I did an experiment recently to compare the impact of fueling on performance. I rode a 40 Km best effort on a turbo trainer fasted. Then a week later I did the same session fueled.
I choose the same flat course on Zwift for both sessions. In both cases I pushed hard for the full 40 Km. In both sessions I drank about 750 ml of water during the session.
Fasted session
I did the fasted session first. The session done in late morning about 14 hours since I last ate the night before. I felt quite strong in the session, but my legs did start to feel quite heavy towards the end of the session. The 40 Km took me 1 Hr 9 Mins 14 Secs to complete. An average speed of 34.8 Km/h. Which was a little better than I expected. I put this down to the positive impact of my training this year. My average power for the session was 192 watts, with average heart rate at 148 BPM, peaking at 167 BPM.
Fueled session
Five days later I did the same session. This time I ate pasta about 3 hours before the session. Then I ate a 35 gram energy bar immediately before the session. During the session I ate another 35 gram energy bar. I ate half at 20 mins and the other half at 40 mins. Again I felt strong in the session and my legs felt noticeably better in the last 15 to 20 minutes.
It took me 1 Hr 5 Mins and 15 Secs to complete this time. 4 mins faster than the fasted session. This was surprise, I had expected to perform better, but not by anywhere near as much. This was also a PR for 40 Km on a turbo. My average power was 217 Watts, quite an increase. My average heart rate was a little higher at 151 BPM, as was my peak heart rate at 171 BPM. Which is a reflection of the extra effort. Overall my average speed was 37 km/h.
Key takeaways
It was fairly obvious that fueling would make a difference in this session. But the impact was huge for me. I can’t understate this. My average power over the session was 25 Watts higher, that equates to a 13% increase. This is massive and reinforces that in sessions where you are working towards improvement in speed, power and capacity you should fuel properly as this will give you the energy to push harder and make better gains.
There is however a place for fasted rides, particularly helping train the body to oxidise fat. But if the aim of a specific session is to maximise performance then fueled is the way to go.
Comparison of performance:
| Measure | Fasted 40 Km best effort | Fueled 40 Km best effort |
| Time | 1 Hour 9 minutes 14 seconds | 1 Hour 5 minutes 15 seconds |
| Average speed | 34.8 Km per hour | 37.0 km per hour |
| Average power | 192 watts | 217 watts |
| Average heart rate | 148 BPM | 151 BPM |
| Peak heart rate | 167 BPM | 171 BPM |
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