The Vatican thinks the removal of feeding tubes from people in vegetative states to be an immoral act.
The Vatican issued the statement in response to questions from bishops in the United States in July 2005 - just months after the case of an American woman, Terri Schiavo, made world headlines.
She died March 31, 2005, in a Florida hospice after her parents unsuccessfully battled a court order to have her feeding tube removed. She died 13 days after the tube was removed.
"A patient in a 'permanent vegetative state' is a person with fundamental human dignity and must, therefore, receive ordinary and proportionate care which includes, in principle, the administration of water and food even by artificial means," the Vatican said in a statement.
At the time, the Vatican condemned Schiavo's death as "arbitrarily hastened" and called the removal of her feeding tube a violation of the principles of Christianity and civilization.
Friday's statement said the Vatican was asked whether the administration of food and water to a patient in a vegetative state was morally obligatory except when they cannot be assimilated by the patient's body or administered without causing significant physical discomfort. The answer was yes.
The statement said exceptional cases, such as the inability of a patient to cope with feeding or food shortages in poverty-stricken or remote areas, "take nothing away from the general ethical criterion."
The Vatican noted that Pope John Paul II told a 2004 medical conference on ethical dilemmas that providing food and water to people in vegetative states should be considered natural, ordinary and proportional care.
Bishop William E. Lori of Bridgeport, Connecticut, chairman of the U.S. bishops Committee for Doctrine, said the American bishops hoped the Vatican's explanations would provide guidance to pastors, ethicists, doctors, nurses and families involved in the issue.