Training for your future brain: effects of exercise training type on cerebrovascular function in young adulthood

Cerebral blood flow (CBF) gradually decreases with normal ageing, therefore there is an overall goal to implement interventions that preserve or increase CBF in older adults. On the other hand, taking measures to preserve CBF at a young age may be a better approach to help slow the natural decline in CBF with age. It has been shown that habitual exercise improves CBF regulation and overall cerebrovascular function. Based on these findings, recent research has aimed to evaluate how various exercise training regimes aid in the preservation of CBF in young adults. Furthermore, conducting this type of research in monozygotic (MZ) and dizygotic (DZ) twin pairs is a gold standard approach to understanding the influence of genetics versus environmental factors on cerebrovascular responses to exercise training. Because MZ twins share nearly 100% of their germline sequence variation and DZ twins share an average of 50% of germline sequence variation, this specific study design helps tease out what proportion of the variation is due to genes and environment (Thomas et al., 2022). Past twin studies have looked at how genetics impacts cerebrovascular disease risk (Bak et al., 2002), but there is a lack of twin studies evaluating the impact of genetics on cerebrovascular function in response to exercise. In a recent publication in The Journal of Physiology, Thomas and colleagues (2022) aimed to fill this research gap. The purpose of their study was to evaluate the contribution of genetics and/or environmental factors to the variation in cerebrovascular function in response to either resistance training (RES) or endurance training (END). This study included apparently healthy, sedentary, young adult, same-sex twin pairs. Participants underwent END and RES in randomized order to better understand the effects of each type of training on various measures of cerebrovascular function. During each intervention block (RES or END), twin pairs trained together for 12 weeks at matching relative intensities. Cerebrovascular function was evaluated using transcranial Doppler ultrasound to obtain intracranial blood velocity of the middle (MCAV) and posterior (PCAV) cerebral arteries and duplex ultrasound to obtain extracranial CBF. Cerebrovascular function and related measures were evaluated at rest, through tests of dynamic cerebral autoregulation, during cerebrovascular CO2 reactivity (CR), and during acute endurance cycling exercise. When evaluating the effect of genetics, shared environment and unshared environment on their findings, the authors operated under the assumptions that MZ twins share 100% of their DNA, DZ twins share 50%, and MZ and DZ pairs share their environments equally. A phenotype that is purely explained by genetics would have a ratio of 2:1, with correlations of 1 for MZ twins and 0.5 on average for DZ twins. If the 2:1 ratio remained the same, but the correlations were lower than 1.0 and 0.5, then both shared and unshared environments may be influencing results. The authors present cerebrovascular results as a change from preto post-training using concordance graphs to depict how each individual participant responded to each training intervention. The authors define ‘responders’ as having a positive response for an outcome variable, whereas ‘non-responders’ were defined as having either no response or a negative response for an outcome variable. Importantly, positive concordance or discordance was observed in 55–89% of participants for all outcome variables related to cerebrovascular function suggesting that most participants were responders to at least one type of training. Heritability (genetic) cross-sectional analyses of cerebrovascular function (specifically MCAV as this was their main variable of interest) at baseline indicated similar contributions from shared and unshared environment for both groups at rest, during CR and during acute exercise, and for MZ twins only during tests of autoregulation. Heritability analyses in response to training indicated that MZ twins had similar contributions from shared and unshared environments for MCAV response to RES at rest. Overall, findings from the present study indicate substantial variation in cerebrovascular responses (i.e. MCAV) to both END and RES exercise training in young healthy adults and such responses are not explained by genetics. An important new finding from this study by Thomas et al. (2022) is that individuals who did not respond to one type of training (i.e. non-responders) had an increased likelihood of responding to the other form of training. However, since the general population likely does not have the resources to evaluate their own cerebrovascular function to determine which mode of exercise is most beneficial to them, it may be recommended that both END and RES are performed in conjunction on a regular basis. Still, more studies are needed to investigate the effects of combined training on different measures of cerebrovascular function. Limited studies evaluating the effect of RES alone on cerebrovascular function tend to rely on cross-sectional and/or self-report data and are inconsistent; for example, older women who self-report engaging in RES at least once per week have significantly greater CBF compared to women who do not, but this relationship was not found in older men (Bliss et al., 2021). Moreover, the authors of the present study conclude that the lack of impact of END training may be because cerebrovascular function of the young healthy volunteers may already have been at near optimal levels (Thomas et al., 2022), highlighting the importance of performing interventional training studies longitudinally throughout the lifespan. Additionally, we recommend that future studies aim to draw the important connection between cerebrovascular function