According to the Mishna (Yoma 8:1), Yom Hakkippurim is forbidden in eating, drinking, washing, anointing, wearing shoes, and marital relations. The Gemara (76b) tries to derive how each of these prohibitions is called "affliction" (עינוי). If the word וצמתם were used, it would not have expressed any of the prohibitions besides eating and drinking.
This answer addresses the question of why the word ועניתם is used instead of וצמתם, assuming that fasting is included, rather than proving that fasting is included.
Some users pointed out that this answer might be less compelling according to some opinions (the Rosh, which Heshy quoted from Tif'eret Yisra'el to the Mishna) who hold that the afflictions besides eating and drinking are not forbidden from the Torah.