1. Today we eat foods/food which come from all over the world.

(Do we have to use 'food' or 'foods'? Are both OK? When do we use 'foods'?

2. I like healthy food.
3. I like health food.
4. I like healthy foods.
5. I like health foods.
(Which ones are correct? Which expression do I have to use?)

1. Either is correct.

Today we eat foods which come . . .
Today we eat food which comes . . .

2 - 4 -- All are correct. 5. is wrong

However, "health food" means food that comes from specialized "health food" stores. Healthy food has a much broader definition.

http://en.wikipedia.org/wiki/Health_food

1. Both "food" and "foods" are grammatically correct, but they are used in different contexts. Generally, "food" is used as a collective noun, referring to different types of edible substances, while "foods" is used to refer to specific types or varieties of food. For example, if you're talking about different types of cuisine from around the world, you might say "today we eat foods from all over the world." On the other hand, if you're discussing the overall concept of eating, you would generally use the singular form, such as "today we eat food from all over the world."

2. The correct expression is "I like healthy food." Here, "food" is being used as a general term to represent all types of nutritious and healthful options.

3. The expression "I like health food" is not grammatically correct. "Health food" refers specifically to food that is believed to be health-promoting or beneficial. However, it is rarely used as a singular phrase in everyday language. Instead, it is more common to use the plural form, "health foods".

4. The correct expression is "I like healthy foods." In this case, "foods" is used to refer to specific types or varieties of food that are considered healthy or nutritious.

5. The expression "I like health foods" is also grammatically correct. It is used to express a preference for multiple types of food that are believed to be health-promoting or beneficial.

In summary, correct expressions would be:
- "Today we eat food from all over the world."
- "I like healthy food."
- "I like healthy foods."
- "I like health foods."