Can I use these both interchangeably?
You are eating junk food before dinnet etc.

You'll lose your appetite.
You'll spoil your appetite.

Question

Can I use these both interchangeably?
You are eating junk food before dinnet etc.

You'll lose your appetite.
You'll spoil your appetite.

Answer

Yes, both are fine. 
I don't know what the other two commenters mean by 'spoil' meaning that you did something wrong. It's not. A spoilt appetite is simply an appetite that is not as strong as it should be.

Answer

Yes, you can use both. As Georgie said, "spoil" implies that you did something wrong, like eating junk food right before dinner. I would also add that "lose" could imply you're not hungry because you feel sick.

Answer

Yes you can use both. 'Spoil' implies more that they're doing something wrong

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