Hcfyk

By saying "mental food" I mean something that you enjoy digesting mentally, like watching a football game or reading a comic book.
(It is in contrast to "physical food" which you physically eating the food).
For example:

Asian U23 football championship is the "____________" for Vietnamese people.

A term for an item of mental interest is "Food for thought" but that would be only a mouthful, not a meal's worth.

I'd suggest intellectual nourishment.

It is defined on thefreedictionary as,

anything that provides mental stimulus for thinking; food for thought.

Note: If you get entertained by some insipid subject: Mathematics documentaries, or some abstract subject etc. (which people normally find tasteless), then there is a word for it.

Pabulum or pablum (noun)
Oxford defines it as,
https://en.oxforddictionaries.com/definition/pabulum

Bland or insipid intellectual matter, entertainment, etc.

It did not seem to me that football is an activity appreciated from an intellectual standpoint. But, thanks to your posing the question, I found a good article in The Irish Times: 'The rise of the Football Intellectual', article by Ian Maleney, Wed. Nov 9, 2016. In it, Maleney references several authors on the subject.

But back to your question. First: Does, in fact, the entire (or nearly so in your estimation), populace of Vietnam enjoy football. "Asian U23 football championship is the "____________" for Vietnamese people." Increasingly, we are told "we all" - when "we all" most certainly do not - for example, have electronic devices.

As to "comic books" - it would depend on the style of comic book, yes? A comic book, like food, can primarily be a visual treat, if you will. Or, it can be to instruct, etc. Or, both. There have been literary comic books, for certain.

"Fodder" is used by the British for what you call "mental food." "Feast" doesn't apply.

How about referencing a literary work you reasonably believe to be known by your target population.

"Asian U23 football championship is the "____________" for Vietnamese people.

Or, make it personal. What is it about football that stimulates you intellectually, and what is your food/book that applies.