Caffeine Shifts Substrate Utilization Toward Fat During Outdoor Running at Fatmax: A Randomized Field-Based Crossover Study
Study Details
This new study tested whether 3 mg/kg caffeine alters substrate use during 1 hour of outdoor running at Fatmax in 14 endurance-trained runners 🔍
Participants consumed 3 mg/kg caffeine or placebo 60 minutes before exercise ☕
Runners completed 1 hour at their individual Fatmax intensity on a ~1.97 km outdoor running course 🏃
Key Findings
🔥 Caffeine significantly increased total fat oxidation from ~33.6 to ~40.5 g across the 1-hour run
📈 This represented roughly a 20% increase in fat utilisation compared with placebo
🍞 Carbohydrate oxidation decreased from ~93.2 to ~77.3 g with caffeine
🔄 Caffeine therefore shifted substrate utilisation toward greater fat and lower carbohydrate use during submaximal running
😮💨 Rating of perceived exertion was also lower with caffeine, averaging 8 versus 9 with placebo
🫀 Heart rate was not significantly affected, suggesting the lower RPE occurred despite a similar cardiovascular workload
🌳 Importantly, the study was performed outdoors rather than under tightly controlled laboratory conditions, improving its relevance to real-world endurance exercise
Conclusion
Caffeine can be effective at acutely increasing fat oxidation during prolonged exercise performed around Fatmax 🔥
Greater fat oxidation does not automatically mean improved endurance performance, as this study assessed substrate use rather than race or time-trial performance ⚠️
For athletes completing longer easy-to-moderate endurance sessions, ~3 mg/kg caffeine taken ~60 min beforehand may increase fat use and make the effort feel slightly easier, although this does not necessarily translate to better performance or mean carbohydrate fuelling should be reduced ✅
Reference
https://pubmed.ncbi.nlm.nih.gov/42686132/
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