A topic in the Open Knowledge Graph — a free, open map of 15,290 topics and the order to learn them in.
Vitamins A, D, E, and K require dietary fat and bile for intestinal absorption and are stored in liver and adipose tissue. Vitamin A (retinol, beta-carotene) is essential for vision, immune function, and cell differentiation. Vitamin D functions as a hormone regulating calcium homeostasis and bone mineralization; synthesis occurs in skin upon UV-B exposure. Vitamin E acts as a lipid-soluble antioxidant protecting cell membranes from oxidative damage. Vitamin K is a cofactor for carboxylation reactions required for clotting factors and bone proteins. Because these vitamins are stored, deficiency develops slowly but toxicity from over-supplementation is possible.
From your study of vitamins overview and dietary fats and lipids, you know that vitamins are micronutrients needed in small amounts for essential biochemical reactions, and that dietary fats are absorbed through a bile-dependent process involving micelle formation and chylomicron packaging. Fat-soluble vitamins — A, D, E, and K — follow exactly this absorption pathway. They dissolve in lipid micelles in the small intestine, are packaged into chylomicrons, and enter the lymphatic system before reaching the bloodstream. This shared mechanism has two important consequences: diets very low in fat impair absorption of all four, and these vitamins can accumulate in liver and adipose tissue to toxic levels if over-supplemented — unlike water-soluble vitamins, which are excreted in urine when in excess.
Vitamin A (retinol and its plant-derived precursor beta-carotene) plays three distinct roles. In vision, retinol is converted to retinal, which combines with the protein opsin to form rhodopsin in rod cells — the pigment that enables dim-light vision. Vitamin A deficiency causes night blindness, the earliest clinical sign, progressing to irreversible corneal damage. Systemically, retinoic acid (the acid form of vitamin A) acts as a nuclear hormone, binding receptors that regulate gene expression for cell differentiation and immune function. This is why vitamin A deficiency also impairs immunity and increases infection mortality in children. Beta-carotene from plants is cleaved to retinol, but the conversion is inefficient and regulated — the body converts only as much as it needs, which is why eating carrots won't cause vitamin A toxicity, but high-dose retinol supplements can.
Vitamin D is unusual because it functions as a hormone rather than a classic vitamin. Upon UV-B irradiation of skin, 7-dehydrocholesterol is converted to cholecalciferol (vitamin D₃), which is then activated by sequential hydroxylation in the liver (to 25-hydroxyvitamin D, the storage form measured in blood tests) and kidney (to 1,25-dihydroxyvitamin D, the active hormone calcitriol). Calcitriol's primary role is calcium homeostasis: it stimulates intestinal calcium absorption and works with parathyroid hormone to maintain serum calcium within its narrow physiological range. Deficiency causes rickets in children (soft, bisfigured bones) and osteomalacia in adults. Because UV-B intensity varies dramatically with latitude, season, cloud cover, and skin pigmentation, dietary and supplemental sources are essential for populations with limited sun exposure.
Vitamin E (alpha-tocopherol) is the primary lipid-soluble antioxidant in cell membranes. Cell membranes are rich in polyunsaturated fatty acids, which are vulnerable to oxidative chain reactions triggered by free radicals. Vitamin E intercepts these reactions by donating a hydrogen atom to the radical, neutralizing it before it can propagate damage across the membrane. Vitamin K exists in two main forms — K₁ (phylloquinone, from leafy greens) and K₂ (menaquinones, from fermented foods and gut bacteria) — and both serve as cofactors for gamma-carboxylation, a post-translational modification that activates several clotting factors (II, VII, IX, X) and bone proteins (osteocalcin, matrix Gla protein). Warfarin anticoagulants work by blocking vitamin K recycling, reducing clotting factor activity. Because newborns have low vitamin K stores and gut bacteria haven't yet colonized, neonatal vitamin K injection is standard practice to prevent hemorrhagic disease.