"My brother doesn't like healthy food."
Translation:Мой брат не любит полезную еду.
I don't think so. For me, «пища» sounds way too formal, I don't think I'd use it in colloquial speech at all.
It is formal indeed, but "здоровая пища" is a set phrase, it reminds every Russian of the famous book "Книга о вкусной и здоровой пище" which was first published in the USSR in 1950s and has come out in lots of editions since then. Besides, any noun derived from a verb sounds a bit formal in Russian; therefore, no Russian would ever say, "мне нравится полезная едa"; instead, we would say something like "я люблю есть то, что полезно для здоровья". In an informal conversation (and in good literature too) the preference is always given to the verb over a related noun.
There are quite a few masculine nouns and nouns that can be either masculine or feminine, depending on the context, that end in -а or -я in the nominative singular. They all have a distinctive accusative form ending in -у or -ю, respectively. Examples: юноша, судья. Она любит этого юношу. Я знаю этого судью. So the rule we are talking about has nothing to do with the gender. And I did mean singular nouns only, when I said, “except singular nouns whose nominative ends in -а/-я”.
Sorry but your last point was a little off base. I'll try to be very specific so we don't wander off on a tangent.
Your original point: accusative and genitive forms are identical for all animate nouns except when the nominative ends in -а/-я.
My point (that only applies to the singular): accusative and genitive animate forms are only identical when the nominative form doesn't end in -а/-я AND the noun is masculine.
So then you started talking about how animate nouns in -а/-я can be masculine or feminine. That is not relevant. We both already agreed that nouns in -а/-я were excluded from the rule, they have an accusative form in -у/-ю so accusative not = genitive.
Instead, the point I've been making all along is that THE ONLY animate nouns where accusative = genitive in the singular is masculine nouns, so gender absolutely matters.
Consider мать (feminine animate but not in -а/-я) or щество (neuter animate). Both of these words have accusative = nominative, not accusative = genitive. So once again, the (singular) accusative = genitive rule is only for animate masculine nouns (excluding those in -а/-я).
@Dmitry_Arch, what I said was absolutely true for singular nouns, where your criteria were too broad.
As for plural nouns, yes all animate nouns meet this condition in the plural, including nominative forms in -а/я. Since you were excluding those it stands to reason you were talking about singular cases.