Gemara

Mishnah: All seven days of separation the Kohen Gadol was not denied food and drink. Shortly before dark on Erev Yom Kipur they would not allow him to eat much, for eating induces sleep.

Beraisa - R. Yehudah ben Nekusa: (In the morning of Erev Yom Kipur) we feed him fine flour and eggs to loosen his bowels (so he will eliminate now and will not need to do so on Yom Kipur).

Chachamim: We do not, for these warm a person (and might induce Keri)!

Beraisa - Sumchus: We do not feed him Esrogim, eggs or old wine. Some also forbid fatty meat; some also forbid white wine, for it leads to Keri.

Beraisa: We attribute Zivah to food (overeating. One does not become a Zav if the second emission can be attributed to a cause.)

R. Yehudah ben Beseira: We do not feed to him (a Zav) milk, cheese, eggs, wine, water of pounded beans, fatty meat, fish oil, or anything that leads to Keri;

These words include garlic, cress, purslane and a plant called rocket.

Rav Gidal: A guest should not eat eggs (it leads to Keri. It would be shameful if his host notices semen on the sheet.)

Bava Kama 82a: Ezra enacted to eat garlic on Erev Shabbos.

This is because (garlic increases a man's seed, and) Shabbos is the ideal time for relations. "Asher Piryo Yiten b'Ito" alludes to relations on Shabbos night.

Beraisa: Garlic does five things. It satiates, it warms a person, it makes his countenance radiant, it increases seed, and it kills intestinal worms.

Some say, it also increases love and dispels envy.

Rishonim

Rambam (Hilchos Avodas Yom Kipur 1:6): Shortly before dark on Erev Yom Kipur they would not allow the Kohen Gadol to eat much, for eating induces sleep; they would not allow him to sleep, lest he see Keri. They do not allow him to eat foods that lead to Keri such as eggs and warm milk.

Perush ha'Mishnayos: Foods that increase seed and cause Kishuy, such as fish and spiced eggs, are forbidden.

Rosh (Hilchos Seder Avodas Yom Kipur): On Erev Yom Kipur they would not allow the Kohen Gadol to eat much, for this induces sleep, which could lead to Keri. They withhold from him foods that increase seed such as eggs and old or white wine.

Rosh (Bava Kama 7:19): Ezra enacted to eat garlic on Erev Shabbos because it increases seed, and is proper to have relations then. The initial letters of "V'Shomru Venei Yisrael Es ha'Shabbos" spell Bi'ah; women are included.

Question (Pilpulei Charifta 10): The Rosh cites a different verse than the Gemara to prove that it is proper for all to have relations on Shabbos, not just Chachamim. It is not clear why he needs to mention that women are included.

Note: Perhaps the Rosh obligates women to eat garlic to increase their seed (see Tosfos ha'Rosh Nidah 13a DH Nashim).

Yam Shel Shlomo (Bava Kama 7:43:5): Seemingly, one should eat garlic during the day to allow time for it to increase seed. However, Nedarim 63b says that it is the custom to eat garlic on Shabbos night. Perhaps the enactment is for Prushim who have relations in the middle of the night. Alternatively, perhaps garlic increases seed very quickly.

Tosfos Yeshanim (DH Ein): R. Yehudah ben Beseira forbids foods that induce Keri to a Zav. We are not stringent to forbid them to the Kohen Gadol. Even though Keri due to these foods is Ones and it is not Tamei regarding Zivah, Keri due to Ones interrupts the seven clean days needed for Taharas Zav.

Likutei Halachos (6a): Presumably, these are forbidden also to the Kohen Gadol.

Hagahos ha'Gra alters the text so that the Beraisa teaches first about the Kohen Gadol, and R. Yehudah ben Beseira adds that the same applies to a Zav.

Poskim

Rema (OC 608:4): On Erev Yom Kipur we do not eat foods that warm the body, lest one see Keri. Similarly, one should not eat dairy, for this increases seed. Our custom is to eat dairy in the morning.

Bach (DH Kosvu): Tosfos (Avodah Zarah 5b DH uch'Divrei) holds that we refrain from beef the entire day. Maharil says that foods that increase seed are forbidden the entire day, but foods that warm the body are forbidden only at the meal before the fast. Even though wine increases seed, the Maharam ate bread soaked in wine on Erev Yom Kipur. Perhaps he was lenient because it is a small amount, or he soaked it in mixed wine.

Source (Magen Avraham 7, according to Levushei Serad): On Erev Yom Kipur it was customary to eat (presumably lean) meat only at the pre-fast meal (Nedarim 63b). Surely people ate dairy in the morning, for it is a Mitzvah to eat! The Kohen Gadol was denied milk (this is like Hagahos ha'Gra), but others were not! A Medrash (Bereishis Rabah 11:5) tells of the great reward of someone who was willing to pay an exorbitant price for fish on Erev Yom Kipur. He would not do so if it was customary to eat meat also in the morning! (Also, the Rambam holds that fish increases seed, nevertheless it is permitted in the morning - Machatzis ha'Shekel DH v'Chen.)

Mishnah Berurah (16,18): Spices and saffron warm the body. It would be proper to refrain from foods that increase seed the entire day (this includes eggs), but the custom is to eat them in the morning. Perhaps this is because they are easily digested. Dairy food is forbidden only if it is hot. Perhaps milk is Batel in a majority of tea or a Tavshil.

Kaf ha'Chayim (34): One should also avoid much fruit with taste and smell like apples and lemons, for they lead to Keri.

Mishnah Berurah (280:1): Ezra enacted to eat garlic on Shabbos night because roasted garlic increases seed. Other foods, i.e. those listed in Siman 608, may be eaten instead. If garlic is not roasted, it decreases lust. All salted foods decrease lust; unsalted lentils increase seed.

Kaf ha'Chayim (1): A single man or one whose wife is Nidah should not eat these foods lest it cause Keri. The Rambam omitted Ezra's enactment because it is not uniform in all places and times.