Abstract:
Population ageing presents profound challenges for individuals and societies alike, unless countered with strategies which promote ‘healthy ageing’ over the expected decline of physical, social and cognitive function. B vitamins (folate, riboflavin, B₆, B₁₂,) are an attractive target given their implication in several diseases projected to confer an overwhelming burden in ageing societies, including cardiovascular and neurodegenerative diseases. This association is attributed to the role of B vitamins in maintaining one-carbon (1C) metabolism, including homocysteine (Hcy) regulation, DNA synthesis and repair, and methylation reactions. A narrow focus on folate, vitamin B₁₂ and Hcy has commanded research attention to date, but the nutrient-function-health relationship concerning B vitamins requires further understanding in light of the complex endogenous and exogenous influences of 1C metabolism in ageing. The overarching aim of this thesis was to explore how B vitamin status can be optimised and monitored in older adults, using 1C metabolites as measure of functional B vitamin status. A mass spectrometry technique was employed to quantify a comprehensive profile of 1C metabolites, including Hcy alongside those in the central methionine (methionine, S-adenosylmethionine, and S-adenosylhomocysteine), choline oxidation (choline, betaine, dimethylglycine, glycine), and transsulfuration (cystathionine, cysteine, serine) pathways. The first objective of this thesis was to describe age-related changes to B vitamin intake and metabolism. This was first achieved by a systematic review in Chapter 3 exploring longitudinal shifts in dietary B vitamin adequacy. Only eight studies (n=3119) were included in the final analysis. While evidence for a decline in riboflavin adequacy with age was provided, this study foremost sheds light on the current lack of understanding regarding changes to intake. In Chapter 4, the acute 1C metabolite response to ingestion of a single supplement containing B vitamins alongside a simple meal was evaluated in healthy younger (19-30y, n=20) and older (65-76y, n=20) adults. Despite divergent baseline metabolite profiles between age groups, the postprandial response was largely similar and again revealed a lack of comparative data in the literature. This research evokes concern around assumptions made concerning B vitamin intake and metabolism in older age, which are the foundations required to inform how and when B vitamin status is optimised. The second objective was to examine the relationship between B vitamins, 1C metabolites and health, which was evaluated in the context of cardiometabolic and cognitive health in Chapter 5. In a population of healthy older community-dwelling adults (n=313, 65-74y), higher glycine concentrations were consistently associated with protective cardiometabolic risk profiles and enhanced cognitive performance. Higher betaine or lower Hcy, dimethylglycine, and cysteine concentrations might also comprise a favourable 1C metabolite profile based on this cohort. However, the relationship between these metabolites and health markers was either less consistent than that of glycine, or dependent on apolipoprotein E genotype, B vitamin intake, or other metabolites. Ultimately, these findings do not point towards a single interpretation of an optimal 1C metabolite profile, and their relevance in light of monitoring habitual dietary intake or interventions seeking to optimise B vitamin status requires further attention. The final objective was to measure the 1C metabolite response to increased B vitamin intake through interventions already recommended to promote longevity, focusing on higher protein intake. Chapter 6 explores the response of 1C metabolites, cardiometabolic markers, and their co-regulation following six months of resistance training alone or with supplementation in seniors recruited across residential care homes in Vienna (65-98y, n=95). Daily consumption of a protein-based supplement containing moderate quantities of B vitamins (Fortifit; 20.7g protein, 200 μg of folic acid, 3.0 μg of vitamin B₁₂, 750 μg of vitamin B₆) alongside resistance training for six-months did not provide further benefits for 1C regulation compared to resistance training alone or a control group in this population of seniors. Regardless of the intervention, choline concentrations transiently increased, while Hcy, cysteine and methionine concentrations were elevated at the six-month follow-up. These findings reflect the nuanced interpretation required for interpreting shifts in 1C metabolites, and highlight the relevance of competing actions of 1C nutrients such as methionine and B vitamins in determining the Hcy response to intervention, particularly in the very old. In contrast, Chapter 7 demonstrated that Hcy concentrations declined in community-dwelling older men (>70y) following ten-weeks of intervention with a whole-food diet containing either the current protein recommendations (0.8 g/kg body weight/day; n=14) or twice that (n=15). This study revealed novel insights into the interplay between choline, betaine, and folate in Hcy remethylation, though it remains difficult to interpret what fluxes in metabolites beyond a decrease in Hcy concentration should be considered favourable. The interactive influence of nutrients within a wider dietary pattern on 1C metabolite status has largely been neglected, and these studies invite future research to address the competing (or synergistic) effect of nutrients like protein and B vitamins on 1C metabolite status. Such research will be instrumental in developing consistent, practical advice for older adults. This thesis has broadened the scope of research pertaining to B vitamin and 1C metabolite status, and brings healthy ageing principles to the forefront. B vitamins have an irrefutable role in maintaining health, but the true challenge will be ensuring their integration within the rapid evolution of nutrition research which is progressing beyond a nutrient-level focus. While a broader profile of metabolites has undoubtedly added complexity to interpreting the findings presented here, this approach does pave the way for future research. Ultimately, the nuanced interpretation of a 1C metabolite profile compared to health status or in response to intervention points towards the need for a personalised approach to optimising B vitamin status. Future research should also consider more sensitive biomarkers of disease progression or ageing, such as the use of novel epigenetic clocks, to understand the true value of B vitamins in promoting a healthy ageing phenotype. However, this thesis foremost calls for better understanding of shifts in nutrient status which are attributable to ageing – this is a necessary foundation from which we can direct recommendations that will have extensive health benefits for ageing populations.