In the UK most of us are more familiar with Calories than kilojoules when speaking about units of energy in a nutritional context. 1 Calorie is approximately equivalent to 4kj. So, the pain au chocolat I was about to eat in the photo contained around 250 Calories. In the sportive example I described in part 1 of this blog, I produced 3800kj of mechanical work over the 160km event which, assuming 25% efficiency, required my body to expend approximately 15,200kj (3800 Calories). For this reason, it’s sometimes easier to match the value for a session’s energy expenditure as recorded in kilojoules with the same value in Calories – a 1:1 ratio – as a rough guide of how much you may need to consume to meet the energy demands of a session.

 How Do Watts Relate To Kilojoules?

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. If you’re using a SRM, the watts recorded for this time period could be as short as 0.25 seconds, or provide an average power measure for an entire Tour de France.

1 watt is equivalent to producing 1 joule of mechanical work per second. A joule is a unit of energy, so where Watts are helpful in describing the rate of mechanical work we are undertaking over a particular time period, joules provide a measure of the total mechanical work which has been done. Because of the magnitude of energy expenditure during exercise, joules are usually expressed in kilojoules (1kj = 1000j). As I suggested in part 1, joules are a useful measure to determine and prescribe energy expenditure in training, racing and experiments in human performance. 

It’s often not possible to consume enough calories during performance to match energy expenditure. In my sportive example, I consumed 1250 Calories during the ride through a combination of carbohydrate, protein and fat, in a mixture of drinks and solid food. Using the 1:1 ratio based on 25% efficiency, we can assume that this was sufficient to generate approximately 1250kj of the mechanical work I carried out during the event. Consequently, I accumulated an energy deficit of 2550kj. During the event, this deficit was provided for through my body’s stored ‘fuel’, primarily in the form of fat and glycogen, so that I was able to carry out this mechanical work. In the 24 hours following the event, I ‘repaid’ some of this energy ‘debt’ through the meals I ate and the rest was likely provided through my ample fat stores!

All Calories Are Not Born Equal

There are a number points that may be worth considering when attempting to balance the energy demands of an effort with your nutritional strategy:

Different foods interact with our bodies in different ways, so their effects and the way in which these foods can be used also varies. A calorie consumed in the form of a simple sugar will have a different influence on the body and may be used in a different way than a calorie consumed in the form of fat from a ‘real-food’ source. The source of nutrients and their timing can be manipulated to achieve many objectives. However, recording energy expenditure is a useful insight to inform the macro and micro-nutrient requirements of an athlete.

Find out more about how changing nutrient type, quantity and timing can enhance adaptations to training and cycling performance.