If you're burning through all of your carbs, then from that point on you are still burning fat even if you're not in a caloric deficit. You're just burning dietary fats rather than body fat. And in that case your body still stores the excess calories as fat on the body. Calories in, calories out still matters, even if you are a certified "fat burning machine" to quote Mark.
The body is constantly burning carbs, yes. The key is to feed it only what it needs based on your relative activity level so that the bulk of the calories you are burning would still be fat. The problem a lot of people run into with becoming "fat adapted" is either they are eating too many carbs and then complain why they aren't adapted yet, but their activity levels don't support the amount of carbs they are eating, or conversely you get the people who go TOO low on carbs that they feel like shit. Different strokes for different folks. You can make a full on ketogenic diet work great for you, or you can adapt to your activity level. And frankly, most people seem to be doing the latter these days. This is fine, but experimenting to find your appropriate level of carbs is essential. You can't just pick some arbitrary number. It takes work and fine-tuning to find out what level you feel best at.
Your body can never truly 100% deplete your glycogen. It does need to preserve a little for emergency situations (think if you had to escape from a predator in the wild), but it can become low enough that the body prefers to use fat EXCEPT in case of emergency. I think the body is smart enough to realize when glycogen is that low and knows that it should be preserved except in dire need. And if you're doing your best to stay healthy, live a stress-free life, and get plenty of sleep, then you hopefully aren't having to dip into these "emergency" stores of glycogen too often and are hopefully maintaining a largely fat-burning state. The body says basically "I am in no need of glycogen right now and it is very low so I'll use fat instead, to preserve as much of that glycogen as I can."
And so what if there is no such thing as being a fat burning beast? Is your body always going to be getting the same amount of fuel from glucose and fat no matter what the macro ratio, it will just get it via different pathways?
If we accept the premise that the body recognizes a low-carb state and immediately uses it to produce its own carbs then consider this: The Daily Lipid: We Really Can Make Glucose From Fatty Acids After All! O Textbook, How Thy Biochemistry Hast Deceived Me!
If the body really CAN turn fat into glucose for fuel, then it might slip into that mode and not necessarily go about catabolizing protein. Then, I'd say that this still counts as a state of fat burning. You're still eliminating excess fat from your body and using it as your primary driver of metabolic processes.
However, even if this is not the case, I'd think that, again, the body being smart as it is, knows the difference in caloric density of the various macronutrients. So, we have, "glycogen is low, must replenish or conserve" and the body says "I can have 4 calories of energy per gram of protein converted to glucose or 7.5 calories per gram of fat burned" (a few calories are lost from fat in the burning process). Still, 4 < 7.5. The body knows that it is more efficient to utilize fat than protein and saves protein catabolism for only the most dire of circumstances. Our body is smart. It knows how to regulate itself in the most efficient matters if we just let it and stop mucking with the controls all the time.
Low carb / ketotenic diets are show time and time again to be beneficial to the body...instead of trying to bait and troll people here do some basic research first please.
Eating primal is not a diet, it is a way of life.
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