The purpose of this additional examination of a larger intervention was to determine the effects of a carbohydrate-rich pre-exercise meal on metabolic and performance-related parameters in a group of male endurance athletes, who had previously followed either a high fat low carbohydrate (HFLC; n = 6) or high carbohydrate low glycaemic (LGI; n = 6) diet for 4 weeks. Participants continued their regular training routine during the intervention. Respiratory exchange ratio (RER), lactate concentrations ([La−]), substrate oxidation rates at rest and during a cycle ergometry with incremental workload were determined in the fasted state and 2 h after consuming a carbohydrate-rich (2 g per kg of body mass) pre-exercise meal. Power output at lactate thresholds, peak oxygen uptake (V̇O2peak), absolute and relative peak power during the incremental test were assessed. After consuming the pre-exercise meal, both groups showed increased levels of RER, [La−] and carbohydrate oxidation at rest and during the incremental test (p < 0.05; Cohen’s d > 0.8), and an improved V̇O2peak (p < 0.05; Cohen’s d > 0.5). Rm ANOVA revealed significant group differences in metabolic parameters. Despite meaningful baseline differences ([La−]: p = 0.133; ηp2 = 0.185; RER: p = 0.037; ηp2 = 0.367; carbohydrate oxidation: p = 0.041; ηp2 = 0.355) upregulation of carbohydrate metabolism was more pronounced in the LGI subgroup compared to the HFLC group after 4-weeks intervention, in both fasted ([La−]: p = 0.130; ηp2 = 0.214; RER: p = 0.001; ηp2 = 0.713; carbohydrate oxidation: p = 0.001; ηp2 =0.684) and postprandial states ([La−]: p = 0.036; ηp2 =0.369; RER: p = 0.001; ηp2 =0.705; carbohydrate oxidation: p = 0.001; ηp2 =0.665). Peak power during the final incremental test was slightly higher in the LGI subgroup compared with the HFLC subgroup (p = 0.320; ηp2 = 0.100). In this group of male endurance athletes, a LGI diet seemed to result in more favourable performance metrics as compared to a HFLC diet despite increases in carbohydrate availability before undertaking the performance test likely due to the ability to sustain higher workloads in training.
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