Excess riboflavin can cause which of the following?

• A. Increased iron absorption
• B. Hypoglycemia
• C. Inhibition of zinc absorption
• D. Nephrotoxicity

Answer: (C)

Riboflavin (vitamin B2) is water-soluble, so excess amounts are typically excreted and toxicity is rare; the usual noticeable effect of very high intakes is bright yellow urine rather than harm. When intake is extraordinarily high, interactions with other nutrients can occur because nutrients compete for absorption or influence gut transport processes. In this context, there is some thought that very large amounts of riboflavin could interfere with zinc absorption, making inhibition of zinc absorption the plausible consequence among the options. The other possibilities—increased iron absorption, hypoglycemia, or nephrotoxicity—aren’t supported as typical outcomes of excessive riboflavin intake.