Gemara

(Reish Lakish): "Zos ha'Torah Adam Ki Yamus b'Ohel" -- Divrei Torah last only in one who kills himself [has nothing to do with the world, except] for Torah.

Berachos 63b: We expound "Haskes u'Shema Yisrael" like "Kesusu" (break yourselves over Divrei Torah);

This is like Reish Lakish taught.

Gitin 57b (Rav Nachman bar Yitzchak): "We are killed for You the entire day" refers to Chachamim who give their lives for learning;

This is like Reish Lakish taught.

Chulin 84a: "Asher Yatzud" teaches proper conduct, that one should not eat meat regularly, only on occasion (e.g. when one happens to trap an animal).

(Beraisa): "When Hash-m will expand your borders (... and you will lust to eat meat)" teaches proper behavior, that one should eat meat only when he has a craving for it.

He should not buy meat to eat. Rather, "you will slaughter from your cattle and flock", and only from them, but not all of them.

R. Eliezer ben Azaryah says, this teaches that if one has 100 Zuz (a poor person), he should buy one Litra of vegetables. If he has 10 Manos, he should buy fish. If he has 50 Manos, he should buy meat. If he has 100 Manos, he should have meat every day.

One with 10 (or 50) Manos should eat fish (or meat) only on Shabbos.

(Rav): We should follow R. Eliezer [ben Azaryah].

(R. Yochanan): Rav was healthy. He could conduct like R. Eliezer. We are not healthy. One who has money should spend it on food.

(Rav Nachman): We are so weak that we should borrow money to buy food.

(Mar Zutra brei d'Rav Nachman): "And life for your youths" - do not accustom your children to meat and wine (rather, to simple foods).

Sanhedrin 111a: Rav saw that Rav Kahana would shampoo his hair before coming to learn. Rav said "Lo Simatzei b'Eretz ha'Chayim."

Rav Kahana: Do you curse me?!

Rav: No. The verse means that Torah is not found in those who look to live comfortably.

Yoma 77a (R. Yitzchak): "Ken Yiten liYdido Shena" refers to wives of Chachamim, who lose sleep in this world (waiting for their husband to return) and merit the world to come.

Eruvin 21b: Keneses Yisrael says to Hash-m "Netzei ha'Sadeh" - I will show You Chachamim who engage in Torah amidst hardship;

22a (Rabah): "Shechoros ka'Orev" one who is MaSHCHiR (blackens) his face like a raven for Torah (i.e. neglects himself) can expound mounds of Halachos from every line [on top of certain letters] in the Torah.

54a: If a person makes his jaws like a stone that is never worn away, his learning will endure. If not, it will not endure.

Avos 6:4 (Mishnah): The way of Torah is to eat bread with salt, drink a small measure of water, sleep on the ground, live a life of pain, and toil in Torah.

Rishonim

Rambam (Hilchos Talmud Torah 3:12): Divrei Torah do not last in one who slackens on them, nor in those who learn amidst eating and drinking, rather, only in one who kills himself and pains his body constantly and does not give sleep to his eyes.

Rashi (54a DH k'Even): A stone is not worn away through people trampling on it. Likewise, a Chacham's jaws do not weary from reviewing his learning and teaching it to others.

Me'iri (57b DH u'Le'olam): One should always exert himself for Torah, for it is found only through exertion, like Reish Lakish taught.

Melamed ha'Talmidim (Yom Matan Torah DH veha'Mashal): Torah is found only in one who exerts himself until the time of death. Chachamim gave a support from a verse ("Zos ha'Torah Adam Ki Yamus b'Ohel"). This greatly helps the masses who hear this to see the truth of this and know in the heart that the Torah commanded about this.

Poskim and Acharonim

Shulchan Aruch (YD 246:21): Divrei Torah do not last in one who slackens on them, nor in those who learn amidst eating and drinking, rather, only in one who kills himself for it and pains his body constantly and does not give sleep to his eyes.

Gra (51): Limited sleep is one of the Midos through which Torah is acquired (Avos 6:5). We learn also from Yoma 77a [that a Chacham must deprive himself of sleep].

Rema: One should not plan to engage in Torah and acquire wealth and honor with learning. One who has this intent will not merit the crown of Torah.

Gra (52): We learn from Eruvin 55a. (It seems that he learns from R. Yochanan, who taught that Torah is not found in wholesalers [who cross the sea] or merchants.

