Overview
Tissues differ in how they handle carbohydrates: each takes up, stores, burns or makes glucose in its own way. Blood glucose is kept steady by hormones and by the coordinated work of the gastrointestinal tract, liver, nervous tissue, muscle, kidney and adipose tissue.
- The body tightly regulates blood glucose levels to ensure a constant supply of energy for cellular functions.
- Glucose homeostasis involves a balance between glucose uptake from the diet, glucose production via gluconeogenesis, and glucose utilization in tissues.
Gastrointestinal tract
Dietary starch is broken down by salivary and pancreatic amylase to maltose and oligosaccharides, which brush-border enzymes of the small intestine (maltase, sucrase-isomaltase, lactase) split into glucose, galactose and fructose.
Glucose and galactose are absorbed into enterocytes by the Na⁺-dependent transporter SGLT1 (secondary active transport) and leave on the blood side through GLUT2; fructose enters through GLUT5 and also leaves through GLUT2. The monosaccharides pass by the portal vein to the liver. Lactase deficiency leaves lactose unabsorbed, so it is fermented by colonic bacteria and causes osmotic diarrhoea and gas. Dietary fibre is not digested but is fermented by gut bacteria to short-chain fatty acids.