Casey Johnson's Death Caused by Neglect

Casey Johnson died because she simply did not take care of herself ... sources connected with the autopsy tell TMZ.

The L.A. County Coroner says the cause of death was "Diabetic Ketoacidosis" and the manner of death was "natural."

But our sources say the Coroner's findings clearly show Johnson -- who died at 30 -- was not taking her insulin shots, was not exercising, did not follow her strict diet and was not taking other medicines she needed.

Diabetic Ketoacidosis is a technical term for lack of insulin, which often leads to a diabetic coma. We previously reported where Johnson was hospitalized for diabetic comas at least twice before her death.