What Are Water-Soluble Vitamins? B and C

Quick answer

Water-soluble vitamins dissolve in water: vitamin C and the B vitamins, such as thiamin, riboflavin, niacin, B6, folate, biotin and B12. The body stores only small amounts of most of them, and absorbed excess is largely lost in urine. Vitamin B12 is the exception, with body stores of about 1 to 5 mg.^1,2,3

Key facts

Riboflavin is one of the B vitamins, which are all water soluble.²
Above 1 g a day of vitamin C, absorption falls below 50% and unmetabolized vitamin C is excreted in urine.¹
The body stores about 1 to 5 mg of vitamin B12, roughly 1,000 to 2,000 times a typical day's intake.³
Upper limits still apply to some: for adults, 2,000 mg a day of vitamin C, 35 mg of supplemental niacin and 1,000 mcg of folic acid.^1,4,5

Which vitamins are water soluble

Water-soluble vitamins (adults)^{1,2,3,4,5,6,7,8}
Vitamin	Storage	Upper limit
C	Absorbed excess is excreted in urine	2,000 mg a day
Thiamin (B1)	Very small amounts, mainly in the liver	None set
Riboflavin (B2)	Small amounts in liver, heart and kidneys	None set
Niacin (B3)	Excess is converted and excreted in urine	35 mg a day from supplements
B6	No storage figure in NIH's fact sheet	100 mg a day (US); EFSA set 12 mg in 2023
Folate	15 to 30 mg in the body, about half in the liver	1,000 mcg a day of folic acid
Biotin	Mostly stored in the liver	None set
B12	About 1 to 5 mg stored	None set

What water soluble means in practice

Because these vitamins dissolve in water, cooking water draws them out of food. About twice as much riboflavin is lost when food is boiled as when it is steamed or microwaved, and vitamin C is also destroyed by heat.^1,2

Absorption also has limits. NIH notes that thiamin absorption declines rapidly at intakes above 5 mg, and that higher doses of vitamin C are absorbed less efficiently than moderate ones, so much of a very large dose is not absorbed.^1,6

B12 and folate are partial exceptions to the low storage rule. The body keeps reserves of both, which is why a shortfall in B12 can take several years to show.^3,5

Water soluble is not the same as harmless

Extra amounts are not always flushed away without effect. Large doses of vitamin C can cause diarrhea, nausea and stomach cramps. Taking 1 to 6 g of vitamin B6 a day for a year or more has caused severe nerve damage, which is why upper limits exist, and high biotin intakes can make laboratory test results falsely high or low.^1,7,8

Stay within the upper limits unless your doctor advises otherwise, and tell your doctor about biotin before any blood test.⁸

Questions people ask

Can you take too much of a water-soluble vitamin?

Yes. Large doses of vitamin C can upset the stomach and gut, and long-term high doses of vitamin B6 have been linked to nerve damage. Upper limits exist for vitamin C, B6, niacin and folic acid.^1,4,5,7

Which water-soluble vitamins are stored in the body?

Vitamin B12 is stored in the largest amounts, about 1 to 5 mg, and folate is also held in reserve, about half of it in the liver. Thiamin and riboflavin are stored only in small amounts, and absorbed vitamin C that is not used is excreted in urine.^1,2,3,5,6

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This page is information, not medical advice. Talk to your doctor or pharmacist about your own situation.