A topic in the Open Knowledge Graph — a free, open map of 15,290 topics and the order to learn them in.
Vitamins are organic micronutrients required in small amounts for normal physiological function. They are divided into fat-soluble vitamins (A, D, E, K), which are stored in adipose tissue and liver and can accumulate to toxic levels, and water-soluble vitamins (B-complex and C), which are generally excreted in urine and require more frequent dietary replenishment. Most vitamins function as coenzymes or precursors to coenzymes that catalyze metabolic reactions; deficiency disrupts those metabolic pathways in characteristic ways.
Create a two-column reference table distinguishing fat-soluble from water-soluble vitamins, noting key functions, deficiency diseases, and toxicity risk for each. Linking each vitamin to a specific enzyme or pathway makes abstract facts more memorable.
From your prerequisite work on digestion and absorption, you know that nutrients are processed differently depending on whether they are water-soluble or lipid-soluble. Vitamins follow exactly this logic. Fat-soluble vitamins (A, D, E, K) travel through the lymphatic system with dietary fat, are packaged into chylomicrons, and end up stored in adipose tissue and the liver. Because they accumulate rather than wash out, they can build up to toxic levels if you take megadoses — this is why vitamin A toxicity (causing liver damage, bone pain, and teratogenic effects in pregnancy) is a real clinical concern, whereas vitamin C overdose mostly just causes diarrhea.
Water-soluble vitamins (the eight B vitamins plus vitamin C) dissolve directly into the bloodstream after absorption and are filtered by the kidneys when in excess. The body cannot stockpile them significantly, which means deficiency develops faster when intake is inadequate — think of B12 as the exception that proves the rule: the liver can store several years' worth, but strict vegans who don't supplement will eventually deplete it. The key functional concept is that most vitamins act as coenzymes or coenzyme precursors — they partner with enzymes to catalyze specific metabolic reactions that the enzyme alone cannot complete. Niacin (B3) becomes NAD+, a molecule you may recognize from cellular respiration as the electron carrier. Riboflavin (B2) becomes FAD. Without these coenzymes, entire metabolic pathways stall.
The practical power of understanding vitamins as coenzymes is that deficiency symptoms become logically predictable rather than things to memorize. Thiamine (B1) is needed for the pyruvate dehydrogenase complex — so thiamine deficiency disrupts carbohydrate metabolism and, critically, glucose utilization in neurons, explaining the neurological devastation of Wernicke's encephalopathy. Folate and B12 are both needed for DNA synthesis (specifically, for recycling of the methyl groups required to build thymidine); deficiency of either causes megaloblastic anemia because rapidly dividing red blood cell precursors can't complete cell division. The cells grow large but can't split — large, immature, dysfunctional cells flood the blood.
The fat-soluble/water-soluble distinction has one more clinically important consequence: absorption requires fat. Conditions that impair fat absorption — Crohn's disease affecting the ileum, cystic fibrosis, cholestatic liver disease, bariatric surgery — predictably cause deficiencies of vitamins A, D, E, and K even when dietary intake is adequate. Vitamin K deficiency from fat malabsorption prolongs clotting times because factors II, VII, IX, and X require vitamin K for their final activation step. Recognizing these patterns lets you use the mechanism to anticipate which vitamins are at risk in a given patient, rather than memorizing disconnected lists.