Some time ago, I was reading the book Metabolic regulation: A Human Perspective by Keith N. Frayn (3rd edition) and in chapter 2.1 and 2.2 I came to a very unusual section that made me jump. It states the following:
“…fatty acids are usually a preferred fuel (over glucose) for skeletal muscle… Fatty acid release is very effectively switched off by insulin, so muscles no longer have the option of using fatty acids… The brain, in contrast, has a pathway for utilizing glucose at a rate that is relatively constant whatever the utilizing glucose concentration, a very reasonable adaptation since we would not want to be super-intelligent only after eating carbohydrate, and intellectually challenged between meals.”
This was revolutionary to me. Skeletal muscles actually prefer to use fat rather than glucose? We have a very carbohydrate-centric view of nutrition, yet our muscles and some other organs too, may prefer fat to glucose as fuel, and also the brain has no problems switching to ketones.
Ketones are not Backup & Glucose is not Primary
Few professionals, even within the field of nutrition, realize that ketones are not our backup fuel. Ketones are not any more backup than glucose is the primary fuel. The human body has no primary or backup fuel: it has two fuels. I presume that the urgency for glucose use — meaning the urgent removal of glucose from the blood — is misleading and makes people think that if the body switches to using glucose the moment it is provided, it must be its primary fuel. But this is a mistaken argument, which assumes that the first task that has to be done is the preferred task. It is true that when we add glucose to our blood by, for example, eating carbohydrates, the body must immediately switch to remove that glucose from the blood.
The maximum comfort level for glucose in the blood is 99 mg/dL (5.5 mmol/L) and if we eat carbohydrates such that this is exceeded, it is literally an emergency and the body must remove it from there. Too much glucose in the blood for extended time is toxic. However, having to remove glucose immediately does not make it into the primary fuel, it just makes it into an urgent task.
As an example, assume you need to go to the dentist because you have a huge toothache. While…