When you don't want eat food

Question

Please imagine your mother insists that you must eat something and you just don't want to eat because of lacking appetite, what would you normally say?

• I don't have appetite.
  
• I'm off my food.

For me, both of the sentences above mean exactly the same, but apparently, based on most dictionary definitions, "to be off one's food" implies the message that you're ill, but rarely it can indicate unhappiness too; in the manner that you don't want eat because you don't feel good.

Please let me know if these sentences are interchangeable or not? If not, why?

Answer 1

You could say:

I'm not hungry.

I'm full.

We say that someone is "off (their) food", "have lost (their) appetite" or "don't have an appetite" when they are sick and the disease makes them not want to eat.

She had a fever on Wednesday and was off her food all Thursday. But on Friday morning she had got her appetite back and ate a big cooked breakfast.

— Mum, I'm full. I don't want to eat the cabbage.
— If you don't finish your cabbage, you won't get ice cream
— But I'm hungry for ice cream; I'm just not hungry for cabbage.

Answer 2

Both, "I'm not hungry" and "I'm full" are good ways to say you don't want to eat. You can also beg off by saying something like:

I just ate before I got here.

Of course, given it's your mother, it's likely that no excuse will work. Every time I go to my mother's house I accept as given that I will eat when I am there, and I will leave with at least one grocery bag full of food.

In any case I couldn't say whether "off my food" or "off my feed" is something you might say in other English-speaking areas (like in the UK) -- but I've always heard it used for animals, not people.

I wonder if the cow is sick, she's off her feed.

This expression sounds very rural, though. I would be more likely to say:

I wonder if the cat is sick, she's not been eating.