Starvation ketosis and "priority" of brain fuels
"Priority" is a funny concept the fields of nutrition, metabolism, etc. For the brain, it is said to be glucose. It'll use ketones when glucose is low and ketones are really high, like during starvation, but otherwise it's just glucose. Why is this? One of my mentors had some great insights...

"It seems that the loss of some energy as ketones in the urine is the price we pay to provide the brain with a suitable fuel. But there are a number of unanswered questions about brain fuel use. Why does the brain not use free fatty acids? The usual answer given is that they are not transported across the blood-brain barrier fast enough to be used as a major fuel and this is probably true. However, why did the brain not develop a suitable transport system, or localized store of glycogen for that matter."


Textbook: Biochemical, Physiological, and Molecular Aspects of Human Nutrition


"In other words, why does the brain rely so heavily on plasma glucose.  The glycogen part is perhaps easiest to answer, a store of glycogen with its attendant water would not fit into the cranium very easily and thus we would lose our protective brain cover, but why not use fatty acids?

Ultimately, there will not be enough fuel left and blood glucose levels will not be maintained.  The brain will not receive sufficient nutrients and coma will occur, followed by death. In practice, complete starvation is seldom the direct cause of death – either micronutrient deficiency will lead to problems and death or the failure to mount a sound immune response results in the subject succumbing to disease (usually pneumonia) and the inability (lack of muscle power) to clear fluid from the lungs."

"Brosnan has proposed a rather interesting idea, that the brain needs to get all of the energy it can from a fuel, r re precisely from a given amount of oxygen. Blood supply to the brain is relatively constant. It certainly doesn't change dramatically as evidence that there is any change with different substrate supply (ie, ketosis). this means that the amount of oxygen available to the brain is constant. We are used to looking at the ATP yield per fuel, ie, 38 ATP from glucose, 129 ATP from palmitate. But Brosnan turned the question around and asked how much ATP do I get per oxygen if I burn glucose, fatty acids, or ketones?"

"Glucose will yield 3.1 ATP per oxygen atom, fatty acids will yield 2.8, acetoacetate 2.9, and beta-hydroxybutyrate 3.1. This means that given a constant amount of oxygen consumption, the brain will get 11% more ATP per oxygen if it burns glucose over fatty acids and about 7% over ketones. Perhaps, this is the difference between brain functioning or not. When people fast for long periods, days, they often describe their thinking as 'different' and there are numerous reports of individuals who had 'visions' while fasting. Perhaps this is due to the brain running on ketones, ie, not getting enough energy and in fact not thinking clearly."

Running on ketones and fat would require more oxygen influx, and even if that were possible, this could induce oxidative stress. Maybe. In any case, COOL THEORY, BRO

That's all for now!

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