I'm not sure that it is very natural to say "the sum of food".... to me it sounds like an oddly formalistic thing to say. You would rather say "the amount of food" IMO.
The word "summen", 'the sum', makes no sense when talking about food - you don't quantify food with numbers, unless you talk about "the number of burgers". It's like "the number of money..." -doesn't work.
"Tilstrækkelig" and "nok" would be just fine using, if the food is an amount, "Mængden".