Okay, I see how that can be an issue.
Can't you just restrict the values to have an absolute minimum? I don't know what sort of programming structure it would have but something as simple as: y = 5 + x, where {x:x=>0} and x is the function by which you determine food production?
Edit: Alternatively, could have an if/then statement sort of modifier.
x = formula for food production
y = food produced.
If x > 0,
z = x,
else
z = 0
end
y = 5 + z
