Before eating something fattening we say “Shabbos calories don’t count!”
I know you mean it as a joke, but in high school, a Rebbe publicly berated me in front of the entire class because my diet included Shabbos. He spent a good ten to fifteen minutes ranting about how Shabbos is not a time for dieting.
And no, I did not listen to him. I continued to diet on Shabbos.
The Wolf