Fat-Soluble vs Water-Soluble Vitamins: Storage, Absorption and Why It Matters for Excess

The difference at a glance

Why "water-soluble" does not mean "safe"

The NHS and MedlinePlus document harm from high supplement amounts of several water-soluble vitamins:

• Vitamin B6: 200 mg or more a day can cause loss of feeling in the arms and legs (peripheral neuropathy), usually improving after stopping, but sometimes permanent after large amounts taken for more than a few months. [8]
• Niacin: high doses of nicotinic acid can cause skin flushes, and high doses over a long time could lead to liver damage. [8]
• Folic acid: doses above 1 mg can mask the symptoms of vitamin B12 deficiency, which matters most for older people because B12 becomes harder to absorb with age. [8]
• Vitamin C: more than 1,000 mg a day can cause stomach pain, diarrhea and gas, which the NHS says should go away after stopping. [7] MedlinePlus says amounts above 2,000 mg a day are not recommended and, rarely, can lead to kidney stones. [9]

These are documented UK and US statements about supplement amounts, not instructions. For the fat-soluble group, the NHS says an average of more than 1.5 mg of vitamin A a day over many years may affect bones, [4] and too many vitamin D supplements over a long time can cause too much calcium to build up. [10] MedlinePlus says high-dose vitamin E supplements (alpha-tocopherol) might raise the risk of bleeding in the brain. [11]

Where the evidence runs out

Not every vitamin has an upper limit. EFSA's overview of Tolerable Upper Intake Levels (version 11, August 2025) lists no adequate data to derive a UL for thiamin (B1), riboflavin (B2), pantothenic acid, biotin, vitamin C and vitamin K, and says no adverse effects have been defined for B12. [12] That is not a finding that high amounts are safe. The NHS makes the same point for several: for thiamin, riboflavin, B12, vitamin E, vitamin K and biotin it says there is not enough evidence to know what high supplement doses do. [8][5][6] Lack of evidence is not proof of safety.

The same EFSA table lists ULs for some vitamins, including A (preformed), D, E, B6, niacin forms and folate from fortified foods or supplements. [12] Compare them on Vitamin Toxicity: Which Vitamins Have Upper Limits.