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A ketone is a specific type of chemical recognised by a carbonyl group (with a double bond between a carbon and an oxygen) in the middle. Our logo is the chemical picture of a carbonyl group. The molecular groups attached to the two carbon atoms at the bottom (the “legs” of the logo) determine the type of ketone it is.
The human body produces three chemicals when in ketosis. Two of them, acteone and acetoacetate, are actual chemical ketones. The third is called β-hydroxybutyrate, and it’s not actually a ketone, but it’s produced by the same processes. So you will often hear or read the phrase “ketone bodies” in recognition of that fact. But people sometimes just call all three chemicals “ketones” for the sake of simplicity.
What is a ketone body? It is made during the process of metabolising a fatty acid. My favourite metaphor is that a ketone is to a fatty acid as charcoal is to wood. They are both partially metabolised/burnt versions of the original item (fat/wood). Most cells, when they metabolise a fatty acid, continue all the way to the end products of CO2 and H2O. However, when we are in ketosis, the liver makes ketone bodies to fuel cells that can use them.
Ketone bodies have many uses throughout the body. They can serve as alternative fuels to glucose or fatty acids, and β-hydroxybutyrate has a role in healing damaged intestinal walls, because it helps restore the tight junctions that are loosened when someone has leaky gut syndrome. The three ketone bodies also have powerful hormone-like effects throughout the body. Acetone has an important role in brain function, and β-hydroxybutyrate, among other things, affects the genes that control our endogenous (built-in) defences against oxidative stress.
We measure ketones primarily as a way of gauging the effect of our low-carbohydrate, high-fat, ketogenic diet. The point of the diet is actually to lower serum insulin levels below a certain threshold by restricting carbohydrate intake, and we know that insulin is low enough when the liver starts producing detectable amounts of ketone bodies. We cannot measure insulin at home, but there are simple home tests for the three ketone bodies. The ketones circulate throughout the blood stream, and they are excreted in breath and urine.
It is easiest to measure acetone in the breath, acetoacetate in the urine, and β-hydroxybutyrate in the blood, but all three ketone bodies are found in all three locations. The definition of nutritional ketosis is a blood level of β-hydroxybutyrate of 0.5 mmol/dL or higher. Fasting levels of ketones tend to be higher, starting around 4.0 mmol/dL and going up to around 7 or 8. Doctors start worrying about diabetic ketoacidosis when β-hydroxybutyrate reaches 10.0 mmol/dL, but symptoms don’t manifest until around 20.0.
When we first embark on a low-carbohydrate ketogenic diet, the liver produces ketones in abundance. The skeletal muscles have to go through a period of adaptation, in which they are busy reactivating certain metabolic pathways and in healing the mitochondria that were damaged by too much glucose (carbohydrate). Skeletal muscles prefer fatty acids, but until they readapt to metabolising them, they have to limp along on ketone bodies. That is why ketones are so easily detectable at the beginning.
As the muscles become fat-adapted or keto-adapted, as we call it, the liver starts matching ketone production more closely to demand, and people often (though not always) see a drop in excreted ketone levels. Even serum β-hydroxybutyrate tends to drop a little, as once the muscles are metabolising fats, there is not such a great need. Some organs prefer ketones, particularly the brain, since fatty acids cannot cross the blood-brain barrier. If ketones are available, the brain will use them in preference to glucose. Some cells, however, require glucose. The big example is our red blood corpuscles; they lack mitochondria, so they cannot metabolise fats or ketone bodies.
All right, that seems like a long enough biology lecture for now. I hope it helped. If I missed something, or you have any further questions, feel free to post them.