The Relationship Between Digit Ratio (2D:4D) and Aspects of Cardiorespiratory Fitness: A Systematic Review and Meta-Analysis

Abstract

Introduction: Digit ratio (2D:4D), a proxy of prenatal testosterone exposure, is a putative marker of physical fitness. No study has comprehensively synthesized studies examining associations between 2D:4D and cardiorespiratory fitness (CRF). We aimed to systematically review and meta-analyze studies reporting associations between 2D:4D and aspects of CRF. Methods: We systematically searched the literature for full text, refereed, cross-sectional studies reporting Pearson's correlation coefficients between objectively measured 2D:4D and at least one aspect of CRF. CRF was objectively assessed using field-based measures (maximal long-duration exercise performance) or laboratory-based measures (maximal oxygen uptake [VO2max], ventilatory threshold [VT], or mechanical efficiency [ME]). We used random-effects meta-analysis to estimate the pooled correlation and 95% confidence interval (95% CI) for aspects of CRF, and moderator analyses to estimate the influence of sex and age. Results: Data from 22 studies, representing 5293 individuals (54% male; mean age range = 10.1–40.2 years) from 12 countries were included. We found a significant strong negative correlation for VT (r = −0.61, 95% CI = −0.78, −0.37) and a significant weak negative correlation for exercise performance (r = −0.18, 95% CI = −0.25, −0.10), indicating that individuals with lower 2D:4Ds had higher VT and better exercise performance. No significant correlations were found for VO2max or ME. Neither sex nor age were significant moderators, except for age which moderated the correlation for exercise performance. Conclusions: 2D:4D is a proxy for some aspects of CRF like exercise tolerance (i.e., VT) and performance, but not other aspects like aerobic capacity and efficiency.