Although mbloch is right that keeping healthy is an important idea in Judaism, it does not seem that one should start diets (in the usual sense) or even give attention to one's excess fat when overweight. Eating לשם שמים and not for one's enjoyment is a fundament principle in Judaism and according to some part of the Positive Commandment קדושים תהיו. As seen in the second letter below, the emphasis is more on why one eats than what he is eating.
See below for the novel approach of the Lubavitcher Rebbe on dieting:
לכתבו על דבר ששוקל יותר מדי ודאגתו על דבר זה וגם על זה שקשה לו למעט
באכילה ושתיה - לדעתי 1) מגזימים הרופאים בהערכתם ההיזק שע"י דיקייט
(תוס' משקל) של האדם, 2) היסח הדעת – על כל פנים שלילת התבוננות תמידית
בזה – תקל ההתאפקות מאכילה ושתיה יתרום – על־כל־פנים, תחזק מנוחת הנפש
ובמילא גם המעטאַבאָליזם (=חילוף החומרים) דהאדם למעט בהשומן שיתר
מהמידה, ועוד וגם זה עיקר – שיעבוד השם בשמחה.
אזכיר עוה"פ עה"צ. להודיעו תוכן המצו"ב
(מכתב אדר"ח אייר ה'תשל"ט)
(Free translation)
The Rebbe is writing to someone who is worried about being overweight
and is having difficulty reducing his eating and drinking.
The Rebbe replies:
- the doctors exaggerate in their assessment of the damage caused by extra weight.
- hesach hadas (purposeful distraction) -- at least not constant consideration in this matter - will strengthen the peace of mind and
also the metabolism which will help with the excess fat.
** The main thing: serve hashem happily.**
The Rebbe writes in another letter about dieting:
Regarding the types of food that should make up your diet:
The types of foods you eat (as long as they are properly kosher) are
not as important as the reason why you are eating. Do not eat in order
to indulge in pleasure, but eat merely to sate your hunger and to be
healthy, thereby enabling you to do good things, etc. See also
Shulchan Aruch, Orach Chayim, ch. 231.1
Shulchan Aruch, Orach Chayim, ch. 231.1
Anything that is pleasant for him in this world, his intent should not
be for his pleasure, but rather for the service of the Creator, may He
be blessed; as it is written (Proverbs 3:6), "In all of your ways,
know Him." And the Sages said, "Let all of your actions be for the
sake of Heaven." As even optional things – such as eating,
drinking...should be for the service of your Creator or for something
that enables serving Him. For even if one was thirsty or hungry – if
he ate or drank for his pleasure, it is not praiseworthy. Rather he
should intend to eat and drink for his sustenance, to serve his
Creator.
The Lubavitcher Rebbe does not need corroboration to support his idea. However, חכמה בגויים תאמין recently there have been more doctors agreeing with this approach see this article.
"The phenomenon that weight regain eventually follows intentional
weight loss is as certain as smoking causing cancer."