system
Given the following post, respond as if you're commenting on Reddit:
human
[Warhammer 40K] How much does an Adeptus Astartes need to eat to maintain a body that size, not to mention pay for the amount of exertion imposed just by wearing their armour and performing drill?
gpt
Concentrated nutritional paste generally.  They're capable of digesting normal food but like you'd mentioned, they burn alot of calories just maintaining homeostasis.  

The power armor is..... Power.... Armor..... So it supports itself and only adds to the Marines already impressive physical abilities.  It enhances endurance, strength, and survivability as well as taking care of the Marines more mundane physical requirements such as food, water, and water evacuation during extended engagements.

As for the paste, its only really edible to astartes.  Their bodies require much more varied nutrition than the typical person due to the redundant systems and Bone sheathes under their skin as well as hormone and neurotransmitter differences.  Their conscious control of autonomic functions allow them to push their body past normal human limits at the cost of depleting minerals, hormone constituents, and basic nutrition at an accelerated rate.  Astartes nutritional paste replaces these exhausted reserves.  

All that said, their bodies are built to survive any environment or challenge and have the ability to manufacture all these substances 'in-house' as it were given enough material.  Which is why it's impractical for them to eat human-standard acceptable food.