show Abstracthide AbstractIntroduction. High cardiorespiratory fitness (CRF) is associated with a reduced risk for metabolic disease across the life course. Evidence suggests this may partly be due to superior mitochondrial respiratory function. Objective. To determine the impact of intrinsic CRF on bioenergetics and metabolic health in adulthood and early life. Methods. Adult rats selectively bred for low (low-capacity runners, LCR) and high (high-capacity runners, HCR) running capacity were metabolically phenotyped prior to being mated. Thereafter, LCR and HCR weanlings were studied at 4-6 weeks of age. Microbial abundance of weanlings' colon content was evaluated by 16S rRNA sequencing.