technically, they don't actually need to eat anything at all to survive, but they have instincts to devour and prey off of humanity. mainly, they feed off of human emotions. they can feed off of any emotions/feelings/sensations, but since people are afraid of alternates… they usually only get to taste fear. and since the more intense the emotion is, the more 'filling' it is, an alternate's first instinct is to amplify whatever emotion a human is feeling to the largest possible degree. it's actually a bit sad because fear actually uh… fear isn't actually the best/most substantial meal for most alternates! emotions/feelings/sensations like pleasure, love, happiness, excitement, etc. are much more filling and satisfying, like a full meal. fear is kind of like straight sugar, tastes good, but it never really satisfies the alternate's hunger, so they're always hungry and always need more. even emotions like sadness, anger, rage, disgust, despair, etc. are more filling than fear, so often times if an alternate knows those emotions exist they will desperately try to get a human to feel those as well.
it's very rare for an alternate to discover positive emotions/sensations, and it's often on accident. if an alternate happened to be feeding on a human's nightmares every night, but unexpectedly got hit with a wet dream one night… it would be very interested in replicating that experience. (do i have a draft for that i'm working on? maybe-)
if an alternate doesn't feed off of human emotions, then it basically becomes stuck in a state of eternal hunger; unable to die and always straving/craving anything to fill the void. not a fun experience for them, they become much more erratic, impulsive, and sometimes even lose higher thought or start being unable to anchor themselves in reality fully.
in terms of actual food, they can eat literally anything, including things that aren't food. in TYBTM, alt!cesar is able to synthesize the chemicals that allow it to make its own bioluminescence and the somnolent/aphrodisiac properties of its saliva by just… eating random shit until it figures out how to do/make those things. sometimes on accident! lock up your pharmacies, live specimens, and literally anything else or else your local curious alternate may decide it looks snack-worthy (not that locking them up would really stop them lol).
also keep in mind i'm not a chemist or biologist or nuclear physicist… however i think the main way these guys achieve this is by being able to just split atoms inside their own bodies without causing a nuclear explosion (or by containing said nuclear explosion). they are multi-dimensional, demonic shadow entities that do not adhere to the laws of physics, space, time, mass, or anything really -- the only thing they need to worry about is their shell of human flesh and whatever else they decide they need to have on a 3rd-dimensional plane. they can literally have pocket dimensions within their own bodies. they're like the humanoid version of the house from House of Leaves/myhouse.wad. they are quite literally unstoppable unless something stronger decides to do something, kinda terrifying! even more terrifying when realizing that they are the lowest on the hierarchy of "interdimensional biblical entities".
sorry for the ramble i love talking about alternate biology and constantly forget i can just Do that at any time lol. and thank you for the question!!!
