Physiological Norms and Dynamics
In humans, the arterial blood glucose concentration remains stable outside of meals, ranging from 3.3 to 5.5 mmol/L (60–100 mg/dL). This is referred to as the basal level.
When a person consumes a carbohydrate-rich meal, a physiological state called alimentary hyperglyglucosedemia (or postprandial hyperglycemia) occurs. Sugar concentration begins to rise, peaking approximately 30 to 60 minutes post-ingestion. At the peak of this postprandial period, values may reach 7–8 mmol/L (120–140 mg/dL). A healthy body requires about two hours to completely clear this spike and return values to the baseline normal.
The entire cycle can be divided into several key phases:
- Absorptive period (fed state): The period of active nutrient absorption from the gastrointestinal tract.
- Postabsorptive period (fasting state): The period when nutrients are no longer absorbed from the gut, and the body shifts to utilizing internal reserves.
- Starvation period: Occurs during prolonged food deprivation.
Biochemistry of the Absorptive Period
Immediately following a carbohydrate meal, the circulatory pattern changes dramatically. In the hepatic portal vein, which drains blood from the intestines, the glucose concentration rises tremendously—up to 10–20 mmol/L (180–360 mg/dL). In response, the pancreas secretes insulin, while glucagon secretion is suppressed. Consequently, the insulin-to-glucagon ratio increases sharply.
Processes in the Liver The liver takes the primary carbohydrate hit. Insulin triggers the following reactions here:
- The glucokinase reaction is accelerated, trapping free glucose within hepatocytes.
- Phosphoprotein phosphatase enzymes are activated, which cleave phosphate groups from key regulatory proteins.
- As a result of dephosphorylation, glycogen synthase transitions into an active state, whereas glycogen phosphorylase transitions into an inactive state (inhibited).
Net result for the liver: the generated glucose-6-phosphate is massively channeled into glycolysis (for energy production) and glycogenesis (for storage as glycogen).
Processes in Muscle and Adipose Tissue These tissues are insulin-dependent. Elevated insulin levels signal specialized carrier proteins known as GLUT-4 transporters. They translocate from intracellular vesicles directly to the plasma membrane. This opens the gates for glucose, allowing it to rapidly leave the bloodstream and enter the cells. In skeletal muscle, as in the liver, insulin additionally stimulates glycogen synthesis. Due to active glucose uptake by these three major tissues (liver, muscle, and adipose), blood glucose concentration normalizes within approximately 2 hours.
Regulation in the Postabsorptive Period and Starvation
Once food has been digested and assimilated, blood glucose levels begin to drop steadily. Along with it, the insulin-to-glucagon ratio falls—and glucagon takes center stage.
Preventing hypoglycemia is the primary goal of this period. To maintain normoglycemia, the body relies on two main mechanisms:
- Glycogenolysis — the breakdown of hepatic glycogen stores.
- Gluconeogenesis — the de novo synthesis of glucose from non-carbohydrate precursors (amino acids, lactate, glycerol).
Temporal Dynamics of Sugar Sources:
- If fasting has lasted for about 12 hours, hepatic glycogen remains the primary supplier of blood glucose. Glucagon activates glycogen phosphorylase, mobilizing carbohydrate reserves.
- If starvation continues for 24 hours, liver glycogen stores are depleted to near zero. From this point onward, gluconeogenesis becomes the sole mechanism protecting the organism from a catastrophic drop in blood sugar.