Chapter 58: Metabolism

The chapter distinguishes between fat-soluble vitamins including A, D, E, and K, which are absorbed alongside dietary lipids through micelle formation and packaged into chylomicrons for transport, and water-soluble vitamins comprising the B-complex group and vitamin C, which depend on carrier-mediated transport mechanisms specific to their chemical structure. The absorption of vitamin B12 receives detailed examination, emphasizing the critical role of intrinsic factor secreted by gastric parietal cells and the site-specific uptake in the terminal ileum, with pathological consequences of deficiency including megaloblastic anemia and neurological dysfunction. The chapter explores mineral absorption with particular attention to calcium homeostasis, which is regulated by vitamin D and parathyroid hormone signaling, iron metabolism involving the regulatory proteins ferritin, transferrin, and the iron-sensing hormone hepcidin, and iodine uptake necessary for thyroid hormone synthesis. Additional coverage includes magnesium and zinc absorption along with other trace elements, describing their specific transport pathways and physiological roles. The chapter integrates discussion of clinical manifestations of micronutrient deficiency states such as rickets and osteomalacia from inadequate vitamin D, scurvy from vitamin C insufficiency, microcytic and pernicious anemias from iron and B12 deficiency respectively, and endemic goiter resulting from iodine deficiency. By connecting molecular transport mechanisms to their systemic health consequences, this chapter demonstrates how the gastrointestinal tract orchestrates the selective absorption and distribution of essential micronutrients required for metabolic regulation and tissue maintenance.