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Full bowls make empty brains.
Full bellies make empty skulls.
Full guts neither run away, nor fight well.
A full belly neither fights, nor flies well.
Source: George Latimer Apperson, Wordsworth Dictionary of Proverbs

Fat paunches have lean pates

The mind shall banquet, though the body pine:
Fat paunches have lean pates, and dainty bits
Make rich the ribs, but bankrupt quite the wits. (Lines 25-27)