I’m trying to gain weight. I know sometimes the Gemara discusses how to do certain things in daily life. Does any text refer to how to gain weight? If it doesn’t, please let me know though. Thanks!
In the Yam shel Shlomo Chullin 86:12 (quoted here in footnote 23), the Maharshal writes that there is a very old ban on following medical advice given in the Talmud, since different places and times have different effects on the body, and we do not want someone to lose faith in the Sages if the advice does not work. Thus, the most halachic answer to any medical question would be to speak to a doctor! But the other answers here are still valuable for pure knowledge, not practice.
The Halacha is that a man only becomes obligated in mitzvos when he is A) at least 13 (and maybe one day), and B) begins showing signs of puberty (the so-called “two hairs”). Likewise for a woman, except that her lower limit on age is at least 12. In either case, if they reach 20 without showing signs of puberty, they automatically become adults and just aren’t able to have children.
This Halacha is brought down in many places, one of which is Yevamos 97a. There, and in Niddah 47b, the Gemara relates the following story, though in Niddah the story is attributed to R’ Chiya instead (translation is mine, and the brackets follow Rashi to the version in Yevamos):
כי אתו לקמיה דרבא אי כחוש אמר להו זילו אבריוהו ואי בריא אמר להו זילו אכחשוהו דהני סימנין זמנין דנתרי מחמת כחישותא וזמנין דנתרי מחמת בריותא:
When they [people who were 20 but without signs] would come to Rava, if they were skinny, he would say to them, “Go and fatten up.” If they were fat, he would say to them, “Go and become skinny.” For these signs, sometimes they fall off due to being skinny, and sometimes they fall off due to being fat.
Rashi (to Yevamos) explains how one accomplishes this (at least the fattening part):
אבריוהו - האכילוהו והשקוהו עד שיהא בריא ושמן:
They would feed him and give him to drink until he would become big and fat.
So, eat a lot.
In case anyone’s curious, the Gemara in Niddah swaps out “falling off” for “coming,” though since it does so in each case, the end result would be the same. Tosfos there nevertheless amend that Gemara to match this one.
Perhaps the Talmud in Kidushin 82a can provide some indirect insight to your situation.
So taught the Rabbis: Ten traits were said about a professional bloodletter: he eats very much and excretes very little.
תנו רבנן עשרה דברים נאמרו בגרע אוכל הרבה ומוציא קימעא
Rashi comments that since he is eating in such bounty of bread, and certain fatty meats, these will cause him to excrete very little [and in turn probably gain weight].
מתוך שמאכלו יפה בלחם נקי ובשר שמן שממעטים את הזבל
For a more modern approach, you could always try Google, indeed one of the methods mentioned is to eat often, as even Rashi mentioned - he is wont to eat with several people everyday.
שמלומד לאכול עם כמה אנשים בכל יום