When it comes to cycling, especially the competitive realm of the sport, the athlete tends to be obsessed with being as lean but also as strong as possible.  This has huge implications in the sport as the lighter and stronger you are, the faster you will be (in most situations).  In order to drop those stubborn last few pounds though, it is necessary to monitor Calories consumed versus Calories expended and ensure there is a deficit.  Of course, this Caloric deficit should be enough to allow for SAFE weight loss, but also not too much so it sacrifices energy and ability to produce power.  There is no more accurate way to judge Calories expended during cycling than by using a power meter.  A power meter has the ability to express how much work the athlete has done during their ride and give them an idea of how many Calories they need to consume to maintain their current weight, or not consume to create a deficit so they will lose weight.  Again, I must reiterate the importance of SAFE weight loss.  I highly recommend seeking out the help of a licensed nutritionist/dietician to have someone to objectively monitor progress and safeguard against any pitfalls, and of course speak to your doctor before embarking on any weight loss program.


You may have noticed in the opening paragraph that I capitalized “Calorie”.  This is because what we consider to be a food Calorie is actually a kilocalorie, or 1000 calories.  Since we are lazy and cannot be bothered to say or write “kilo”, we dropped it and instead capitalized the C, problem solved!

A Calorie is the amount of energy needed to raise 1 kg of water from 15-16º Celcius. (1)

So, a Calorie is actually a unit of energy that the body uses.

Oxygen and fuel are needed for fire and combustion; your body needs oxygen and Calories for energy production and to produce mechanical force.  Compare the human body to a car engine…

  • The car engine needs gasoline for fuel.  The body needs food that contains Calories for fuel.
  • The car engine needs oxygen for combustion and is taken in via the air intake.  The body needs oxygen for energy production and is taken in via the lungs inhaling.
  • The car engine combines the oxygen and gasoline to create combustion which moves the pistons in the engine, resulting in mechanical force that drives the car forward.  The body combines oxygen and Calories, creating energy and heat, that results in muscular contractions and concludes with you applying force to the pedals, propelling the bicycle forward.
  • The car engine removes exhaust through the tail pipe.  The body removes exhaust by exhaling through the wind pipe.


A kilojoule (kJ) is another way to express energy, and in some countries kilojoules are actually found on the nutrition label instead of Calories.  You may have noticed when you upload your rides that there is a section of “total work” that displays the ride in terms of kilojoules.


So, the power meter converts how many watts we produce (remember that a watt = 1 joule/second), multiplies this by the seconds we produce them, and expresses this in kJ.  Put simply, if you produced 100 watts for 100 seconds you would have produced 10,000 joules, or 10 kJ.  As you can see above, I expended 4,921 kJ during my ride which is another way to say how much work it took to finish it.

A joule is equal to the work done by a force of one newton when its point of application moves through a distance of one meter in the direction of the force.  (2)

Total work done is relevant for training purposes and race preparation.  For example, if you know a rider that is of similar weight and fitness to you and has done an event you are training for, you are able to look back at their previous data and see how many kJ (how much work) it took to finish the event.  Then, with this knowledge you are able to plan workouts based upon kJ expenditure to better approximate the needs of the event, pretty cool, right?

How do kJ and Calories relate to cycling?

Cyclists consume Calories by eating, and expend kJ by riding.  If a Calorie is equal to 4.183 kJ, that must mean for every 4 kJ expended we only burn 1 Calorie, right…?

The human body is an incredible machine and continues to boggle my mind every day, but it is rather inefficient at converting food into mechanical energy, i.e. turning that gel you just hastily downed into wattage to the pedals.  So inefficient in fact that it only converts about 25% of the food we consume into actual mechanical energy (1).  The other 75% is dissipated as heat.  So, the people who say you only burn 1/4 of the kJ expended during a ride as Calories are incorrect.  This would mean athletes would be able complete an entire Ironman on only a handful of gels, obviously this is not the case.

Remember from above, the pedals and power meter are only receiving about 25% of each Calorie we burn due to the inefficiency of the body.  Thus, if 1 Calorie is equal to ~4 kJ, but it takes 4 Calories to produce 1 kJ of mechanical energy, for all intents and purposes, kJ expenditure during your ride is equal to Calories burned.

Weight loss from cycling

Now, armed with the knowledge of understanding how many kJ you produce during a ride is approximately how many Calories you burn we can play around with our total Calories consumed for the day to promote weight loss.  Here is what I do myself when I am looking to achieve race weight and has worked very well for me the past few years…

  1. Figure out what your basal metabolic rate (BMR) is.  This calculator gives you an estimate of what your BMR is, if you want a more precise number here I recommend a VO2 BMR test.
  2. Use a Calorie tracker.  I use My Fitness Pal and find it to be excellent.  The best part is it communicates with many other apps (Strava, Training Peaks, etc.) and automatically modifies your Calories and macronutrients for the day based on your exercise.
  3. Keep your total Calories consumed for the day SAFELY below your Calories expended to promote weight loss and keep this process going until you reach your target weight.  Safe daily Caloric debt is ~500-1000 Calories per day, or 1-2 lbs lost per week.  (3)
    1. Remember though, you also need to meet your macronutrients for the day to keep yourself healthy, help your body repair itself after intense workouts, and make those fitness gains.  So, choose foods that have the highest nutrient density without the added Calories, i.e, you can meet your Calorie goals by eating nothing but ice cream and french fries, but good luck performing at the level you want to by eating that garbage!
  4. Monitor yourself closely.  If you are becoming sick more often, irritable, light-headed/dizzy, aren’t recovering from your workouts, or just plain don’t feel like yourself you may be losing weight too fast or have gone below your weight loss threshold (too lean).
  5. Hire the help of a licensed dietician/nutritionist.  Again, the above is what works for me and most of my athletes, but may not work for you.  As always, talk to your doctor before embarking on a weight loss program and seek professional help if you have any preexisting medical conditions.

So, a kilojoule is simply a way to express energy or work and has many uses in cycling ranging from gauging the difficulty of a workout to helping with weight loss.  Make this the year when you trade that steak & cheese sub for a spinach, beet, and goat cheese salad, drop those stubborn pounds, achieve race weight, look even better in Lycra, and push your watts per kilogram to new heights!


(1) Buccholz, A., & Schoeller, D. (2004). Is a calorie a calorie? The American Journal of Clinical Nutrition, 79(5), 899S-906S.
(2) http://dictionary.reference.com/browse/joule
(3) http://www.cdc.gov/healthyweight/losing_weight

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