Your brain only represents 2% of your bodyweight, yet uses 20-25% of your total resting energy expenditure. Roughly 500 calories per day are needed just for the brain. If you have always eaten a high carb based diet your brain runs on 100% glucose (carbohydrates). And you only have a very small reservoir of glucose available to you. I will try and show you why your brain is really the key piece of the puzzle in keto-adaption. First I need to explain a few things. Our body`s cells can use 2 sources of fuel for energy, glucose (carbohydrate), or fat. There are a few cells that can only use glucose for energy. It is because of these cells, that the human body must maintain a certain level of glucose, at all times. This is called glucose homeostasis.
Glucose homeostasis simply means this. The human body is required to maintain tight control of your blood sugar levels 70-110. It is required to maintain this level of blood sugar for the few cells, that can only use glucose for energy. These cells are found in the brain (if not keto-adapted), the red blood cells, a few in the kidneys, and your retinas. Also if your blood sugar goes much over 110, this becomes toxic to your cells, and if your blood sugar goes to low, you will not have the necessary glucose needed, to maintain fuel, to these cells. So this is glucose homeostasis. Why is this important? The brain, being the main control center of the body, must be keep going every second, of every day. The red blood cells carry the oxygen in the bloodstream, You must be able to breath, to remain alive. Your kidneys filter your blood, No more blood, and you are no longer alive, And I would say your eyesight is pretty important also.
Here are the amounts of glucose needed daily, for the cells that can only run on glucose. 1- The brain uses 130 grams of glucose daily 2- The red blood cells use 40 grams of glucose daily 3- The gonads of the kidneys, and your retinas, use roughly 30 grams a day 4- This is a total of 200 grams of carbohydrate needed daily to supply these cells that can only run on glucose. 5- And if you eat a high-carb diet, you are a glucose burner, and you will need a lot more carbohydrate to supply the remainder of your body`s cells , with their daily energy needs. 6- If you would eat a ketogenic diet, you would become a fat burner, all of your other cells could run on fat. And part of your brain could run on ketone bodies.
Your liver only stores, at the most, 100 grams of glucose, in the storage form of glycogen. The liver is the only storage depot of glucose available to the brain, and these other cells, that can only run on glucose. This amount of glucose is only enough for 12 hours supply for these cells that can only use glucose. Your muscles store 300-400 grams of glucose, but this glucose is reserved for these specific muscles. These stores of glycogen, in the muscles, lack a specific enzyme (glucose-6-phosphatase), needed to allow re-entry of the glucose back into the bloodstream. So this means that your body, to maintain glucose homeostasis, for these cells, can only use the glucose stores in the liver of 100 grams. This is a very, very small amount of glucose available to you, to keep these cells going. This is why, a glucose (sugar) burner has to eat so frequently. You have to be tied to that constant glucose drip, a slave to carbohydrate.
If you are still insulin sensitive, you still maintain your metabolic flexibility, and can easily switch from glucose to fat as needed. You maintain low insulin levels, so you can also draw on your fat reserves for energy. But, as the work of Dr. Joseph Kraft has shown us, up to 83% of us, has some form of insulin resistance. When we are insulin resistant, our cells have become resistant to insulin, and glucose can no longer enter our cells as easily. We also have higher circulating levels of insulin. This denies us access to our fat stores for energy. In this situation, we have to eat frequently, to keep the cells that can only use glucose going.
As I showed you earlier, our brains need 130 grams of glucose per day. We need at least 200 grams of glucose daily for the cells that can only use glucose. What if we could cut this amount in half? What if the brain could use ketone bodies for some of its energy needs. The brain can also run on ketone bodies, only problem is, the circumstances have to be just right, for this to happen.
When you are insulin resistant, and eat a high-carb diet, you will have high circulating levels of blood sugar, and high levels of insulin. These constant high insulin levels will keep your fat stores locked up in storage. Your brain will be dependent on glucose for 100% of its energy needs. Your liver can use fat from your fat cells to make ketone bodies, for your brain, but insulin must be kept low for this to happen. When insulin is low, this activates a specific enzyme called, hormone-sensitive lipase. This enzyme allows your fat stores to be mobilized, and be released into the bloodstream, your liver then takes this fat and makes ketone bodies with. Your brain cannot use free fatty acids, they can only use ketone bodies, for a fuel source.
The free fatty acids in the bloodstream are bound to plasma (blood) albumin. The brain has a protective barrier surrounding it. These fatty acids that are bound with albumin cannot cross the blood-brain barrier. Ketones can cross the blood barrier as they are not bound with albumin. All of your other cells, that can run on fat, can use these free fatty acids that are bound to albumin.
If you follow a ketogenic diet, and your insulin levels are kept low, your liver will start to make ketone bodies. After the adaption period of 2-4 weeks, once you have made the conversion from glucose burner to fat burner. Your liver is now making enough ketones to supply your brain with 50-70% of its energy needs. As your brain gets better at using ketones for fuel, after 3-6 months, your liver can make enough ketones to supply your brain with 80% of its energy needs. This would cut the glucose needs for the brain from 130 grams to 25 grams of glucose per day. This would cut your daily need of 200 grams of carbohydrate, for glucose homeostasis, down to 95 grams per day, you now have created a low glucose environment. A low glucose environment, also means a low insulin environment. With this small amount of glucose needed of 95 grams daily, the liver can easily make this glucose, through the process of gluconeogenesis. Gluconeogenesis, is when the liver takes protein amino acids and converts them to glucose. The liver can also take by-products of fat metabolism , for gluconeogenesis to make the necessary glucose. All of your other cells that can run on fat, will use free fatty acids. You will no longer have blood sugar spikes from high carbohydrate foods. Your blood sugar levels will stay in the normal range of 70-100. Your brain will have a constant energy supply, mainly from ketones. All of your other cells, that can run on fat, will use free fatty acids, and the remainder of your cells that can only run on glucose, your liver can easily supply, through gluconeogenesis. And you will be finally free of that constant glucose drip.