Fats are ketogenic (except odd chain FA), so they produce ketones for energy production (Acetyl-CoA) rather than glucose. If the question asked what the primary source of energy production was, it would still be glycogen (and not ketones), because this is within 24 hours. However after 24 hours the answer could be ketone bodies. Regardless, the question specifically said the pt had a serum glucose of 100, indicating that we are looking for something that provides a substrate for gluconeogenesis.
During periods of starvation, substrates for gluconeogenesis come from two sources: (1) breakdown of existing muscle, or (2) via odd-chain FA through propionyl-CoA. (*Valine also feeds into propionyl CoA, but is not involved during starvation --> see below)
(1) The alanine-pyruvate cycle provides this (glutamine in muscle + pyruvate --> alanine --> goes to liver --> transamination to alpha-ketoglutorate --> pyruvate is separated from glutamine --> glutamine goes to urea cycle, pyruvate goes on to gluconeogenesis). Lactate can also be used (this could have been a right answer if it were listed).
(2) Odd chain FAs are also glucogenic, but stearic acid (provided in the answer choice) isn’t odd chain, so it is only ketogenic and can be ruled out.
Although valine (and other branched a.a.) feed into Propionyl-CoA, they are not used in starvation because starvation strictly relies on hepatic gluconeogenesis. These a.a. are not metabolized in the liver because the liver lacks branched-chain a.a. transferase enzyme. In First Aid, Biochem section, under Fasting/Starvation, in both the “fasting state” (which is within the time frame of this question), or the “starvation state,” both utilize hepatic gluconeogenesis. My assumption is that valine is used during regular metabolism, and not during periods of starvation.
@frimmy_11 Why would protein break down after only 20 hours? Shouldn’t fat be the major contributor now? Also if protein is being used, then why isn’t valine the choice? It’s also glucogenic.
P. 91 of FA has a quick explanation for this!
Basically once you're in a starving state there's still hepatic gluconeogenesis going on (as well as using FFA) but the gluconeogenesis is coming from peripheral tissue lactate and alanine.
Muscle protein breaks down into the amino acid alanine which enters Cahill Cycle to form glucose ...