Associations between fitness components and brain health in young adults: A cognitive and brain volume MRI study exploring sex differences. The YoungFit study

• Evidence integrating fitness, cognition, brain volume, and sex in young adults is scarce. • Higher cardiorespiratory fitness is associated with faster processing speed. • Cardiorespiratory fitness shows an inverse association with cingulate cortex volume. • Associations between fitness, cognition, and brain volumes may differ by sex. • Brain volumes do not mediate associations between fitness and cognitive functions. Evidence linking fitness components to cognition and brain structure in young adults is limited and mixed, yet early adulthood is a key life-course stage when health behaviors consolidate, potentially shaping later brain health. This exploratory study examines the relationship between physical fitness components, cognitive functioning, and brain volumes, investigating sex differences, and assessing whether brain volumes mediate the fitness–cognition relationship. This cross-sectional study included 94 university students (mean age 21.6 [2.1] years; 45 [47.9%] females). Fitness, cognitive, and brain volume measures were obtained. Multiple linear regression models examined associations between fitness components and cognitive and brain outcomes. Sex moderation was tested by adding sex-by-fitness interaction terms to the regression models, and mediation analyses examined whether brain volumes mediated fitness–cognition associations. In the total sample, higher cardiorespiratory fitness was associated with better processing speed and smaller cingulate cortex volume. Formal moderation analyses showed that sex significantly moderated the associations between flexibility and attention–speed and processing speed, with positive associations in males and negative or null associations in females. No significant sex moderation was observed for brain volumes. Brain volumes did not mediate the relationship between physical fitness components and cognitive outcomes. Different physical fitness components were associated with cognitive functions and brain volumes in young adults, and some associations—particularly those involving physical flexibility and attention/processing speed—varied by sex at birth. Brain volumes did not explain the observed fitness–cognition associations.