If you are off your food, you do not want to eat, usually because you are ill.
"It's not like you to be off your food."
To lose one's appetite = to no longer feel hungry.
"That movie made me lose my appetite."
"More cake, Henry?"
"No thanks. I don't feel hungry right now"
あなたが "off your food"だとすると、それはあなたは食欲がないという事になります、たいていの場合、それは病気だったりします。
"It's not like you to be off your food."
（食欲がないなんてあなたらしくないね）
"That movie made me lose my appetite."
（あの映画を見ると食欲がなくなる）
To lose one's appetite = お腹がすいていない、食欲がない
"More cake, Henry?"
（もう少しケーキどう、ヘンリー？）
"No thanks. I don't feel hungry right now"
（いらない。今お腹すいてない）
Not feeling well and my appetite has disappeared today.
▪Not in the mood to eat today.
Mood=a temporary state of mind or feeling
This is saying directly that you do not feel like eating
▪Not feeling well and my appetite has disappeared today.
This means that you feeling a bit sick and because of that you lost your appetite.
▪Not in the mood to eat today.
(今日は食べる気分ではありません)
Mood = 一時的な心や感情の状態。これは直接的にあなたが食欲がないと言っています。
▪Not feeling well and my appetite has disappeared today.
(気分が悪く今日私の食欲は消えてしまいました)
これは少し気分が悪いため食欲がなくなったという意味です。
When you want to explain that you are in no mood to eat today, you may say it in the following ways:
-I have no appetite today
-I have absolutely no appetite today.
-I do not feel like eating today.
食欲がないときは、以下のフレーズが使えます。
-I have no appetite today（今日は食欲がありません。）
-I have absolutely no appetite today.（今日は全く食欲がありません。）
-I do not feel like eating today.（今日は食欲がありません。）
Your appetite or interest in eating can be affected by many things, you can use the phrases above followed by, because I ate too much yesterday or my stomach is unwell, or I am unwell etc, It explains why the food does not appeal to you today and you simply cannot eat.
食欲はいろいろなことの影響を受けます。
上記のフレーズに続けて、
because I ate too much yesterday（昨日食べ過ぎた）
my stomach is unwell（胃の調子が悪い）
I am unwell（体の調子が悪い）
などと言えます。これらは、なぜ食欲がないのかを説明します。
Your 'appetite' is when you have a feeling/feel like eating food. So if you don't feel like eating much today as well as saying 'I don't feel like eating much today' you could also say 'I don't have an appetite/have much of an appetite today' which means you don't feel like eating much food today
appetite'は「食欲」という意味です。
ですから、食欲があまりないときには、'I don't feel like eating much today'（今日はあまり食欲がない）に加えて、
'I don't have an appetite/have much of an appetite today'
と言うこともできます。「今日はあまり食欲がない」という意味です。
おっしゃられている内容は、以下のように表現できると思いました(*^_^*)
I don't have appetite today.
「私は今日食欲がない」
I don't feel like eating much today.
「今日はたくさん食べる気がしない」
I'm not so hungry today.
「そんなにお腹がすいていない」
以上です。
少しでも参考になれば幸いです(*^_^*)
日本 
イギリス 
南アフリカ 
アメリカ合衆国 
アイルランド 
