The 196th prohibition is that we are forbidden from eating on Yom Kippur.

There is no verse in the Torah that explicitly prohibits this act.1 However, since the punishment – that one who eats is punished by kores – is mentioned, we know that eating is counted as a prohibition.

The source which describes the punishment is G‑d's statement,2 "If anyone does not fast on this day, he shall be punished by kores."

In the beginning of Tractate Kerisus, all those who are punished by kores are listed, and one who eats on Yom Kippur is listed among them. It also explains that all mitzvos which are punishable by kores are prohibitions, except for the Pesach sacrifice and circumcision. Therefore, clearly, eating on Yom Kippur counts as a prohibition.

Therefore, if one intentionally transgresses this commandment, the punishment is kores, and if the act was unintentional, he must bring a sin-offering, as explained in the beginning of Tractate Kerisus.

This [i.e., that eating on Yom Kippur counts also as a prohibition] is also explained in the Tractate Horiyos,3 which rules that one is required to bring a sin-offering only if one violates a prohibition. The proof for this is G‑d's statement4 (may He be exalted and elevated) regarding those who are required to bring a sin-offering, "[And they violate] one of the prohibitory commandments of G‑d."

The Sifra says5: "The verse, 'If anyone does not fast on this day, he shall be punished by kores," describes the punishment for not fasting. However, we do not have a verse to serve as the actual prohibition.

But [there is an "extra" verse that serves as the actual prohibition;] we do not really need a verse to tell you the punishment for doing melachah on Yom Kippur, because we could derive it from the following kal vechomer:6 if for the prohibition of fasting, which [applies only on Yom Kippur, and] not on Shabbos and holidays, one receives punishment, then certainly for the prohibition of melachah, which applies on holidays and Shabbos [and is therefore more strict] one should receive punishment. If so, why is there a verse stating the punishment for doing melachah? From it we learn the actual prohibition of eating on Yom Kippur: just as the punishment for melachah follows its prohibition, so too the punishment for eating follows its prohibition."

The details of this mitzvah are explained in Tractate Yoma.