Toolbox: Kilojoules and Calorie Counting
Delving into Matt Hayman’s Paris-Roubaix winning power file, we can see that pushing on the pedals over 6 hours, 9 minutes and 22 seconds required 6696 kilojoules of energy. What are the implications of kilojoules for cyclists and nutritional intake?
Many pro-cyclists, coaches and scientists use kilojoules as a metric in analysing training, guiding nutritional strategies, prescribing sessions and controlling experiments. As the price of power meters drops and increasing numbers of cyclists begin to measure and record their power output, awareness and discussion of the use of kilojoules, as a training measure, is growing.
In winning Paris-Roubaix, Mat Hayman rode a canny race, making efforts where he needed to, but riding relatively consistently thanks to being in an early break. Still, he averaged a staggering 302 watts (347 watts normalized). (1)
So How Do Watts and Calories Relate To Kilojoules?
The amount of energy a cyclist has applied to the pedals over the course of a ride may also be used to help to determine their nutritional strategy. In fact, some pro cyclists have become renowned for their predilection for weighing their food in an attempt to balance energy intake with expenditure. The kilojoule readout on your power meter gives you the energy output part of the equation, and it’s generally a fairly simple conversion to turn this number into the more commonly-known kilocalorie or Calorie.
Most power meter users are much more familiar with watts as a measure. Essentially, watts describe our work rate: how much and how quickly we are producing mechanical work on the bike over a given time period – the work that results in the pedals moving.
1 watt is equivalent to doing 1 joule of mechanical work per second. Because of the magnitude of energy expenditure during exercise, joules are usually expressed in kilojoules: 1kj = 1000j. So riding at 200 W (200 joules per second) for 5 seconds requires 1 kJ of energy.
However, the human body is inefficient. Relatively little of the energy we consume in our food and drink is actually converted into useful mechanical (pedal turning) work. In fact, human beings generally operate at less than 25% efficiency (most people operating well below this) and this figure varies significantly between individuals.
1 Calorie is approximately equivalent to 4 kj (actually 4.17). Therefore, this ratio is also very helpful for quick calculations as, assuming 25% efficiency, the energy recorded in kilojoules roughly corresponds to the same value in Calories – a 1:1 ratio.
Practically, assuming Matt Hayman is 25% efficient, the 6696kj supplied to his pedals for his Paris-Roubaix ride (as recorded by his SRM) likely required around 26,784kj of energy expenditure by his body, but this gets converted back to 6696 kCal expended.
How Much Do I Need To Eat?
So does this mean that Hayman needed to consume 6696 Calories to be able to supply 6696kj to the pedals? In a race, it’s often not possible, or even advantageous, to consume enough calories during performance to match energy expenditure.
For example, providing the intake was through a combination of carbohydrates that are absorbed via different mechanisms (e.g. glucose or maltodextrin combined with fructose in a 2:1 ratio) it’s likely that the very highest rate of carbohydrate intake that a pro rider could absorb and utilise is 90 grams per hour (3) (most cyclists would ‘max out’ at 60 grams per hour). 90 grams of carbohydrate per hour corresponds to being able to consume and utilise 360 Calories per hour, which would correspond to 2160 Calories over the course of his 6 hour ride.
However, we know that he was required to expend approximately 6696 Calories, resulting in an energy deficit of 4536 Calories. During the race, this deficit was provided for through his body’s stored energy, primarily in the form of fat and glycogen, so that he was able to carry out the necessary mechanical work of pedalling. Being a big guy, Hayman’s muscle and liver glycogen stores may have been around 2000 Calories, leaving 2536 Calories to be supplied by stored fat. This equates to fat supplying 38% of his overall energy expenditure during the event.
This rough estimate is likely inaccurate to some degree. However, it corresponds to the rates of fat oxidation described in the literature. For example, Achten and Jeukendrup’s 2003 paper (4), describing maximal rates of fat oxidation in a group of 55 endurance-trained individuals, suggested rates up to 0.67 grams per minute, equating to 40.2 grams per hour, 241.2 grams in a 6 hour event, which would suggest it’s possible to supply 2170.8 Calories from stored fat.
