A meal: the fed state
Picture breakfast. As glucose is absorbed, blood glucose climbs. Beta cells sense the rise and pour out insulin; alpha cells quiet down, so glucagon falls. Insulin moves GLUT4 to the surface of muscle and fat so they soak up glucose, and tells the liver to store the surplus as glycogen. Within a couple of hours glucose settles back toward baseline. This coordinated swing is [[glucose-homeostasis|glucose homeostasis]] in action — the loop from guide one, now staffed by the cells and hormones from guides two through four.
A fast and a sprint
Skip lunch and the loop runs in reverse. Insulin falls, glucagon rises, and the liver releases glucose — first by glycogenolysis, then increasingly by gluconeogenesis as the hours pass. Your glucose barely dips, even though you ate nothing, because the warehouse is being drawn down on schedule.
Now sprint for a bus. Exercising muscle suddenly needs fuel — but here is a beautiful detail: muscle contraction itself moves GLUT4 to the surface, drinking in glucose *without* needing extra insulin. At the same time, epinephrine and glucagon make the liver release more glucose to keep supply matched to demand. Three different situations — fed, fasting, exercising — and one steady output: blood glucose that barely budges.
When the loop breaks
Everything above assumes a working loop. [[diabetes-mellitus|Diabetes mellitus]] is what happens when it breaks, and there are two broad ways. In [[type-1-diabetes|type 1 diabetes]], the immune system destroys the beta cells, so there is little or no insulin — the lowering signal is simply absent. In [[type-2-diabetes|type 2 diabetes]], beta cells still make insulin but the target tissues stop listening: [[insulin-resistance|insulin resistance]] means it takes more and more insulin to get the same effect, until the beta cells cannot keep up. Either way the result is the same headline — glucose that stays too high.
How do you read months of glucose control from a single blood test? Through [[hba1c|HbA1c]]. Glucose slowly and permanently sticks to hemoglobin inside red blood cells; the more glucose around, over the roughly three-month lifespan of a red cell, the more gets stuck. So HbA1c is a running average of recent glucose — a memory that cannot be faked by skipping breakfast before the test.
Rule of thumb: HbA1c (%) -> estimated average glucose Formula: eAG (mg/dL) = 28.7 x A1c - 46.7 A1c 5% -> ~97 mg/dL (normal) A1c 6% -> ~126 mg/dL (prediabetes range) A1c 7% -> ~154 mg/dL (common treatment target) A1c 8% -> ~183 mg/dL A1c 9% -> ~212 mg/dL Quick mental version: each 1% step up in A1c is roughly +29 mg/dL of average glucose.