Fair question, and it comes down to knowing when you can use a noun in countable or uncountable form. Some nouns seem to skip between forms a little too easily. A nice rule-of-thumb is to establish which nouns are "basically uncountable" (such as food and fruit). So your first option should be to use the non-plural form for these things. The rule is "broken" when you mean "different types of..." So the question is, are you suggesting that there are different types of the uncountable noun in the context you mean? Will you tell us more about each type? If not, then use the uncountable form.

The first of each of those (food, forest, fruit) is singular, meaning only one. Foods, forests, and fruits are plural, meaning more than one.

типичная ошибка рускоязычника -- незнание какие существительные в анг яз countable, а какие uncountable Нет такого зверя как "There is a lot of forest".

I'll specify: What's the diff. in the meaning of words above in these sentences: There is a lot of forest. There are a lot of forests. What food do you buy? What foods do you buy? Fruit grows on trees. Fruits grow on trees.