Why do some foods have greater caloric content per unit mass?

Fat has more calories per gram than protein has. Fat can be effortlessly hide always as a deposit in body while protein can't . Our body does not have a protein stockpiling station like fat cells are for ingested fat. In this way, our body needs to separate protein into amino acids - the building squares of proteins - and revamp proteins when our body needs them.

I’d say a : water molecules

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