The literature doesn't seem to be clear on that question, or maybe it is and I'm simply failing to find the truth.
Officially, all protein (animal, vegetable, etc) is assumed in labelling to be 100% converted to energy at a rate of 4 calories per gram. Most amino acids are converted to glucose, and some are converted to ketones.
In practice, the rate is sometimes as low as 2 calories per gram, and it will often be 0 calories per gram if the amino acids are instead used as building blocks rather than converted to fuel, which obviously happens in nature. The fuel effect might be metabolically neutral for body composition, as some studies suggest that energy from protein goes directly into higher metabolism / body heat, and thus doesn't get converted into fat.
It is interesting how babies are very efficient at using protein. Breast milk is around 7% protein, that means babies get 10 grams of protein a day about. With that, they put on 10 lbs of mostly lean tissue of all types (bone, muscle, brain, skin, organs, etc) within 6 months.
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