food作为食品的统称时为不可数名词，单数形式；指“一种食品时”可数，可以用复数。比如I like all kinds of foods.这里把一个food当成是一种食品，foods指各种食品。
例句
1、Enjoy your food.
用餐愉快。
2、It's not like you to be off your food.
胃口不好，这可不像你了。
3、Lord Fraser's speech offers much food for thought
弗雷泽勋爵的讲演发人深省。
4、Developments in your career may give you food for thought.
事业上的发展可能会引发你很多的思考。
5、Supplies of food and medicines are inadequate
食物和药品供应不足。