It’s very well established that fluid intake has both a positive and negative impact on cycling performances.
When you, the athlete remains in a relatively hydrated state (<2% loss in body mass), thermal and cardiovascular strain is reduced, and power output is subsequently maintained to have improved performances over your dehydrated counterparts.
Knowing the exact amount needed is a complex matter, however a review by Holland et al (2017) can point us in the correct direction.
To keep the math consistent, I’ll use a 75kg male for example (only 10% of the data was derived from females) where riding condition
were between 22-30 degrees Celsius.
DURATION: <1 hour
Fluid intakes between 0.15 and 0.34mL/kg body mass/min did not improve high intensity cycling performance. Under these training conditions (85% V02max), fluid loss will not be great enough to warrant very high fluid intakes, if any. And in some studies, high amounts of fluid intake <1 hour impaired power output by 2.5%.
During high intensity exercise, gastric emptying becomes compromised at intensities >75% V02 max, therefore if fluid intake exceeds the rate at which we can absorb it, gut issues can happen. That being said, if you consume a smaller volume of fluid/hour, I wouldn’t expect gut issues or negative performance outcomes.
75kg example = 675-1,530mL/hour
DURATION: >1 hour, <2 hour
For moderate cycling intensities, intakes of 0.15-2mL/kg body mass/min were associated with improvements in power output by 2%.
75kg example = 675mL-900mL/hour
DURATION: >2 hours
For longer duration, lower intensity rides, an intake of 0.14-0.27mL/kg body mass/hour improved performance by 3%. Ad libitum intakes (drink to thirst) equally improved performance.
However, the present study was unable to detect noticeable differences between the two. So perhaps personal preference wins here?
75kg example = 630mL-1,215mL/hour
These fluid recommendations for >1 hours of cycling were sufficient to prevent fluid losses of 2%. It’s worth noting that these studies were under laboratory conditions, and perhaps don’t reflect normal racing conditions?
When examining riders during a race lasting >1hour, <2 hours, average intake was in the range of 0.09-0.13mL/kg body mass/min (405-585mL/hour). From experience, this appears to be a tolerable limit for many cyclists during training rides and races.
On the other hand, cyclists during >2 hour road races have reported to consume up to 0.3mL/kg body mass/min (1,350mL/hour), which is in alignment with the above recommendations.
If you wished to be more specific, I would recommend using the ‘in field’ testing method of weighing yourself pre and post ride (1kg loss = 1L of fluid). Then divide the total loss over the number of hours cycling…but don’t forget to factor in the total amount of fluid consumed also. That is very important.