Why does the risk of toxicity arise specifically from high supplemental intake of Vitamins A, D, E, and K?

Because they are stored in body fat and the liver, allowing harmful buildup

The key factor differentiating the toxicity risk between the two vitamin classes lies in storage capability. Fat-soluble vitamins (A, D, E, K) are absorbed in the presence of dietary fat and, crucially, are stored long-term in the body’s fatty tissues and the liver. When individuals consume supplements providing these nutrients far in excess of the Recommended Dietary Allowance (RDA), the body accumulates the surplus rather than excreting it readily. This consistent buildup can eventually push nutrient levels into a range where they cause adverse physiological effects, leading to toxicity, a phenomenon rarely observed with water-soluble vitamins which are easily cleared by the kidneys.