supplies to maintain glucose homeostasis. What is glucose homeostasis?
Your body is required to maintain a certain level of blood sugar 70-110. It is required to maintain this level of blood sugar so the few cells that can only use glucose as their fuel source will always have glucose available to them. This is glucose homeostasis. The cells that can only use glucose are found in the brain, the red blood cells, the retinas, and a few in the kidneys. Your brain can also use ketone bodies as a fuel source but these ketone bodies need to be made in the liver from your stored body fat. The only way this will happen is if insulin is kept low enough to activate the enzyme hormone sensitive-lipase (HSL). This really is the key part of keto or fat adaptation. If you can furnish part of your brains energy needs with ketones this will spare glucose even more. In other words you will need a lot less glucose in reserve if your brain can run on ketones. Our brains use 20-25% of our energy, 500-600 calories per day.
Our liver is the only storage tank that can supply glucose to the brain. The liver only stores 400 calories worth of glucose which is not even enough fuel to last the brain one day. The 1400 calories of glucose stored in your muscles are reserved for those specific muscles. They lack a certain enzyme needed to convert the stored glycogen to glucose. Since your brain is the master control center of your body it must be kept running at all times. When you are a glucose burner this keeps your brain dependent on glucose. And this keeps you locked into glucose and carbohydrate as a fuel source. And of course eating this way keeps insulin levels high which denies you access to your stored body fat. Your liver cannot make ketone bodies for the brain because of high insulin. High insulin stops the enzyme HSL from being turned on.
The only way you will become truly keto-adapted is if you can make glucose a scarce fuel supply. You must stop eating carbohydrate and if you are insulin resistant you will have to limit your protein somewhat. Protein was never meant to be used as a fuel source. Our bodies need protein for to many other things than to be wasting it as a fuel source. The only way you will convert to using fat as your primary fuel source is if you eat high fat. If you limit your glucose intake as much as you possibly can and convert your brain to running mostly on ketones. Then you will need very little glucose in reserve. The small amount of glucose needed to maintain glucose homeostasis for the few cells that can only run on glucose your liver can easily make with the by-products of fat metabolism. When glucose is in low supply your cells and tissues that can burn fat will burn fat and preserve the little bit of glucose for those cells that can only run on glucose. This will also keep insulin levels low so your fat stores can be released into the bloodstream to be used for energy. If you are insulin resistant and over eat protein this will keep you from becoming fully keto-adapted. When you are insulin resistant or type 2 diabetic your liver will make more glucose with the excess protein. You want to make glucose as scarce as you possibly can. Once you have been following a ketogenic diet for 6 months or longer your liver will store less glucose it will go from 100-50 grams of storage. It does this because you now need a lot less glucose in reserve. Your brain is running primarily on ketones. Also, while on a ketogenic diet if you exercise in a specific way you will replace some of the stored glucose reserves in your muscles with fat droplets. Your body knows what it is doing you just need to give it the right instructions. The macro nutrients needed to give your body the right instructions are fat and protein in the right amounts for you.