Chapter 2: The Fed or Absorptive State

The fed state, or absorptive state, represents the metabolic period immediately following nutrient intake when the body actively digests, absorbs, and processes dietary macronutrients for distribution and storage. This chapter examines how insulin and glucagon serve as the primary hormonal regulators during this phase, with insulin promoting anabolic processes including glucose uptake across tissues, glycogen synthesis in the liver and skeletal muscle, fatty acid synthesis and triacylglycerol storage, and protein building, while simultaneously suppressing catabolic fuel mobilization. Carbohydrate metabolism begins with enzymatic breakdown by salivary and pancreatic amylases, followed by monosaccharide absorption into the hepatic portal circulation, where glucose serves as the central biosynthetic precursor and primary energy substrate for the brain and erythrocytes. Protein digestion proceeds through sequential action of pepsin and pancreatic proteases like trypsin, liberating amino acids that are incorporated into new protein synthesis, converted to neurotransmitters or heme, or oxidized for energy production. Lipid processing involves bile salt emulsification of dietary triglycerides, pancreatic lipase-mediated hydrolysis, micelle formation within the intestinal lumen, and packaging into chylomicrons that transport dietary lipids to peripheral tissues, particularly adipose depots. The liver emerges as the metabolic hub during absorption, converting excess glucose into hepatic glycogen and de novo fatty acids while exporting triacylglycerols as very low-density lipoproteins. The chapter explores the lipoprotein transport system encompassing chylomicrons, VLDL, LDL, and HDL particles and their roles in triglyceride and cholesterol distribution, establishing how dysregulation contributes to hyperlipidemia and atherosclerotic risk. Clinical integration through metabolic syndrome and type 2 diabetes illustrates how excessive energy storage and abdominal adiposity, assessed through BMI and waist circumference measurements, increase cardiovascular morbidity. The chapter concludes by synthesizing these coordinated metabolic processes to demonstrate how nutrient excess is efficiently stored as glycogen and fat while establishing the biochemical foundation for understanding metabolic disease development.