Complex sentence: Diabetes mellitus, or simply diabetes, is a group of metabolic diseases in which a person has high blood sugar, either because the pancreas does not produce enough insulin, or because cells do not respond to the insulin that is produced.
Split into simpler sentences: Diabetes mellitus, or simply diabetes, is a group of metabolic diseases in which a person has high blood sugar. This is either because the pancreas does not produce enough insulin, or because cells do not respond properly to the insulin that is produced.