(And we all know how well frogs work after they're disected.)
What I meant was that the humor doesn't depend on whether there actually is any fat to reduce. It's based on ... oh shit, now I get it.
Umm, here in the U.S. we have a "low-fat" buttler-like [link|http://www.tasteyoulove.com/|spread] that is called "I Can't Believe It's Not Butter." The commercials for it always involve someone tasting it and exclaiming ... well you've probably already figured out what they exclaim.
Point being, it's not about the wafers actually being low-fat. It's a play on a famous commercial, and the (somewhat sacreligious) idea of the wafer not actually being Jesus, as some denominations (notably Roman Catholic) claim.