Riboflavin and vitamin B6: metabolic interactions and impacts for population health

Abstract

Vitamin B6 and riboflavin are interrelated nutrients which play fundamental biochemical roles including in one-carbon metabolism, however, their roles in health and disease, and the consequences of deficient status of either nutrient, are generally under-investigated. This is particularly so in the case of riboflavin, where research is hampered by the lack of biomarker measurements in human studies, with most reliant on dietary intakes only as a basis to assess nutritional status. Therefore, the overarching aim of this thesis was to investigate riboflavin and vitamin B6, and their interactions, in adult cohorts in relation to biomarker status, function and health outcomes.

Following a systematic review of the available evidence, a meta-analysis was conducted to investigate the influence of the common C677T polymorphism in the gene encoding the riboflavin-dependent enzyme methylenetetrahydrofolate reductase (MTHFR) in relation to endothelial function. The results showed a non-significant trend towards lower endothelial function in individuals with the variant MTHFR 677TT genotype. Of note, none of the studies identified in the systematic review had considered the potential influence of riboflavin on the MTHFR-endothelial function relationship (despite emerging evidence that riboflavin has an important modulating effect on blood pressure in individuals with the variant TT genotype in MTHFR). Although the erythrocyte glutathione reductase activation coefficient (EGRac) assay is widely considered to be the gold-standard method of assessing riboflavin status, the main challenge associated with this biomarker measurement relates to specific and labour-intensive pre-analysis sample processing of fresh washed erythrocytes, greatly limiting the widespread use of this biomarker for research or nutrition survey purposes. Data presented in this thesis provides the first evidence to show that removal of this pre-analysis step does not significantly affect the resulting EGRac values, a finding which would make this functional assay much more accessible for use in future population studies. The sensitivity and specificity of the assay were also confirmed, and EGRac was shown to respond to riboflavin supplementation in a dose-dependent manner. In addition, a strong level of agreement was found between the performance of the EGRac assay as used in this laboratory at Ulster University and the only other external laboratory worldwide also measuring riboflavin status on a routine basis and using the same biomarker assay (Cambridge UK).

Analysis of EGRac data from a large observational cohort of over 5000 Irish adults (aged 18 – 102 y) identified deficient riboflavin status in 39 % of younger adults, and 29 % of older adults. Moreover, vitamin B6 as measured by plasma pyridoxal 5′-phosphate (PLP)iii was shown to be strongly associated with riboflavin status in both males and females in this cohort, independently of dietary vitamin B6 intake. Furthermore, observational analysis of data from over 6000 adults found that plasma PLP was inversely associated with three separate pro-inflammatory biomarkers across adulthood and showed, for the first time, that this association was strongest in older adults, particularly females.

In conclusion, this thesis presents novel data that sub-optimal riboflavin status, evident across adulthood, may negatively impact on vitamin B6 status, and that riboflavin maybe the more limiting nutrient for maintaining adequate B6 status particularly among older adults. Furthermore, PLP concentrations were inversely associated with pro-inflammatory biomarkers, particularly in older females. Whether this finding is a consequence of inflammation adversely affecting vitamin B6 status or indication that better vitamin B6 is protective against inflammation, needs to be investigated in future intervention trials. Overall, these findings have important implications for emerging dietary recommendations for both riboflavin and vitamin B6 in age- and sex-specific subgroups.

Date of Award	May 2021
Original language	English
Supervisor	Kristina Pentieva (Supervisor), Helene McNulty (Supervisor), Mary Ward (Supervisor) & Catherine Hughes (Supervisor)