Extra vitamin C is harmless because you just pee it out
MisleadingThe Claim
Because vitamin C is water-soluble, many people assume megadoses are completely safe since the excess simply leaves in urine.
Origin
Water-soluble vitamins are generally considered low-risk, and high-dose vitamin C has been promoted for decades.
What Science Says
Excess vitamin C is partly converted to oxalate, a main component of kidney stones. In large US cohorts (over 40,000 men followed for about 11 years), supplemental vitamin C of 1,000 mg/day or more was linked to a 19% higher risk of kidney stones in men (HR 1.19), while no association was seen in women. A meta-analysis of four studies similarly found higher stone risk with ascorbic acid supplements in men but not women.
Verdict
Misleading