Chandogya Upanishad
Prapathaka VI, Khanda 5
'The earth (food) when eaten becomes threefold; its grossest portion becomes feces, its middle portion flesh, its subtilest portion mind.
'Water when drunk becomes threefold; its grossest portion becomes water, its middle portion blood, its subtilest portion breath.
'Fire (i. e. in oil, butter, &c.) when eaten becomes threefold; its grossest portion becomes bone, its middle portion marrow, its subtilest portion speech .