water-soluble vitamin
Meaning of water-soluble vitamin: The vitamins that dissolve in water and leave in urine, so the body holds only a small working supply of most of them.
Definition of water-soluble vitamin
A water-soluble vitamin is a vitamin that dissolves in water rather than in fat, so it travels freely in blood and leaves the body in urine. Vitamin C and the eight B vitamins make up the group. Because little is held in reserve, a regular intake matters, and doses far above what the body can use are largely excreted rather than stored.
What does dissolving in water change?
Fat-soluble vitamins need bile and dietary fat to be absorbed, then park in the liver and fatty tissue for months. Water-soluble vitamins skip all of that. They are absorbed straight from the gut into blood, circulate unattached, and the kidneys filter out whatever is surplus. That is why bright yellow urine follows a high dose vitamin B2 tablet. It also means these vitamins leach into cooking water and are lost when that water is poured away, so steaming or roasting preserves more than long boiling.
How do they compare with fat-soluble vitamins?
| Water-soluble | Fat-soluble | |
|---|---|---|
| Members | C and the B group | A, D, E and K |
| Stored | Barely, apart from B12 | Yes, in liver and fat |
| Absorption needs fat | No | Yes, and bile too |
| Excess removed by | The kidneys, in urine | Slowly, if at all |
| Deficiency appears | Within weeks to months | Usually after longer |
Is vitamin B12 an exception?
It is, and the old teaching that none of these vitamins are stored is wrong because of it. The liver holds enough B12 to last for years, which is why deficiency from a diet lacking it develops very slowly. B12 also needs a protein made by the stomach in order to be absorbed at all, so stomach surgery or an autoimmune attack on those cells causes deficiency even when the diet is rich in it. Folate stores, by contrast, run down in a matter of months.
Can there be too much?
Excretion protects against most excess, but not all of it. Sustained high dose vitamin B6 damages sensory nerves. Large doses of niacin cause flushing and can affect the liver. Heavy vitamin C loads bring diarrhea and, in susceptible people, kidney stones. The reference figures published as dietary reference intakes include upper limits for exactly this reason, and a dietary supplement can exceed them easily where food alone cannot.
Used in a sentence
Because thiamine is a water-soluble vitamin, stores run down within weeks of poor intake.
Good to know
The common claim that no water-soluble vitamin is stored does not hold for vitamin B12, which the liver keeps in reserve for years, so dietary deficiency of it appears far later than deficiency of folate or thiamine.
Word origin
Where “water-soluble vitamin” comes from
Vitamin joins the Latin vita, meaning life, with amine, the chemical family the first vitamin identified was thought to belong to.