(1) Just as in plain 'food', both forms are possible, depending on whether the speaker is considering the nutritive substance in general, or as several discrete types. It is often in the industry or in nutritional discussions that we speak of the different natural/health foods-- muesli, royal jelly, or whatever-- and their relative merits. In more general conversation, we would be satisfied to say that we abhor health food, and love junk food.
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