Why storage changes the toxicity risk
The fat-soluble vitamins A, D, E, and K dissolve in lipids and are stored in the liver and fatty tissue. Because the body holds a reserve, daily intake matters less day to day, but the same storage means excess intake (usually from high-dose supplements rather than food) can accumulate to harmful levels over time, especially for vitamins A and D.
Water-soluble vitamins, the B group and vitamin C, are not stored to the same degree and surplus amounts are largely excreted in urine. This makes acute toxicity uncommon from normal intake, though it also means stores deplete faster and regular intake matters more.
What this means for supplementing safely
The contrast explains a common safety asymmetry: it is easier to reach a harmful level with fat-soluble vitamins than with most water-soluble ones, which is why upper intake limits are a particular concern for A and D supplements. Some water-soluble vitamins still have practical limits, such as vitamin B6 at sustained high doses.
This is general educational information, not medical advice. Dosing decisions, particularly around fat-soluble vitamin supplements or any high-dose regimen, should be made with a qualified professional who can account for individual needs and existing intake.