Qualitative and quantitative assessment of flavins in cow's milk.

Although milk is one of the primary sources of riboflavin in the North American diet, the amounts of riboflavin and other flavin derivatives in milk have not been accurately quantified. In this study, a comprehensive assessment of milk was performed to identify and quantify greater than 95% of the flavins in this food. Riboflavin and flavin adenine dinucleotide (FAD) were the predominant flavins in all milk samples; 10-(2'-hydroxyethyl)flavin was next most common. The latter is potentially an antivitamin that may exert its effect at either the absorption or utilization level. Therefore, the presence of this analog should be considered when evaluating the riboflavin efficiency of milk. The large quantity of FAD indicates that precautions against underestimating FAD are warranted when determining total flavin content by fluorescence. This is particularly important because the fluorescence of FAD at pH 7.0 is only 10-20% of that for riboflavin. The presence of 7 alpha-hydroxyriboflavin (7-hydroxymethylriboflavin) in milk was ascertained by extensive testing. Traces of 8 alpha-hydroxyriboflavin (8-hydroxymethylriboflavin) were also detected. These metabolites may serve as indicators of cellular biochemical activity.