Feinman and Fine demonstrated that there was up to 27% energy loss in converting, as an example, amino acids to protein and back. (They note that higher inefficiencies have been found in other studies). They don’t mention Bunsen burners, but they go to the heart of this issue when they say “1g carbohydrate =4 kcal; 1g fat = 9kcal, these relations only apply to the reaction with oxygen...if an ingested macronutrient undergoes some other reaction in vivo, e.g. conversion from amino acid to carbohydrate, or multiple metabolic cycles before oxidation, then calories cannot be directly substituted
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