Complex sentence: Of increasing concern in the western world is a rise in nut allergies; as royal icing is often used together with marzipan to decorate cakes, it is becoming increasingly common for these to be substituted with fondant.
Split into simpler sentences: Of increasing concern in the western world is a rise in nut allergies. As royal icing is often used together with marzipan to decorate cakes, it is becoming increasingly common for these to be substituted with fondant.