「太りにくい体質」は英語で「fast metabolism」とよく使われています。
例文：
A: Wow, you eat so much, but you're so skinny!
(すごい、よく食べられるけど、身体はめっちゃ細い！）
B: I know. I have a fast metabolism.
（そう。私は太りにくい体質なんです。）
他の言い方は：
I can eat as much as I want without gaining weight.
どんなに食べても、私は太らないよ。
Good luck!
I find it hard to gain weight. / I don't gain weight easily.
I'm naturally slim.
I generally don't put on weight. / My weight stays constant. / My weight doesn't change.
英訳１：「なかなか体重が増えないんです」
英訳２：「自然とやせるんです」
英訳３：「あまり体重が増えないほうなんです」
上記以外には、
I have a fast metabolism.
などと言ってもいいかもしれません。「代謝がいいんだよね」という意味なので、つまり、脂肪が燃焼されやすく、太りにくいということになります。
太ろうと頑張ってもなかなか体重が増えなくて困っている人なら、
It's really hard for me to gain weight.
などと言えます。
既にセインさんが例として出している文ですが、
It's hard for me to gain weight.
がいいと思います。
直訳は、
”私にとって、体重を増やすことはきついんだ” です。
要は太りたくても太れないって感じ。
なんか嫌味な感じですね♪
ただ、この
”It is 形容詞 for 人 to~" の構文は見たことある方、多いのではないでしょうか。
会話中に思い出しやすい構文だと思い、載せてみました。
A casual way of saying this might be,
"I just can't put on weight".
A more common expression is
"built like a whippet". meaning to be built like a skinny lean dog
A more formal way would be to say
"I am naturally thin, I don't tend to put on weight.
or
"i am lean, and find it difficult to gain weight".
カジュアルな言い方として
"I just can't put on weight".（私は太りにくい体質です。）
もっと一般的な表現として、下記があります。
"built like a whippet".
(細くて引き締まった犬のような体だという意味です。)
もっとフォーマルな言い方として、次のようなものもあります。
"I am naturally thin, I don't tend to put on weight."
(私は元から細いです。私は太りににくいです。)
"I am lean, and find it difficult to gain weight."
（私は細くて、太りにくいようです。）
He can eat like a horse...but never gets fatter! Its down to his fast metabolism!
Some people can eat whatever they like and still NOT GET FATTER!
This is a due to their accelerated metabolism... or their high metabolic rate.
These lucky individuals burn fat as fast as it arrives...and stay slim!
So they don't "pile on the pounds" like the rest of us;-D
好きな物何を食べても、太らない人がいます。
これは速い新陳代謝や代謝速度が速い為である。
このようなラッキーな人達はすぐに脂肪を消費し、スリムに保つことが出来るのです。
彼らは私達のように"pile on the pounds"（重量が蓄積される）ことがないのですね。
People don't always put on weight when they eat a lot
People with a fast metabolism burn more calories, and stay thin!
Everyone's genetics vary and sometimes it's hard for someone to get fat
The above expressions detail the reasons why people may not always put on weight in direct proportion to eating a lot. Some people have a 'fast metabolism' and obviously naturally burn more calories than someone with a slow metabolism. There is also an association with the 'genetics' or family history that someone has. If your parents are large and overweight, there is a high probability that you will be also.l
Even though you eat so much, you never put on weight!
You eat a lot of food, but you don't seem to put on weight
*It's impossible for you to put on weight! (this is a joke of course :)
*Even though you eat so much, you never put on weight!
*You eat a lot of food, but you don't seem to put on weight. don't seem to= you don't know for a fact whether they have put on weight... like a guess)
*You can also say*
*Wow how do you eat so much and never get fat? (only say this to close friends)
*I wish I was like you and could eat everything and not put on weight
*NB, with a subject like this I would advise caution and perhaps say these phrases to close friends so you don't offend people. Lots of people don't like to talk about their weight and even if they 'look skinny' they might be unhealthy from eating lots of any kind of food so just be careful :)
Hope this helps! ^ ^
*It's impossible for you to put on weight! (this is a joke of course :)
あなたが太るのは不可能だわ！（もちろんこれは冗談です。）
*Even though you eat so much, you never put on weight!
たとえどんなにたくさん食べても、あなた太らないね。
*You eat a lot of food, but you don't seem to put on weight.
たくさん食べるけど、太らないわね。don't seem toとは、事実はわからないけれど、推測するようなニュアンスです。
以下のようにも表現できます。
*Wow how do you eat so much and never get fat? (only say this to close friends)
まぁ。どうしてそんなに食べて太らないの？（近い友達にのみ言うことのできる表現です。）
*I wish I was like you and could eat everything and not put on weight
あなたみたいに、たくさん食べても太らないならいいのにな。
注意：このようなフレーズは、仲の良い友達にのみ話しましょう。このような話題を話したくない人もたくさんいるので、相手の気分を害さない配慮が必要です。たとえ、良かれと思って、痩せて見えると相手に言った場合でも、健康上の問題でそういう風になっている人もいるので、注意してください。
I have a naturally small body type, my whole family finds it hard to gain weight.
If you don't gain weight easily it means that your body is able to burn the fat from your food quicker than usual and so you would have to have excessive amounts in order to gain weight whereas someone else would easily gain weight from a much smaller portion.
"I am lucky, I can get away with eating a lot because I have a fast metabolism."
"she is always so slim no matter how much she eats, it must be that she has a fast metabolism."
体重が増えにくいなら、それは脂肪の燃焼が他の人よりも早いということです。ですから、太りたいなら、他の人よりもたくさん食べないといけません。
"I am lucky, I can get away with eating a lot because I have a fast metabolism."（私はラッキーです。代謝がいいので、たくさん食べても太りません）
"She is always so slim no matter how much she eats, it must be that she has a fast metabolism."
（彼女はどれだけ食べても全然太りません。きっと代謝がいいんだと思う）
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