In the hours and even days following the event, Hayman likely ‘repaid’ this energy ‘debt’ through the meals he ate and beers he may (or may not) have consumed.
It’s also worth noting that an energy deficit following a session or race is not necessarily a bad thing. We’ve been indoctrinated into thinking that we must replace every calorie we expend in exercise as soon as possible. If maximising exercise capacity in the next 24 hours is the objective, for demanding training sessions and competitions that may be taking place for example, eating to replenish an energy deficit quickly, is probably a good approach.
However, there is good evidence to suggest that training in a glycogen depleted state and NOT restoring expended muscle glycogen immediately following exercise may actually improve the body’s adaptation to exercise (5). The question is whether you are trying to achieve maximal performance in a training session or race, or maximal adaptation afterwards.
It’s Easy To Over-Eat
Eating for performance requires many considerations. Athletes must eat to meet the demands of:
• Adapting to training stimulus
• Maximising exercise capacity for competition
• Protecting immune function
• Maintaining health
• Achieving body composition targets
• Integrating socially.
• Enjoying their food!
It’s not all about the bike. An athlete’s energy expenditure may be described as comprising 3 components:
1) Basal metabolic rate: The amount of energy required to stay alive makes up about 60-80% of total energy expenditure and is related to the amount of active tissue that the individual has.
2) Thermic effect’ of feeding: The metabolic cost of the body processing food.
3) Normal daily activity, training/competition: This is usually the most variable component in the energy ‘mix’.
Adapted from Broad & Cox, 2008 (6)
In addition, this example from Paris-Roubaix illustrates why it can be relatively difficult to lose weight through cycling, and why many pro-riders control their diets so closely and don’t appear to eat very much, particularly during training.
Amateurs and even professionals on training rides would not be expending anywhere near as much energy as Hayman did during Paris-Roubaix. However, it’s common to see riders drinking concentrated energy products and eating a lot to ‘reward’ themselves after long days in the saddle. This is certainly one of the fun parts of cycling, but serious riders who are trying to adjust body composition and power:weight ratio should perhaps take the time to estimate their energy expenditure and intake during training and racing.
Recording kilojoules using a power-meter provides a useful means to understand the energy demands of cycling. It can help us to fuel adequately before, during and in the recovery phase, as well as providing another method of objectively quantifying how demanding a session or event has been and what the resulting fatigue may be.
If you would like to experiment with recording and applying kilojoules as a measure in your training and racing, track both your energy expenditure and energy intake over the course of a week and see whether there are gross imbalances high or low.
1. Johnson, A.J (Retrieved 04/2016) Power Analysis: Mathew Hayman’s Victory at Paris-Roubaix; https://home.trainingpeaks.com/blog/article/power-analysis-mathew-hayman-s-victory-at-paris-roubaix
2. Gibala, M.J. et al. (2006) Short-term sprint interval versus traditional endurance training: similar initial adaptations in human skeletal muscle and exercise performance. The Journal of Physiology. 575 (3) p. 901-911
3. Jeukendrup (2014) A Step Towards Personalized Sports Nutrition: Carbohydrate Intake During Exercise; Sports Medicine, 44 (1): Supplement p. 25-33
4. Achten, J. & Jeukendrup, A.E. (2003) Maximal fat oxidation during exercise in trained men. International Journal of Sports Medicine. 24( 8) p. 603-608
5. Baar, K. & McGee, S. (2008) Optimizing training adaptations by manipulating glycogen. European Journal of Sport Science. 8 (2) p. 97-106
6. Broad, E.M & Cox, G.R. (2008) What is the optimal composition of an athlete’s diet? European Journal of Sport Science. 8 (2) p. 57-65
James Hewitt is Sports Scientist and Performance Coach with HINTSA Performance based in Geneva, Switzerland. In a previous life he was an Elite racer but now focusses on avoiding caffeine overdose and helping other people achieve their goals. You can contact James through twitter @jamesphewitt and find out more at his website www.jameshewitt.net