While in ketosis, the body utilizes fatty acids as an alternative source of energy due to the glucose deficit. To accomplish this, acetyl CoA, which is normally oxidized into H2O and CO2 as part of the citric acid cycle, is converted by the liver into acetoacetate and 3-hydroxybutyrate, also known as ketone bodies. These ketone bodies are now free to flow where needed and be used as fuel for metabolic processes. It is worthwhile to note that ketone bodies are very effective respiratory fuels; whereas 100 g of glucose generates 8.7 kg of ATP, 100 g of 3-hydroxybutyrate can yield 10.5 kg of ATP, and 100 g of acetoacetate 9.4 kg of ATP. Most areas of the body, such as the brain, will use ketones whenever provided to them (in fact the blood-brain barrier has a very effective transporter for ketone bodies). However, there are still some processes that are partial to glucose for energy metabolism. In these cases, glucose is supplied by hepatic gluconegenesis, where the liver converts non-carbohydrate sources (such as fatty acids and amino acids) into glucose.



Correlation between breath acetone and blood beta-hydroxybutyrate.

A common misunderstanding is that breath acetone directly correlates to the blood test ketone beta-hydroxybutyrate. It is easy to make that assumption, and in some contexts it appears to be correlating.

Fatty acids break down in liver to acetoacetate. Acetone is spontaneously released from the AcetoAcetate. Excess AcetoAcetate is stored in blood as beta-hydroxybutyrate. 

The concentration of Beta-hydroxybutyrate is a product of the ketosis and time.

The concentration of Acetone in breath is indicating the strength of ketosis.

The Acetone in breath is correlating well to your blood glucose. So in the case of fasting, your breath Acetone will more or less be constant (high burning fat) until your glycogen stores are depleted. Then it will raise even higher to produce more AcetoAcetate.

The Beta-hydroxybutyrate does not correlate to your glucose as the breath Acetone. Beta-hydroxybutyrate indicates the product of your ketosis strength and time.

Being in high ketosis (high breath acetone) will over time build a higher concentration of beta-hydroxybutyrate.


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