Water-soluble vitamins are the vitamins that dissolve in water — the B-complex vitamins (B1, B2, B3, B5, B6, B12, folate, biotin) and vitamin C. Because they dissolve in the body’s water, they are not stored in significant amounts (B12 is the exception, stockpiled in the liver) and excesses are excreted in urine — which is why they must be supplied regularly in food and why megadoses are less toxic than fat-soluble vitamin overdoses, though not harmless. They leach into cooking water, so steaming and using cooking liquids preserves them. Their shared chemistry explains their shared behaviour: fragile to heat and light, easily lost in processing, and essential to energy metabolism.