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Diabetes Mellitus
Type 1 vs Type 2 Pathophysiology
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Diabetes mellitus is a group of metabolic disorders defined by chronic hyperglycemia caused by problems with insulin production, insulin action, or both. The two main forms, type 1 and type 2 diabetes, can both raise blood glucose, but they arise from very different pathophysiologic mechanisms. Understanding those mechanisms helps explain the age of onset, body habitus, laboratory findings, and treatment strategies seen in patients. This distinction is essential for early diagnosis and prevention of acute and chronic complications.
In type 1 diabetes, autoimmune destruction of pancreatic beta cells causes an absolute insulin deficiency, so glucose cannot be effectively taken up by insulin-sensitive tissues and ketone production may become excessive. In type 2 diabetes, the earliest problem is usually insulin resistance in muscle, liver, and adipose tissue, followed over time by progressive beta cell dysfunction and relative insulin deficiency. Both conditions increase hepatic glucose output and impair normal fuel metabolism, but type 2 diabetes is more strongly linked to obesity, chronic inflammation, and excess free fatty acids. The resulting hyperglycemia damages blood vessels, nerves, kidneys, and retina through long term metabolic stress.
Key Facts
- Type 1 diabetes: autoimmune beta cell destruction leads to absolute insulin deficiency.
- Type 2 diabetes: insulin resistance plus progressive beta cell failure leads to relative insulin deficiency.
- Normal fasting plasma glucose is about 70 to 99 mg/dL; diabetes is diagnosed at fasting plasma glucose greater than or equal to 126 mg/dL.
- HbA1c greater than or equal to 6.5% is consistent with diabetes mellitus.
- Insulin promotes glucose uptake in muscle and adipose tissue by increasing GLUT4 translocation to the cell membrane.
- In uncontrolled insulin deficiency, increased lipolysis and hepatic ketogenesis can produce diabetic ketoacidosis, with ketone body production rising as insulin falls and glucagon rises.
Vocabulary
- Beta cell
- A beta cell is an endocrine cell in the pancreatic islets that synthesizes and secretes insulin.
- Insulin resistance
- Insulin resistance is a reduced tissue response to normal or high levels of insulin, especially in muscle, liver, and adipose tissue.
- Ketogenesis
- Ketogenesis is the liver process that converts fatty acid breakdown products into ketone bodies during low insulin states.
- Hyperglycemia
- Hyperglycemia is an abnormally high concentration of glucose in the blood.
- HbA1c
- HbA1c is glycated hemoglobin that reflects average blood glucose over roughly the previous 2 to 3 months.
Common Mistakes to Avoid
- Assuming all diabetes is caused by eating too much sugar, which is wrong because type 1 diabetes is primarily an autoimmune disease and type 2 diabetes involves complex insulin resistance and beta cell dysfunction.
- Thinking insulin levels are always low in type 2 diabetes, which is wrong because early type 2 diabetes often has normal or high insulin levels before beta cell failure progresses.
- Confusing diabetic ketoacidosis with all forms of hyperglycemia, which is wrong because marked ketone production is much more typical of severe insulin deficiency in type 1 diabetes than in most type 2 cases.
- Believing glucose cannot enter any cells without insulin, which is wrong because some tissues such as brain and red blood cells use insulin-independent glucose transporters.
Practice Questions
- 1 A patient has a fasting plasma glucose of 132 mg/dL on two separate tests and an HbA1c of 7.1%. Do these values meet criteria for diabetes, and which thresholds support your answer?
- 2 A person with type 1 diabetes misses insulin injections for 24 hours. Explain why blood glucose rises and why ketone production also increases, using the roles of liver, adipose tissue, and glucagon.
- 3 Two patients both have chronic hyperglycemia. One is lean, has positive autoantibodies, and low C peptide. The other has central obesity, acanthosis nigricans, and high fasting insulin early in disease. Identify the most likely diabetes type for each and explain the pathophysiology behind your choice.