Our normal healthy cells do prefer glucose as their primary energy source, obtained from the sugar circulating in the blood. That “blood sugar” comes from a variety of sources, including dietary carbohydrates occurring in fruits and starchy vegetables like potatoes, and grains. The complex carbohydrates in such foods are broken down into glucose during the digestive process, catalyzed by a variety of carb-specific enzymes like amylase.

We also maintain a certain amount of stored sugar as glycogen, found in the liver and muscle and formed when glucose molecules link up to one another in complex chains. In times of need, and if deprived of dietary carbohydrates, our liver and muscle cells can break down glycogen into glucose for release into the blood stream. Our liver cells can also, when necessary, convert certain amino acids such as alanine into glucose.

However, our glycogen supplies in the liver and muscle are quite limited, providing only an 8-12 hour emergency supply. So during a fast, or starvation, or on a diet providing no carbohydrates in any form, we quickly run out of glycogen. In this situation, through a variety of neural and hormonal signaling, our fat cells, or adipocytes, begin releasing free fatty acids into the blood stream. These fatty acids can in turn be used by our cells in the alternate energy pathway to produce the energy molecule Adenosine Triphosphate (ATP) in the process of beta oxidation. The end result of this series of reactions, acetyl coenzyme A, can then be shunted into the citric acid cycle and the electron transport chain, to produce maximum amounts of energy-rich ATP.

Though most of our cells can utilize fatty acids of all types, via beta oxidation to create ATP energy, our central nervous system is at somewhat of a disadvantage. In fact, long chain fatty acids with 14 or more carbons, which can yield the most ATP from beta oxidation, do not cross the blood-brain barrier. However, in a state of prolonged dietary carbohydrate depletion, the liver begins converting acetyl coenzyme A into various ketone bodies, such as acetoacetate and beta hydroxy butyric acid, which easily penetrate into the brain and which can, like acetyl coenzyme A, be shunted into the citric acid cycle and then the electron transport chain, providing the brain with ATP. (See – Chart on next page).

On a low carb or a nearly no carb diet, the billions of cells in all our tissues and organs switch their energy mechanics from a process driven by glucose to one propelled by fatty acids and ketone bodies. The term “ketosis” simply means the state in which, in the absence of sufficient glucose, our liver synthesizes ketones.

Nutritional ketosis, which occurs with carbohydrate restriction and is further enhanced with calorie restriction, forces the physiological shift from a glucose-based metabolism to a fatty acid and ketone metabolism. When your body is, shall we say, keto-adapted, your brain energy metabolism is more stable and your mood is more stable. It may take a few weeks to adapt physiologically to this restriction. You don’t need ketones in urine, only in blood. Ketones in urine mean that the brain and the body have no more need for such excess ketones.

Also proteins need to be moderated. Protein is gluconeogenic. There are gluconeogenic amino acids in protein that can be converted into blood sugar. If protein is at, say, for example, two or three grams per kilogram per day that is probably going to feed in through the gluconeogenic pathway and contribute to glutaminolysis. It will then be hard to deplete your glycogen stores, which is necessary to drive the ketogenesis in your liver. Excess protein is hence anti­ketogenic.

So to summarize, in order to maintain and sustain nutritional ketosis, you need to decrease both carbohydrates and protein. But how much protein is enough, or too much? (See – ‘Food – Fat – Energy Cycle’).

However, even on a nearly no carb, all meat, high-fat diet, we will still be consuming some glucose in the form of glycogen stored in muscle and organ meats, and our livers will continue to convert some dietary amino acids into glucose, so blood sugar levels never hit zero on such a diet. But in such cases, the amounts produced will be minimal.

Ketones are produced if you eat very few carbs say only about 5 percent of your total daily calories (that are quickly broken down into blood sugar) and only moderate amounts of protein about 5 to 10 percent of your daily dietary calories and the rest all from fat.

Ketones are produced in the liver, from fat. They are then used as fuel throughout the body, including the brain. The brain is a hungry organ that consumes lots of energy every day, and it can’t run on fat directly. It can only run on glucose or ketones.

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