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The acute impact of breakfast consumption and omission on postprandial metabolic responses in adolescent girlsBreakfast consumption (BC) frequency declines from childhood to adolescence and is associated with poor metabolic health. This research aimed to analyse whether BC versus breakfast omission (BO) affects substrate oxidation during rest in adolescent girls. Secondly, it examined whether BC vs BO influences postprandial and 5 h glycaemia and insulineamia. Lastly, it evaluated the effects of BC vs BO on Fatmax, MFO, rate of perceived exertion and physical activity (PA) enjoyment during an exercise bout performed 2 h after lunch. Seventeen breakfast consuming girls (13.2 ± 0.7 years old) were recruited. Two experimental trials were completed in a randomised counterbalanced order: BC and BO. A standardised lunch was provided three hours after breakfast (BC) or after breakfast omission (BO). Finger prick blood samples for the analysis of plasma glucose and plasma insulin and expired gas samples for the analysis of substrate oxidation were taken throughout the trials. An incremental 7-stage cycling test was performed 2 h after lunch for the determination of maximum fat oxidation (MFO) and intensity at which MFO occurred (Fatmax). OMNI Scale was used to evaluate the perceived exertion at the end of each cycling stage. PA enjoyment was evaluated after the cool-down using Physical Activity Enjoyment Scale (PACES). There was a significant main effect of condition (BC vs BO) for fat (p= 0.008) and carbohydrate (p< 0.001) oxidation after lunch. Fat oxidation was significantly higher during BO compared to BC, while carbohydrate oxidation was significantly higher during BC compared to BO. The main effect of condition for glucose and insulin incremental area under the curve (iAUC) (p= 0.509; p= 0.603, respectively) and total area under the curve (tAUC) for glucose and insulin (p= 0.738; p= 0.665, respectively) throughout the whole day was not significant. However, post lunch glucose and insulin iAUC (p= 0.05; p= 0.001) and tAUC (p= 0.05; p= 0.001) were significantly higher during BO compared to BC. There was no significant difference in MFO (p= 0.104) or Fatmax (p= 0.945) between conditions. Physical activity enjoyment was higher during BC vs BO with an almost significant difference (p= 0.055). The main effect of condition for perceived exertion (p= 0.307) was not significantly different. In conclusion, BC resulted in lower fat oxidation and lower second meal glycaemic and insulineamic responses. Ultimately, the findings of this study will assist in understanding further the effects of BC vs BO on adolescents’ metabolism. This may have important implications in prevention of obesity and type 2 diabetes.