If FIMS could be related to Sugar, Carbs, Fat,  and protein.

I'd say that:

  Sugar is Money - getting the instant energy you need in a pinch.

  Carbs is Industry - getting a better bang for the buck than sugar, but isn't instant.

  Fat is Food - Extra population for extra population sake is a good resource to have, but getting too fat comes at a cost.  Better to be lean.

  Protein is Science - it is needed for muscle development, so that heavy weights can be lifted.

Given the different body types:

   Agrarian, Militarist, Builder, Scientist, Trader, Expansionist, and Aestete.

What does each of their diets generally look like?

Does it depend on their age?  (ie. Era level), or other factors?

Does it depend on their activity level?  (ie. Districts, Infastructure, and Units they produce).

What is a general and good way to determine it?

Should we eat more steak and potatos, avocados,  and snickers, or something else?  Maybe that depends on its cost  and calories. Nuts and berries seem to be the balance in the Neolithic Era, except when going on the hunt.   I  wonder what factors would  represent  cost and calories  in  humankind game.  Hmm...   And, what the winning competition is eating before their matchup?  I wonder what a famous movie actor body builder Arnold Schwazenager would choose getting hsi Fame?  That's another thought.

All in all, often people in Humankind game choose their own story, given their taste preferences.  But, we also know that Lucy was a meat eater before  going vegetarian!   So, its not all  about having a suculent steak either.