What is the central question of this study? Does achieving energy balance mainly with ingested fat in a `sleep-low` model of training with low muscle glycogen affect the early training adaptive response during recovery? What is the main finding and its importance? Replenishing the energy expended during exercise mainly from ingested fat to achieve energy balance in a `sleep-low` model does not enhance the response of skeletal muscle markers of early adaptation to training and impairs glycaemic control the morning after compared to training with low energy availability. These findings are important for optimising post-training dietary recommendations in relation to energy balance and macronutrient intake. Training with low carbohydrate availability (LCHO) has shown to acutely enhance endurance training skeletal muscle response, but concomitant energy deficit (ED) in LCHO interventions has represented a confounding factor in past research. This study aimed at determining if achieving energy balance with high-fat (EB-HF) acutely enhances the adaptive response in LCHO compared to ED low-fat (ED-LF). In a crossover design, nine well-trained males completed a `sleep-low` protocol: on day 1 they cycled to deplete muscle glycogen while reaching a set energy expenditure (30 kcal/kg of fat free mass (FFM)). Post-exercise, low carbohydrate, protein-matched meals completely (EB-HF, 30 kcal/kg FFM) or partially (ED-LF, 9 kcal/kg FFM) replaced the energy expended, with the majority of energy derived from fat in EB-HF. In the morning of day 2, participants exercised fasted and skeletal muscle and blood samples were collected and a carbohydrate-protein drink was ingested at 0.5h recovery. Muscle glycogen showed no treatment effect (P < 0.001) and decreased from 350 ±98 and 192 ±94 mmol/kg dry-mass between rest and 0.5 h recovery. Phosphorylation status mTOR and AMPK pathway proteins showed only time effects. mRNA expression of p53 increased after exercise (P = 0.005) and was higher in ED-LF at 3.5h compared to EB-HF (P = 0.027). Plasma glucose and insulin AUC (P < 0.04) and peak values (P=0.05) were higher in EB-HF after the recovery drink. Achieving energy balance with a high-fat meal in a `train-low` (`sleep-low`) model did not enhance markers of skeletal muscle adaptation and impaired glycemia in response to a recovery drink following training in the morning.
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