Two Glucose Curves: When the Signal Is Loud but the Response Is Weak
Two sedentary adults, both 45 years old, drink 75 g of glucose after an overnight fast. Blood is drawn at 0, 30, and 120 minutes and assayed for glucose (mg/dL) and insulin (µU/mL).
Person A - glucose: 88, 142, 96. Insulin: 6, 45, 22.
Person B - glucose: 112, 198, 164. Insulin: 22, 105, 130.
Neither has taken medication. Both have a parent with type 2 diabetes. The clinician explains that insulin signals muscle, liver, and fat cells to take up glucose from the blood, and that a rising blood glucose concentration normally stimulates the pancreas to release more insulin. A result of 140 mg/dL or higher at 120 minutes is considered abnormal.
1. Identify which person's negative feedback loop is functioning and which is not. For each person, name the sensor, the signal, the target cells, and the response, and cite the specific numbers that support your conclusion.
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The loop: sensor = pancreatic beta cells, which detect glucose; signal = insulin; targets = muscle, liver, and fat cells; response = increased glucose uptake and reduced liver glucose output, which lowers blood glucose and therefore reduces the stimulus.
Person A - the loop closes. Glucose rises to 142 at 30 minutes, then falls to 96, within 8 mg/dL of the fasting value. Insulin rises fourfold to 45 and then falls to 22 as the stimulus disappears. The response was adequate and the signal was then switched off. This is negative feedback operating as expected.
Person B - the loop does not close. Glucose peaks at 198 and is still 164 at 120 minutes, well above the 140 threshold and 52 mg/dL above the fasting value. Insulin is not low; it is 105 at 30 minutes and still 130 at 120 minutes - roughly three to six times Person A's values, and still rising while glucose remains high.
The critical comparison: a strong insulin signal is present and glucose is still not clearing. The defect is not in sending the signal but in the target cells' response to it, which is insulin resistance. B's beta cells are compensating by secreting far more insulin than A's to achieve a worse result.
2. A classmate concludes, "Person B's pancreas must be failing - that is why the glucose stays high." Use the insulin data to evaluate that claim, and describe what the numbers would look like if the pancreas truly did begin to fail.
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The claim is contradicted by the data. If the pancreas were failing, insulin output would be low, not high. Person B's insulin rises to 105 and reaches 130 at two hours, far above Person A's 45 and 22. A failing pancreas cannot produce a value three to six times higher than a healthy person's.
What is actually happening: the beta cells are working harder than normal, secreting extra insulin to overcome target cells that respond poorly. Fasting insulin of 22 versus 6 is itself a sign of chronic compensation - the pancreas is running high all the time just to hold fasting glucose at 112, which is already above normal.
If the pancreas did begin to fail, the pattern would invert in a specific way: insulin values would fall toward or below normal while glucose rose higher and stayed elevated longer. The appearance of high glucose with low insulin is the signature of beta-cell exhaustion, and it marks a later stage in the progression. Person B is currently in the earlier, compensated stage - which is exactly why the finding is worth acting on now. A person can spend years in this state before insulin secretion drops, and complications can accumulate during those years.
3. Both adults are offered a program of 30 minutes of brisk walking, five days a week. Explain, in terms of membrane proteins and feedback, how this could lower Person B's blood glucose even before any weight loss occurs. Then state one real limitation of the claim.
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The mechanism: in skeletal muscle, insulin normally raises glucose uptake by moving GLUT4 transporter proteins from internal vesicles to the cell membrane. Muscle contraction triggers the same GLUT4 translocation through a separate, insulin-independent pathway, so working muscle pulls glucose out of the blood even when insulin signaling is sluggish. This adds a second route for glucose disposal, lowering blood glucose without requiring more insulin and reducing the load on the beta cells that are overproducing it.
Feedback consequence: better glucose uptake means the stimulus to the pancreas is smaller, so insulin secretion can fall toward normal and target tissues become more responsive over weeks of training. The loop starts closing again from both ends.
Limitations to state honestly: the insulin-independent effect of a single session lasts only hours to a day or so, so the benefit depends on sustained activity rather than any one walk. Exercise also does not remove the underlying genetic and metabolic contributors to insulin resistance, and it will not by itself restore normal glucose tolerance in everyone. It is a genuine, mechanistically sound intervention - not a cure - and it should be presented as something that must be maintained to keep working.