Caffeine enhances performance regardless of fueling strategy; however, high carbohydrate availability is associated with improved training speeds compared with ketogenic diet.

The purpose of this study was to confirm reduced training metrics previously associated with a ketogenic low-carbohydrate (CHO) high-fat diet (LCHF) and investigate their attenuation with caffeine supplementation. At baseline, n 21 elite race-walkers followed a high CHO availability (HCHO) diet and performed a tempo hill session (14 km with a 450 m elevation gain). Athletes were then assigned to either the HCHO or LCHF in a parallel groups design for 3 weeks, where the 14 km tempo hill session was repeated each week. On weeks 2 and 3, in a randomised crossover allocation, all participants received 3 mg/kg caffeine or placebo (gum), 20 min before the session. Race-walking speed, heart rate, ratings of perceived exertion, blood metabolites and Stroop word-colour test metrics were collected. Although LCHF athletes walked faster at baseline compared with HCHO (P = 0·049), the HCHO group improved by week 2 (P = 0·009) and week 3 (P = 0·007), whereas the LCHF group was significantly slower in Week 1 (P < 0·001) and Week 2 (P = 0·026) compared with baseline. During the 14 km hill session, within-group analysis shows that athletes walked significantly faster (P = 0·010) and at a higher percentage of vVO₂max (P = 0·007) when using caffeine compared with a placebo. Between-group differences remained present, with HCHO athletes walking at a higher percentage of vVO₂max than those adhering to the LCHF diet (P = 0·035). No interaction between supplement treatment and dietary group occurred (P = 0·640). Caffeine supplementation partially reversed the performance impairment associated with an LCHF diet, but training quality remained lower than the combination of caffeine and high CHO availability.