Riboflavin is an important determinant of vitamin B-6 status in healthy adults

Background: Riboflavin is required to generate the active form of vitamin B-6 (pyridoxal 5’-phosphate; PLP) in tissues, but the relevance of this metabolic interaction for nutritional status of vitamin B-6 is unclear because riboflavin biomarkers are rarely measured in human studies. Objectives: The purpose was to identify the determinants of biomarkers of vitamin B-6 and riboflavin status and to examine the relationship between these nutrients in healthy adults. Methods: Multiple linear regression was performed on observational data in 407 healthy adults aged 18–92 years who did not use B-vitamin supplements. Vitamin B-6 status was assessed by plasma PLP concentrations and erythrocyte glutathione reductase activation coefficient (EGRac) was used as a functional indicator of riboflavin status. Results: Dietary intakes of vitamin B-6 and riboflavin were below the average requirements in 10% and 29% of participants, respectively. Suboptimal status of vitamin B-6 (PLP ≤30.0 nmol/L) was more prevalent in adults aged ≥60 years than in younger participants (i.e. 14 vs. 5%), whereas a high proportion (i.e. overall 37%) of both age groups had deficient riboflavin status (EGRac ≥1.40). In multiple regression analysis, EGRac (P = 0.019) was a significant determinant of plasma PLP, along with dietary vitamin B-6 intake (P = 0.003), age (P <0.001), BMI (P = 0.031), and MTHFR genotype (P <0.001). Significant determinants of EGRac were dietary riboflavin intake (P <0.001), age (P <0.001) and MTHFR genotype (P = 0.020). Plasma PLP showed a stepwise decrease across riboflavin status categories from optimal (EGRac ≤1.26) to low (EGRac 1.27–1.39) to deficient status (P = 0.001), independently of dietary vitamin B-6 intake. Conclusions: The findings are consistent with the known metabolic dependency of vitamin B-6 on riboflavin status and indicate that riboflavin may be the limiting nutrient, particularly in older people, for maintaining adequate vitamin B-6 status.