When blood glucose is >180 mg/dl, the ability of renal sodium-dependent glucose transporters (SGLT) to reclaim glucose is impaired, and glucose “spills” into urine. The loss of glucose is accompanied by the loss of water, resulting in the characteristic polyuria (with dehydration) and polydipsia of diabetes.
B. Metabolic changes
The metabolic abnormalities of T1D result from a deficiency of insulin that profoundly affects metabolism in three tissues: liver, skeletal muscle, and white adipose (Fig. 25.3).
1.
Hyperglycemia and ketonemia: Elevated levels of blood glucose and ketone bodies are the hallmarks of untreated T1D (see Fig. 25.3). Hyperglycemia is caused by increased hepatic production of glucose via gluconeogenesis, combined with diminished peripheral utilization (muscle and adipose tissue have the insulin-regulated glucose transporter GLUT-4; see p. 97). Ketonemia results from increased mobilization of fatty acids (FA) from triacylglycerol (TAG) in adipose tissue, combined with accelerated hepatic FA β-oxidation and synthesis of 3hydroxybutyrate and acetoacetate (ketone bodies; see p. 196). [Note: Acetyl coenzyme A from β-oxidation is the substrate for ketogenesis and the allosteric activator of pyruvate carboxylase, a gluconeogenic enzyme.] Diabetic ketoacidosis (DKA), a type of metabolic acidosis caused by an imbalance between ketone body production and use, occurs in 25%–40% of those newly diagnosed with T1D and may recur if the patient becomes ill (most commonly with an infection) or does not comply with therapy. DKA is treated by replacing fluid and electrolytes and administering short-acting insulin to gradually correct hyperglycemia without precipitating hypoglycemia.
2.
Hypertriacylglycerolemia: Not all of the FA flooding the liver can be disposed of through oxidation and ketone body synthesis. These excess FA are converted to TAG, which are packaged and secreted in very-lowdensity lipoproteins ([VLDL] see p. 230). Chylomicrons rich in dietary TAG are secreted by the intestinal mucosal cells following a meal (see p. 227). Because lipoprotein TAG degradation catalyzed by lipoprotein lipase in the capillary beds of adipose tissue (see p. 228) is low in diabetes (synthesis of the enzyme is decreased when insulin levels are low), the plasma chylomicron and VLDL levels are elevated, resulting in hypertriacylglycerolemia (see Fig. 25.3).
C. Treatment