The urban myth (something repeated often enough that people believe it): Inserting marshmallows in one's butt spikes blood sugar levels and is, thus, dangerous for diabetics to attempt.
Marshmallows are mostly sugar, corn syrup, gelatin, and air. They "melt" at around 98° (F)/36° (C) primarily because the gelatin (which is the ingredient that gives the treat it's melts just a few degrees below that.
There are no studies I could find that claim, one way or the other, that taking a marshmallow enema raises blood glucose levels. But, who would expect to find evidence of such studies? It IS a rather esoteric issue - yes?
A typical serving (1 oz/28g) of marshmallows contains about 15 grams of sugar, an amount that definitely spikes my blood glucose level when taken orally and without eating something that can dilute the sugar.
There IS considerable information on the internet about whether or not sugar can pass from the colon into the bloodstream as it does in the small intestine. The overwhelming concensus is that if any sugar DOES pass, it will not be enough to affect blood glucose/sugar levels.
As a T2D (Type 2 Diabetic) I keep a close eye on what I ingest. Three large marshmallows (about one serving) taken anally and retained for 15 minutes did not raise my blood glucose level. Expulsion WAS a sticky mess that required additional cleaning.
Conclusion: Marshmallow enemas are no less safe for diabetics than any other type of enema.
Caveat: I am not a medical doctor, biochemist or associated in any way (except as a patient) with any medical profession. This post is my personal reasearch and experimentation. If you are diabetic and have concerns about this type of enema, don't do it.