During a multi-day cycle event or a stage race it is important to maintain a positive energy balance (i.e. consume at least as many calories as your body has burned) in order to keep your body’s energy levels topped up. So it would be really useful to know how many calories you did burn. In this article we will explain how to convert Watts into kilocalories and what the limitations of this method are. Although there are some calculations and formulas involved, it’s not rocket science.
Look at your power.
Let’s look at a regular endurance training of two hours, done with an average power of 200 W. One Watt equals one joule per second (1 J/s). So every second during this ride, 200 joules of energy is produced. However, that is only a relatively small part of the total energy that your body has produced. When you cycle, a lot of your energy production is lost in the muscles in the form of heat. The basic functioning of body processes like the maintenance of cells, the brain functioning and breathing, gobbles up another chunk of this total energy production. The ratio of the total energy production in relation to the energy that actually reaches the paddles is called the Gross Efficiency (GE). For male cyclists, the average GE is about 21% while for women it is slightly lower at about 19-20%.
Back to the rider above. The 200 J/s pedal power output is only 21% of the total energy produced. So the total energy production was in fact 952 J/s (200/0.21) during the ride. Over two hours this equals (120 minutes x 60 seconds = 7200 seconds; 952 Joules x 7200 seconds = 6,854,400 Joules = 6854 kilojoules (KJ). One kilocalorie equals 4.1868 KJ. So during this 2-hour ride, 6854 KJ has been produced, which equals 1637 kcal. Are you still with us?
We have to realize that the GE of 21% is only an average. There are people with a lower or higher GE, ranging from 16 to 25%. The more efficient you are, the less work your body will have to do to produce the same Watts. If you are not trained yet and start training, your GE will automatically improve. This is mainly because the power you can deliver increases. As the power you can deliver increases, the percentage of the basal, metabolic energy needs in relation to total energy consumption decreases. The result is a better GE. Furthermore the GE improves when your power delivery is improving. The power delivery eventually plateaus at around 60% of your VO2 max power. When you are already well trained it is difficult to improve your GE by much.
Efficiency falls during training.
Your GE can decline during training. Intensive intervals around or above your FTP can cause a lower GE for the rest of that workout. This means that after a number of VO2 max intervals, you will ride the rest of that workout with a lower GE. This, by the way, is also the reason why you should not ride the first few kilometers of a time trial at power readings above your FTP, as that will leave you with a lower GE for the rest of that TT. The lower GE will also lead to underestimating the total energy produced. This does not matter too much for long extensive endurance rides, but at intensive interval training sessions in a hilly area, this can lead to quite big differences.
Crunching the numbers.
You’re probably wondering now: what is the impact of a lower GE? Let’s compare an extreme example. We take a (talented) rider with a GE of 24% and a rider with a GE of 17%. This is a big difference but certainly not uncommon. When you cycle two hours with a power of 200 W with a 24% GE you will burn 1433 kcal. When you cycle two hours with a power of 200 W with a GE of 17% you will burn 2023 kcal. That is a substantial difference of 590 kcal. A difference that is bigger than a Big Mac!
How do we solve this?
Unfortunately, science has not yet development a tool or a method that can calculate your GE at every moment during a workout. An exact GE can only be measured in a laboratory with (in)direct calorimetry. Until a good alternative is found, a GE of 21% is used as an approximation. But it is recommended for experienced cyclists to keep the time above FTP in mind when approaching their GE. Because then we can make a fairly accurate estimates of how much granola, cottage cheese, fish or beer a rider should consume to replenish his energy supplies.