With insulin absent and glucagon unopposed, the metabolic map of type 1 diabetes can be read directly from the normal pathways you already know, run in reverse. In the liver, the missing insulin signal means glycogen synthase is no longer activated and gluconeogenic enzymes such as PEPCK and G6Pase are no longer suppressed, so the liver releases glucose into blood instead of storing it. In muscle and adipose tissue, GLUT4 never reaches the plasma membrane, so post-meal glucose uptake is lost. In adipose tissue, unrestrained lipolysis releases free fatty acids; the liver converts them to ketone bodies through beta-oxidation and ketogenesis, and without insulin to restrain that production, ketone bodies accumulate faster than peripheral tissues can use them. The combined effect is hyperglycemia with osmotic diuresis and, when ketone production outpaces clearance, ketoacidosis.
Two laboratory markers separate this picture from type 2 diabetes. Islet autoantibodies, including glutamic acid decarboxylase (GAD65), islet antigen-2 (IA-2), zinc transporter 8 (ZnT8), and insulin autoantibodies, indicate that the immune system is targeting beta cells. C-peptide is released in equimolar amounts with insulin from the same proinsulin precursor, so a low or undetectable C-peptide confirms that the pancreas is not secreting meaningful insulin. Together, positive autoantibodies and low C-peptide establish absolute deficiency rather than resistance.