Category: transport
Explains the bright yellow-green, almost-glowing urine that lands 1-2 hours after a B-complex, multivitamin, or standalone riboflavin dose — the most-photographed supplement side effect on the internet. Riboflavin (vitamin B2) is natively fluorescent (peak excitation ~450 nm, peak emission ~525 nm) — the same property that makes it useful as a tracer in pharmaceutical disintegration tests. Intestinal absorption is via the SLC52 family of riboflavin transporters (RFVT1/SLC52A1, RFVT2/SLC52A2, RFVT3/SLC52A3), characterized by Yonezawa 2008 and Yao 2010; the apical-side RFVT3 in enterocytes is the rate-limiting step and is saturable at modest single doses (Zempleni 1996 showed ~27 mg as the practical upper limit before disproportional excretion). Beyond that ceiling, unabsorbed riboflavin moves into stool while absorbed-but-unneeded riboflavin (above the tissue-storage requirement of ~1.3-1.6 mg/day) is filtered at the glomerulus and excreted in urine essentially unchanged, often within 1-2 hours — producing the characteristic fluorescent stream. The phenomenon is benign and a useful real-time marker of GI absorption: cloudy/lighter urine after a riboflavin-containing supplement suggests effective absorption-then-overflow; complete absence may signal severe absorption impairment. Color intensity scales with dose (visible at ~50 mg+ for most users), hydration state, and the time-since-dose; it fades within 4-6 hours. The yellow color of B-complex tablets and energy-drinks themselves is also riboflavin (E101).