Glucose in humans and most animals primarily comes from the diet, but there are additional internal sources and pathways that maintain blood glucose levels. Direct dietary sources
- Carbohydrates in the foods eaten are digested to monosaccharides, predominantly glucose, which are absorbed into the bloodstream.
- Starch and other complex carbohydrates are broken down by digestive enzymes in the mouth and small intestine, yielding glucose and other sugars that are absorbed into the blood.
Internal production (gluconeogenesis and glycogenolysis)
- Glycogenolysis: The liver and, to a lesser extent, skeletal muscle break down stored glycogen into glucose-6-phosphate, which is converted to glucose and released into the bloodstream to maintain blood glucose during fasting or between meals.
- Gluconeogenesis: The liver (and to a smaller extent the kidneys) generate new glucose from non-carbohydrate precursors, including lactate, glycerol, and certain amino acids. This becomes especially important during prolonged fasting or intense exercise.
Other notes on glucose handling
- Glucose uptake into cells is mediated by glucose transporters. Some transporters act independently of insulin (e.g., in the liver, brain, red blood cells), while others (such as GLUT4) are insulin-responsive and prominent in muscle and fat tissues.
- In ruminants and some other animals, gluconeogenesis from propionate (a volatile fatty acid produced in the gut) can be a major source of glucose.
In summary, the glucose in humans and other animals mainly derives from:
- dietary carbohydrates
- hepatic glycogen breakdown (glycogenolysis)
- hepatic and renal gluconeogenesis from non-carbohydrate precursors
If you’d like, I can tailor this explanation to a specific species (e.g., humans, nonhuman mammals, or birds) or focus on particular metabolic states (fasting, postprandial, or exercise).
