Caffeine Can't Save Keto: Why High-Carb Fuelling Dominates Elite Endurance Training
Elite race walkers on ketogenic diets walked slower during intense training despite caffeine supplementation, revealing limitations of low-carb approaches for high-performance athletes.
Numerous sports nutrition strategies that seem promising on paper have fallen flat during hard training or preparation. Keto is one such approach. This study reveals what happens when elite endurance athletes attempt to maintain high-intensity training quality on a ketogenic diet and whether caffeine can rescue performance when carbohydrate availability drops off. What this also shows us is that we’ll often try anything for a performance edge, except for following basic, proven nutrition practices.
PMID: 40964913
Overview
This investigation examined how a ketogenic low-carbohydrate, high-fat (LCHF) diet affects training performance in elite race walkers and whether caffeine supplementation could offset any negative effects. Twenty-one elite race walkers (15 male, 6 female) participated in a three-week training camp where they performed weekly 14-kilometre tempo hill sessions with 450 meters of elevation gain. Participants were assigned to either a high-carbohydrate (HCHO) diet or a ketogenic LCHF diet based on preference, with baseline measurements taken before the dietary interventions began.
Aim
The researchers had two primary objectives: first, to confirm whether reduced training quality previously associated with ketogenic diets persists when baseline performance is measured before diet assignment; and second, to investigate whether low-dose caffeine supplementation could rescue the training impairments observed on LCHF diets. An additional focus examined whether caffeine improved cognitive performance during high-intensity endurance sessions.
Methods
The HCHO group consumed approximately 8 grams of carbohydrate per kilogram of body mass daily (63-65% of total energy), with pre-session carbohydrate intake and sports drinks during exercise. The LCHF group followed a ketogenic protocol providing less than 50 grams of carbohydrate daily (approximately 5% of energy), with 78% fat and 16% protein, consuming the diet for at least four days before their first hill session.
During weeks two and three, all athletes participated in a randomised crossover protocol where they received either 3 milligrams per kilogram body weight of caffeine via chewing gum or a placebo, administered 20 minutes before the hill session. Researchers measured race-walking speed, heart rate, perceived exertion, blood metabolites (glucose, lactate, ketone bodies), and cognitive performance before and after each session.
Results
Training Performance
At baseline, the LCHF group walked faster than the HCHO group, completing the course approximately 6 minutes quicker. However, this advantage disappeared during the intervention period. The HCHO group improved their completion times significantly by week 2 and week 3 compared to baseline, while the LCHF group walked significantly slower in week 1 (approximately 7 minutes slower) and week 2 (approximately 3 minutes slower) compared to their baseline performance.
When speed was normalised to maximal aerobic capacity, no differences existed between groups at baseline. By weeks 2 and 3, however, the HCHO athletes walked at a higher percentage of their maximum capacity (5.2% higher in week 2 and 4.4% higher in week 3) compared to the LCHF group.
Caffeine Effects
Caffeine supplementation improved hill session completion time by approximately 2.4% across all athletes, regardless of diet group. Athletes walked significantly faster and at a higher percentage of their maximum aerobic capacity when using caffeine compared to a placebo. However, between-group differences persisted: HCHO athletes still walked at a higher percentage of their maximum capacity than LCHF athletes, even with caffeine supplementation.
Perceived exertion and heart rate remained similar between caffeine and placebo trials. Blood lactate concentrations were higher during caffeine trials compared to placebo. Ketone body concentrations decreased from pre-exercise to post-exercise in the LCHF group but remained significantly higher than the HCHO group throughout.
Cognitive Performance
Caffeine ingestion improved test outcomes regardless of diet. Athletes answered more questions, provided more correct answers, and achieved faster response times per correct answer after exercise when they had consumed caffeine compared to placebo. These cognitive benefits were absent in the placebo condition.
Practical Takeaways
For athletes engaged in high-intensity endurance training, maintaining high carbohydrate availability appears essential for preserving training quality and speed. While a ketogenic diet can be sustained during training, it comes with the trade-off of reduced absolute training speeds during key high-intensity sessions, even as athletes maintain similar perceived effort levels.