I recall reading that much of the energy a cat produces from its food comes from proteins which I assume would produce energy via being catabolised into amino acids which in turn, if glucogenic, would undergo gluconeogenesis and then, by means of being metabolised into glucose, enter glycolysis and the resulting pyruvate would be converted to acetyl-CoA and enter the TCA cycle to produce energy. I'm wondering whether this recollection is accurate and whether cats could, like humans, use ketone bodies for their sole fuel source if need be.


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You are correct that cats consume a lot of protein, and it's safe to assume a large share of this is broken down to amino acids, which are then oxidized to produce energy. But amino acids in general are not catabolized via glucose; instead, they are converted into compounds that feed into the TCA cycle via other biochemical routes.

Almost every of the 20 proteinogenic amino acids have their own specialized catabolism pathway, so this is a large topic. The general theme is that the amine nitrogen is first removed by a transaminase to yield a keto-acid, which is when converted into a carbohydrate that can enter the TCA cycle. To give but one example, leucine is converted in the mitochondrion by a series of CoA-coupled intermediates into acetyl-CoA and acetoacetate (a ketone body; see below). The acetoacetate in turn yields two more acetyl-CoA. A description of this pathway is found at the HumanCyc database. This is a great resource for learning amino acid (and other) biochemical pathways, and provides literature references.

Some amino acids are indeed used for gluconeogenesis, mainly by the liver, but this is not a requirement for their catabolism by most tissues. (It would be energetically inefficient to first generate glucose and then break it down again!) An exception is the central nervous system, which does require glucose and cannot subsist on fat or amino acids (for reasons that, to my knowledge, are still not well understood). So in order to support the CNS on amino acids, the route your describe via gluconeogenesis does occur.

Ketone bodies (acetone, acetoacetate and beta-hydroxybutyrate) are the main alternatives to glucose accepted by the CNS. Gluconeogenesis is energetically expensive in the long run, and if deprived of dietary sugar for a long time, the body will increase its usage of ketone bodies instead of glucose. However, ketone bodies can never fully replace glucose: even during prolonged starvation in humans, blood glucose is kept > 2mM. The CNS would not function otherwise. I'm not an expert in cat physiology but I would bet the situation is similar.


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