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If you take graduate level biochemistry, you study every aspect of the various metabolic pathways. If you don't have the precursors from food, the body will rob Peter to pay Paul from human tissue to keep things going for awhile. Eventually it cannot do so and so has prioritize just to do basic things like make the body control temperature and maintain respiration and heart rate to perfuse organs and maintain brain function. Eventually the patient loses their consciousness,then loses eyesight, goes comatose, then organ failure, then death.