Introduction: Flavonoids are small molecules that can be found ubiquitously in plants (e.g. cocoa, berries, grapes, apples) and can protect humans against vascular disease, as evidenced by improvements in peripheral endothelial function, likely through nitric oxide (NO) signalling. Emerging evidence also suggests that diets rich in these compounds may be beneficial for cognitive health, but the underlying mechanisms are not well established.
Aims: To investigate the effects of dietary flavonoids on executive function in young healthy adults and their underlying mechanisms of action within the vasculature.
Methods: In a series of acute (1-2 h, within subject) and chronic (8 weeks, between-subject) randomized, counterbalanced, double-blind, placebo-controlled studies, healthy young adults consumed either a high or low flavonoid intervention in the context of different cerebral physiological challenges, such as hypercapnia (5% CO2), incremental exercise (low, moderate, high intensity) and cognitive tasks. Pre-frontal cortical oxygenation (TOI) and total haemoglobin (THI, index of blood flow), was measured using Near Infrared Spectroscopy (NIRS) before and after the flavonoid interventions. Executive function performance was also assessed by Stroop/Double Stroop, Modified Attention Network Task (MANT) and Task Switching Test (TST).
Results: Our studies suggest that flavonoids are effective at improving executive function acutely (Double Stroop: p=0.045, N=18) and chronically (TST: p=0.019, N=50) in a resting state, but can also enhance cognition post-exercise (MANT: p=0.026, N=58), with benefits being apparent particularly when cognitive demand is high. This may be driven by more efficient cerebral oxygenation (sparing of oxygen) after flavonoid consumption, as some of our data suggests flavonoid intake leads to faster (by approx. 1 min; p < 0.001, N=18) and greater cortical oxygenation (p=0.030) in response to an hypercapnic challenge, as well as higher oxygenation during low-intensity exercise (p=0.042), without further changes in total cortical blood flow (THI). In agreement with this, during post-exercise cognitive performance, acute flavonoid intake also resulted in higher TOI in high-fit individuals (p=0.015, N=58). In contrast, low-fit individuals experienced a reduction in TOI (p=0.024, N=58), despite similar cognitive benefits, likely suggesting improved capacity to utilise available oxygen (i.e., greater oxygen extraction).
Conclusions: Together our data suggests that flavonoid-rich foods might be an effective dietary strategy to improve cognitive function acutely and chronically in young healthy adults. These benefits may be particularly important in the context of tasks with high cognitive demand. Interestingly, intake of flavonoids modulates cortical oxygenation, but specific mechanisms may be dependent on the levels of cardiorespiratory fitness of the individual. These findings have important implications for future research to explore the relationship between flavonoid-rich food choices in situations in which brain physiology is challenged, such as cognitive performance and/or exercise.