Efficiently convert wattages into calories

Cycling Tips

During a multi-day cycle or stage race it is important to maintain a positive energy balance in order to keep your energy level high. In this article we will explain how to convert wattages 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

As an example we take a normal endurance training of two hours with an average power of 200 watts. One watt equals one joule per second (1 J/s). So every second 200 joules of energy is delivered to the pedals. However, that is not the energy that is released in the body. When you cycle your body consumes much more energy, that energy is mainly lost in the muscles to the heat production. Another part of the energy is needed to maintain basic body processes such as the maintenance of cells, brain functions and breathing. The ratio of the total released energy to the energy you deliver to the pedals is called the Gros Efficiency (GE). For male cyclists the average GE is 21%, for women it is slightly lower and the GE is on average 19-20%.

Example

Back to the rider. This means that 200 J/s is only 21% of the total energy released. So in fact the body burns 952 J/s (equal to 100%) during the ride. Two hours is 120 minutes, times 60 seconds = 60 x 120 = 7200 seconds. That makes 952 Joules x 7200 seconds = 6,854,400 Joules = 6854 kilojoules (KJ). One kilocalorie is equal to 4.1868 KJ. We have burned 6854 KJ, so 6854 KJ divided by 4.1868 = 1637 kcal. Do you still have it?

Efficiency in cycling

Although this sounds very useful, 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 for the same wattages. 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 basal metabolic rate decreases in relation to total energy consumption. The result is a better GE. Furthermore the GE becomes better when your power delivery is better reached it reaches a plateau around 60% of your power at VO2 max. When you are already well trained it is difficult to improve your GE much more.

Efficiency goes down during training

But, more importantly, your GE can get lower 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 drive the rest of that workout with a lower GE. In between, this is also the reason that it is not wise to ride the first kilometers of your time trial above your FTP. You will have a lower GE for the rest of the time trial. The lower GE will lead to an underestimation of the consumed energy. This will not matter much for long extensive endurance rides, but for intensive interval training in a hilly area, this can cause big differences.

Crunching numbers

You’re probably wondering now: how much does a lower GE matter? 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 BigMac!

So how do we solve this?

Unfortunately, science has not yet reached the point where you can calculate a GE for every moment of a workout. An exact GE can only be measured in a laboratory by means of (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. In this way we can make a fairly accurate estimate of how much granola, curd cheese, fish or beer a person can consume to bring his energy supply back up to standard.
If you want more information nutrition advice you can contact our sports dietician Mariette or check out our personal nutrition plans over here. If you want to be better prepared for your event you can check out our coaching services our download our app JOIN Cycling.