Quantification of the bioavailability of riboflavin from foods by use of stable-isotope labels and kinetic modeling.

BACKGROUND Discrepancies have been reported between estimates of the prevalence of riboflavin deficiency based on intakes of riboflavin and estimates based on measures of riboflavin status. One reason for this may be an overestimate of the bioavailability of riboflavin from foods, about which relatively little is known. OBJECTIVE We aimed to quantify the bioavailability of riboflavin from milk and spinach by using stable-isotope labels and a urinary monitoring technique and by a plasma appearance method based on kinetic modeling. DESIGN Twenty healthy women aged 18-65 y were recruited for a randomized crossover study performed with extrinsically labeled (13C) milk and intrinsically labeled (15N) spinach as sources of riboflavin. An intravenous bolus of labeled riboflavin was administered with each test meal to assess the apparent volume of distribution of riboflavin in plasma. RESULTS No significant differences were noted in riboflavin absorption from the spinach meal and from the milk meal according to either the urinary monitoring technique (60 +/- 8.0% and 67 +/- 5.4%, respectively; P = 0.549) or the plasma appearance method (20 +/- 2.8% and 23 +/- 5.3%, respectively; P = 0.670). CONCLUSIONS A large fraction of newly absorbed riboflavin is removed by the liver on "first pass." The plasma appearance method therefore underestimates riboflavin bioavailability and should not be used to estimate riboflavin bioavailability from foodstuffs. Urinary monitoring suggests that riboflavin from spinach is as bioavailable as is riboflavin from milk.