Rethinking The Role Of Fat Oxidation Substrate Utilisation During High Intensity Interval Training In Well Trained And Recreationally Trained Runners(1)

This study compared how well-trained versus recreational runners use different fuel sources (fat and carbohydrates) during high-intensity interval training. Researchers had 18 male runners perform a challenging workout consisting of six 4-minute high-intensity intervals with 2-minute recovery periods, while measuring their oxygen consumption to determine what fuels their bodies were burning.

The key finding was striking: despite both groups feeling like they were working equally hard and having similar blood lactate levels, the well-trained runners burned fat at nearly three times the rate of recreational runners during these high-intensity efforts. Specifically, fat provided about 33% of the energy for well-trained runners versus only 16% for recreational runners. Both groups used similar amounts of carbohydrates, meaning the better-trained athletes were simply able to access and burn more total fuel.

This research challenges the common belief that high-intensity exercise relies almost exclusively on carbohydrate burning. Instead, it suggests that as fitness improves, the body becomes remarkably more efficient at burning fat even during very demanding workouts. The ability to burn more fat was strongly linked to higher VO2 max (a measure of cardiovascular fitness).

For metabolic health, this finding is significant because it shows how training can fundamentally rewire your metabolism to become more metabolically flexible - the ability to efficiently switch between and utilize different fuel sources. In clinical practice, this supports the value of progressive fitness training not just for performance, but for optimizing how efficiently your body can access and use its energy stores, which has implications for weight management, metabolic health, and longevity.