Food has nothing to do with reducing unhealthiness. And actually, the point is not to reduce unhealthiness, but to have more health points than unhealthy points.
To do so:

from population: Only way to reduce it is to reduce the population of your city. Not the best way.

from flood plain: No way to reduce it

from jungles: clear jungles

from buildings: No way to reduce it until very late in the game
best way (after clear jungles) is still to raise

. How to do so?
Buildings (aqueduct, grocer...)
Civic (environmentalism)
Ressources (rice, clam, cow...)
Fresh water, forest
Expansive trait
Someone has written in the strategy forum an article about unhelthiness and unhapiness; I suggest you take a look at it.