When you are talking about specific"types of food" There are many foods which are rich in iron.

Food is uncountable when it is used as a general idea in the context of the sentence. For example: "We can't live without food." However, if you are talking about food as a collective noun (aka a collection of different foods), then it can be considered as a countable noun, and therefore would need to have its corresponding verb conjugated to match the number of the noun. For example: "The food from the restaurant was excellent." "There were rows of food on the table last night."

Do you mean the word "food" alone? The word "food" by itself (alone) is not a countable noun in American English if you mean that by adding an "s," it can be made plural and used in common parlance. The term "food groups" is the closest I can think of to make "food" plural, but "food groups" does not denote simple "food." Good luck.