Objectives: To investigate the relationship of childhood cardiorespiratory fitness with early markers of chronic kidney disease, glomerular hyperfiltration and albuminuria, in midlife.

Design: Prospective cohort study.

Methods: This study included 1371 participants aged 36-49 years who participated in the 1985 Australian Schools Health and Fitness Survey when they were 7-15 years of age. Childhood cardiorespiratory fitness was estimated by the time taken to complete a 1.6- km run. Blood and urine samples were collected at follow-up. Log-binomial regression was used to determine the associations of childhood cardiorespiratory fitness with glomerular hyperfiltration [estimated glomerular filtration rate (mL/min/1.73 m²) > 95th percentile standardized for age and sex] and albuminuria (urine albumin-to-creatinine ratio ≥ 2.5 mg/mmol in males or ≥3.5 mg/mmol in females) in midlife.

Results: Compared with women with high childhood cardiorespiratory fitness, those with lower childhood cardiorespiratory fitness had a higher risk of glomerular hyperfiltration in midlife after adjusting for childhood age, the duration of follow-up, and midlife smoking status [adjusted relative risk = 2.86, 95% confidence interval, 1.04-7.86 for individuals with moderate childhood cardiorespiratory fitness (P = 0.04), and adjusted relative risk = 3.38, 95% confidence interval, 1.13-10.14 for individuals with low childhood cardiorespiratory fitness (P = 0.03)]. Further adjustment for childhood and midlife body mass index resulted in a slightly attenuated and statistically non-significant association. No significant associations were found with glomerular hyperfiltration in males or albuminuria in either males or females.

Conclusions: Low cardiorespiratory fitness in childhood may increase the risk of glomerular hyperfiltration in midlife in females, possibly via a path through adult cardiorespiratory fitness.