A witch or wizard in Animagus form will eat the food of their animal form:
An Animagus is a witch or wizard who can transform at will into an animal. While in their animal form, they retain most of their ability to think as a human, their own sense of identity and their memories. They will also retain normal human life expectancy, even if they take their animal form for long periods of time. However, feelings and emotions are simplified and they will have many animal desires, feeding off whatever their animal body craves, rather than demanding human food.
The above quote comes from the e-book: Short Stories from Hogwarts of Heroism, Hardship, and Dangerous Hobbies, Chapter One: Animagi.
One would then assume that if they ate something toxic to their animal form it would indeed be harmful to them.
However, since most Animagi are not in their animal forms for that long, I would expect they would merely transform back into their human form prior to eating.