The formula is Food Required for Pop N+1 (where N is your current pop) = 15 + 8*(N-1) + (N-1)^1.5 (rounded down to the nearest whole integer).
So, for N=1 (food required to get to N+1 pop (2 pop)), it's just 15 food. For N=2 (to get to 3 pop), it's 24 food. For N=3 (for 4 pop), it's 33 food. For N=4 (for 5 pop), it's 44 food. Etc.
Don't have immediate access to a spreadsheet program, or I would post a more complete listing. Perhaps an enterprising forum member might take care of that....
As an aside, vanilla originally provided for a "6" in the second element of the formula, but the exponent in the last piece was "1.8", which generally made for lower food requirements early in the game and higher requirements later in the game.