Let's get some food in you first.
It's an expression, a way of saying "Let's get you some food" or "Let me feed you." Another close expression would be "Let's get some food in your belly."
I means "let's get some food inside you", in your stomach, in your body. It's something you might say to someone who hasn't eaten for a long time or who might not be able to eat again for a long time.
I'm not a native either so I just give my feelings.
You can use both prepositions but the tonality is different and it depends on the context. You should give a little bit more context for the next questions. Imho.
The meaning is the same, in this case who is talking wants to stretch INSIDE your body. Like if the person was empty or tired or fatigued. So probably who is talking is referring more to the physicality or food, body, etc.. It's an expression.
Yes, to my ears, it's ok to say also: "let's get you some food first".
Thanks a lot. Sorry for not giving you more context.
I wrote down their conversation below, I hope it helps.
A: Jimmy? You okay?
B: Yeah, what time is it?
A: It’s 6:30. I’m sorry. You want to go back to sleep?
B: No. I…I’m up.
A: You can still go to a doctor, you know. No harm in getting checked out.
B: I’m fine. I, uh, think I just need food. I haven’t eaten since, um… well, really before everything.
A: Okay. Maybe go easy, just some soup at first.
B: I’m okay, Kim, really.
A: I know. Good.
B: Good. What about you? You seem, uh… worried, maybe?
A: No, no, no. It’s just… I have news, but it can wait. Let’s get some food in you first.
B: What news? What kind of news?
A: Nothing bad.
B: So, tha… that means it’s good news?
A: Yeah. I think so.
B: Great. Let me hear it.
Sorry, I'm not a native, but for me it's like a funny way to say the same. The subtle difference is that she's proposing not just to get [him] some food, but to actually have him to eat the food right after they've got it, that is "to get [food] in [him]".
This expression is one that acknowledges the hierarchy of human needs.
If one has basic biological needs, such as hunger, one is not going to be very capable of higher-level thought for problem solving or complex human interactions.
It means, take a moment and take care of yourself first.
It means, relax before moving on to the bigger thing.
It means, literally, to take a break and eat something before taking on what's next.