The usually ingredients used to make marshmallows consists of water, sugar and gelatin. They are mixed together into a consistency that is spongy, molded into cylindrical pieces, and coated with corn starch.
Some other recipes for marshmallows consist of eggs. Marshmallows are a sugar-based confection.
So can guinea pigs eat marshmallows at all?
No, they cannot eat them at all as they are far too sweet for guinea pigs to eat.
If they were to eat marshmallows it would give them a stomach ache.
Additionally most guinea pigs would probably stick up their nose to marshmallows anyways and not want to eat them.
But anyways it is important to avoid giving marshmallows to guinea pigs.