Fat-Soluble Vitamins (A, D, E, K)
Vitamin D and its analogues, vitamin K and coagulation, and vitamins A and E in deficiency and toxicity.
Vitamin D: Activation, Deficiency and Which Form to Prescribe
We call it a vitamin, but vitamin D behaves like a hormone — one your skin manufactures from sunlight and then hands to the liver and the kidney to switch on in two deliberate steps. Get those steps and everything else follows: why we measure one form but the body uses another, why a child's legs bow and an adult's bones ache when it is missing, and — the whole point at the bedside — why one patient needs a cheap capsule of plain vitamin D while another needs an expensive activated analogue. The prescribing decision is not "how much?" so much as "which form?", and the answer is written in the activation pathway.
Vitamin K: Clotting Factors, Warfarin and the Newborn
The "K" comes from the German Koagulation — and that single letter tells you almost everything this vitamin does. Vitamin K is the switch that turns a handful of freshly made clotting factors from useless protein into working machinery. Take it away and the blood loses its ability to clot; that is exactly what warfarin does on purpose, and exactly why every newborn gets an injection of it in the first minutes of life. Learn vitamin K and you have quietly learned the backbone of anticoagulation, one classic poisoning, and one of the most successful public-health interventions ever devised.
Vitamins A and E: Vision, Retinoids and the Toxicity Trap
Water-soluble vitamins forgive you. Take too much vitamin C and your kidneys flush the surplus away by morning. The fat-soluble vitamins — A, D, E and K — are different animals. They dissolve in fat, ride with dietary lipid, and lodge in the liver and adipose tissue for months. That storage is a gift in famine and a hazard in excess: what accumulates can also poison. Nowhere is this double edge sharper than with vitamin A, a molecule the eye cannot see without, that a global child-health programme distributes by the billion of doses — and that, in the wrong dose or the wrong trimester, deforms an unborn child.