Rema (ibid.): Rather, he makes his Torah primary and his Melachah secondary. He diminishes business and engages in Torah, and expels pleasures of this world from his heart. Each day he does what is needed to live, if he lacks food, and the rest of the day and night he engages in Torah.

Gra (54): Avos d'R. Noson (28:5) says that whoever accepts for himself pleasures of this world is denied pleasures of the world to come, and whoever does not accept pleasures of this world is given pleasures of the world to come.

Mishbetzos Zahav (YD 28:1): In Chulin, we conclude that we are not healthy, so we should eat meat. The Yam Shel Shlomo (6:3) says that in any case one should not indulge in pleasures, and cites the Rambam. Keneses ha'Gedolah brings from Damesek Eliezer that if one is very finicky like Rav Yosef, he may indulge himself, for this guards the health of his body.

B'Tzel ha'Chachmah (2:31:6): The Radam wrote that nowadays we are not accustomed to wash the face, hands and feet in hot water. Therefore, this is not a pleasure that applies to everyone [so one may not heat water for it on Yom Tov]. This is unreasonable. Tosfos (Beitzah 21b DH Lo) says that also Beis Hillel forbid heating water to bathe the entire body, for this is only for pampered people. Shabbos 33b says that [Edom made] bathhouses to indulge themselves. Shlomo made for himself "v'Sa'anugos Benei Adam", i.e. springs and bathhouses (Gitin 68a). However, it says in Sanhedrin (17b) that a Chacham may not live in a city without a bathhouse! Surely, a Chacham need not live amidst pleasures. Divrei Torah last only in one who kills himself for Torah! The way of Torah is to eat bread with salt...! In a city with Nochrim and Yisre'elim, one may not bathe in a bathhouse immediately on Motza'ei Shabbos if the city is mostly Nochrim (Machshirim 2:5). If one finds bread, Pas Isah (clean bread, or home-baked bread) or coarse bread, we follow the majority of bakers, or eaters of Pas Isah or coarse bread. We do not say so regarding the bathhouse [because everyone bathes]!

Divrei Yatziv (YD 134 DH v'Chol): Regarding the Mitzvah to kill the seven Kana'ani nations and Amalek, the Minchas Chinuch (425, 604) says that the Torah does not rely on miracles. One must put himself in danger for these. Since through Torah we will merit the ultimate redemption and destroy the seed of Amalek, one must enter himself to danger and kill himself in the tent of Torah.

Divrei Yatziv (EH 111 DH v'Hinei): R. Chiya used to request that Torah be our occupation, that our hearts not mourn, and that our vision not wane (Berachos 16b). This is because they used to learn while standing, which is bad for the heart (Kesuvos 111a), and speaking is bad for the eyes (Nedarim 41a). Also, great exertion in deep learning and lack of sleep is dangerous for the eyes. Therefore, he requested for these. One can acquire Torah only if he kills himself for Torah, i.e. endangers himself through great investigation and meager sleep.

Mishnah Halachos (7:154 DH uvi'Gemara): Surely, one can learn temporarily amidst pleasures, but Torah learned in this way will not last, unlike other Chachmos.

Netzach Yisrael (7 DH she'Mar'in): The Torah and the body are opposites. How can the Torah, which is intellectual, last in a physical body? They cannot coexist. Therefore, Torah lasts only one who kills himself for Torah. Such a person's body is not considered at all. It is as if he is not physical, offer he kills himself and removes his [physical] self for Torah, therefore Torah lasts in him. If one does not do so, he is physical, so Torah, which is intellectual, cannot endure in him.

Shlah (Toldos Adam, introduction 362): If man would not sin, his intellect would be clear. He would understand written Torah, and through introspection he would know everything. He would not need Oral Torah. However, since he sinned, he removed the light and brought death to the world. He needs to toil and he needs Oral Torah, which is the Perush, and to kill himself over it. Darkness and death caused that Oral Torah needed to be written. If the Beis ha'Mikdash were intact, the light would increase, like it will in the future, and Torah would be light, which gladdens the heart. We would not have darkness, and killing oneself for Torah. This is the secret behind "Shnei ha'Me'oros" (the two luminaries that Hash-m created on the fourth day). The letters of Me'oros spell Ohr Maves. In the future, our written Torah will be understood through the light of intellect, and it will be "Torah Achas", for we will not need the Oral Torah.

Chasam Sofer (83b DH Zos): What is the connection of the verse to engaging in Torah? One might have thought that a Tzadik who kills himself over Divrei Torah and does not refrain even at the time of death, he does not have Tum'as Ohel after death. The verse teaches that this is not so.