Fat-Soluble Vitamins

Definition, classification & the fat- vs water-soluble divide

• Vitamins — non-energy-yielding organic compounds essential for normal metabolism that must be supplied in small quantities in the diet; the pharmacological importance is chiefly prophylaxis and treatment of deficiency states, with some empirical uses at supra-physiological doses.
• Vitamers — the different chemical forms/precursors of one vitamin (analogous to isomers) — e.g. retinol (A₁), dehydroretinol (A₂) and β-carotene (provitamin) are all vitamers of vitamin A.
• Two-group classification — Fat-soluble = A, D, E, K vs water-soluble = B complex, C. The fat-soluble group (except K) is stored for prolonged periods and is liable to cause cumulative toxicity; A and D act through nuclear receptors of the steroid/thyroid superfamily (hormone-like). Water-soluble vitamins are meagrely stored, renally cleared and act as enzyme cofactors (out of scope — cross-reference only).
• Central pharmacological contrast — because A, D and E are lipophilic and stored in liver & adipose tissue, regular excess accumulates → cumulative hypervitaminosis; the water-soluble surplus is simply renally cleared. Vitamin K is the fat-soluble exception — stored only to a limited extent and turned over fast, so its deficiency can appear relatively quickly.
• Over-use myths — vitamins as a class are over-promoted, over-prescribed and over-used; the myth that "vitamins are harmless" is specifically dangerous for the fat-soluble group, which carries a real accumulation/toxicity risk.
• Vitamin D — pointer only — vitamin D (ergocalciferol D₂ / cholecalciferol D₃ / calcitriol) is a fat-soluble vitamin, but its full pharmacology belongs to calcium homeostasis / bone mineral metabolism and has its own dedicated site topic — included here only as a class member for the storage/toxicity contrast.