How vitamins are classified and function in the body Vitamins are organic compounds required in small amounts to regulate metabolism, enzyme activity, and cellular signaling. They are categorized by solubility—either in fat or water—which determines how they are absorbed, transported, and stored in the body. 1️⃣ Fat-soluble vitamins (A, D, E, K) These dissolve in lipids, require bile for absorption, and are stored in the liver and adipose tissue. Because they accumulate, toxicity can occur with