please note, not all buildings which provide pop cap are population enhancements. Slaver Work Camps provide 1 pop cap, but are Social Construction. The Food Dist won't work on that one either.
(edit) to be clear. If you put a city next to your Food Dist, the city would take +2 adjacency from it, and your pop cap will go to 7.2 (3 from city, 3 from capital, +20% from Food Dist adjacency to your city).