1 Department of Health, Kinesiology, and Applied Physiology, Concordia University, Montreal, Quebec, Canada.
2 Metabolism, Obesity, and Nutrition Lab, School of Health, Concordia University, Montreal, Quebec, Canada.

Adults who have had obesity from childhood are at greater risk of obesity-related comorbidities compared to those who only develop obesity in adulthood. The main way of mitigating these risks in obesity is with weight loss, which has been shown to positively affect the cardiorespiratory fitness (CRF) and body composition of adults. However, it is unclear whether the response of these outcomes to weight loss may be influenced by age of obesity onset. The objective of our study was to investigate how age of obesity onset mitigates the responsiveness of CRF, muscle strength and body composition to modest weight loss. Measurements were conducted at baseline and 12 weeks. In total, 37 participants (childhood-onset = 19, adult-onset = 18) lost 3.7% ± 0.4% through aerobic exercise and diet. The YMCA cycle ergometer test (YMCA) and the 20-m shuttle run test (20MSR) were used to estimate CRF (mL kg^-1 min^-1 ) and a handgrip dynamometer was used to estimate muscle strength. Total body composition was assessed by dual-energy x-ray absorptiometry (DEXA). Overall, CRF and body composition improved (time effect: p < 0.05) after 12 weeks. There was no group-by-time interaction for YMCA, 20MSR, muscle strength and body composition variables. Therefore, the present study suggests that individuals with childhood-onset obesity and adult-onset obesity can improve their CRF and body composition similarly after mild weight loss.