Something has bugged me that I could never figure out. When you select a site for your capital city before you start a game you get the message:

"Local food can sustain X healthy population (Y maximum)"

Y is basically the sum of all the food produced.

But how is X calculated?

I understand how food works. Workers eat grain-fruit-grain-meat in this order. Still it doesn't help me figuring out how the X number is calculated. I feel I'm missing something obvious...

OK, I figured it out, it simply tells you out of Y population, how many are going to be healthy according to the food rules.

The confusion arose from the fact that I found the statement slightly misleading. "Local food can sustain X healthy population" does not mean that if you have X population, all will be healthy.
It simply means that out of maximum supported, X will be healthy.

e.g. 3 Grain 1 Meat.
Local food can sustain 3 healthy population (4 maximum)
It doesn't mean that if you have 3 population, all will be healthy.
It means that out of 4 people, eating the 4 units of food, 3 will be healthy (and 1 won't).