Here is the closet thing I found:
Rabbi Chaim Rubin wrote a excellent article about dieting on Shabbos and mentions that there is a tradition "from Sefarim and Sofrim" that Shabbos food does not make one fat. Although he implies there are many sources he only brings one (since it's not the main point of his article) and even that is a Maaseh Rav
In the book Toldos Yisrael (The Alexander Rebbe Yismach Yisroel) it's brought down that in his time many bachurim would fast in order to lose weight so they would be deemed to weak to be taken into the army. The Rebbe would tell them not to fast on Shabbos since "Shabbos food does not make one fat"
Rabbi Rubin mentions elsewhere in the article that one needs to be on a great level of "Emunas Chachamim" in order for these segulas to work. So if you are on that level enjoy ! unfortunately the rest of us have to watch what we eat - even on Shabbos.
I encourage you to read the whole article