Energy Expenditure: Calories, Kilojoules, and Power in Cycling

How many calories do you burn cycling? It’s a question asked by general exercisers and novice cyclists, all the way up to professional bike racers. People ask for different reasons, though. General exercisers may be primarily interested in burning calories for weight loss or to meet daily energy expenditure goals. Some cyclists may ask for similar reasons. Fitness and performance-oriented cyclists typically ask so they can evaluate caloric intake based on energy expenditure during rides and energy needs for post-workout recovery. No matter why you’re wondering, it’s important to understand what calories and energy expenditure mean in cycling, and how you can use them to maximize performance.

What is a Calorie?

If you remember from physics, one calorie is the energy required to raise the temperature of a gram of water by 1 degree Celsius. The energy in food is expressed as kilocalories (kcal) or Calories with a capital ‘C’. One kilocalorie = One Calorie.

When you consume “a 100-calorie gel”, you mean a gel containing 100 kilocalories or 100 Calories. For the sake of simplicity, we’re going to use “calories”, as see on a nutrition label, in the remainder of this article because that’s the most commonly used spelling outside of scientific literature.

Measuring energy expenditure with wearable devices

Wearable tracking devices and apps have exploded in popularity. Many track your steps, heart rate, sleep behaviors, time spent active, and more. According to recent review studies, like this 2020 study by Fuller et al. and this 2022 study by Germini et al., wrist-worn trackers like those from Fitbit, Garmin, Apple, and Nike track steps and heart rate reasonably accurately (10-25% error). But, as Fuller succinctly concluded, “For energy expenditure, no brand was accurate.” Germini was not quite as harsh, but noted, “For energy expenditure, the mean absolute percentage of error was >30% for all the brands, showing poor accuracy across devices.”

The challenge to estimating energy expenditure through movement data and heart rate is that small errors add up. The more accurately you input your height, weight, age, gender, and max heart rate, the better a wearable device can estimate caloric expenditure.

For athletes, however, the devices don’t do a great job of accounting for the fact fitness improves economy of movement. As you gain fitness, the oxygen cost for movement at a given pace decreases. In other words, you expend less energy to walk, run, or ride at a given pace.

Fortunately, cyclists can measure power output and thereby calculate a more accurate estimation of energy expenditure and calories burned cycling.

Kilojoules and calories burned cycling

Power output is calculated on bicycle with the equation: Power in watts = Torque x Cadence. This is a cycling-specific version of the general equation Power = Force x Velocity. A kilojoule is unit of energy representing the work performed by producing watts in a given amount of time: Kilojoules = (watts x seconds)/1000. For example, riding at an absolutely constant 200 watts for 3600 seconds produces 720 kJ of work in an hour.

Calories are the potential energy in food, and kilojoules are kinetic energy of turning the pedals. Your body burns calories so your muscles can produce force, and the work performed by muscles to move a bicycle is expressed as kilojoules.

Kilojoules to Calories Conversion

The burning question, so to speak, is how many calories are burned to produce how many kilojoules? Both are units of energy and 1 kilocalorie = 4.184 kilojoules.

Here’s where things get tricky. Your body tries to burn 1 food calorie (1 kilocalorie) to produce 4.184 kilojoules of work, but a cyclist’s Gross Metabolic Efficiency (GME) is only 20-25%. That means only 20-25% of the energy from fuel (calories) ends up doing mechanical work. The rest is lost as heat. So, if only one quarter of a food calorie ends up as work, then only 25% of 4.184 kilojoules of work is produced. Hence, for practical reasons we estimate the ratio of calories burned to kilojoules produced to be 1:1 during cycling.

The effect of fitness and experience

For better and worse, greater aerobic fitness and improved economy on the bike can reduce the caloric expenditure of maintaining a given power output. On the positive side, this means you can do more work per calorie burned. And because improved aerobic fitness increases the rate at which you can burn calories, you both burn more calories per minute and produce more work per calorie burned. As a result, you go faster!

If you are counting calories burned cycling, however, improved fitness can affect the estimation of kilojoules to calories. A novice and/or less fit cyclist may burn more than 1 calorie for each kilojoule produced. Completing a 1500 kJ ride might burn 1700 calories. A highly experienced and/or very fit cyclist may burn less than 1 calorie for each kilojoule produced. That same 1500 kJ ride might burn only 1300 calories. However, with greater fitness, it would also take less time to complete that 1500 kJ ride and burn those 1300 calories (on similar terrain and conditions). On a percentage basis, the differences may be small. Practically, over the course of hours and weeks of training, the cumulative difference can be substantial.

Caloric Overcompensation

Athletes often overestimate their energy expenditure and overestimate the amount of food they need to consume to fuel workouts and support post-workout recovery. To keep your caloric expenditure and energy consumption in perspective, consider the following:

You start with full glycogen stores

If you are rested and adequately fed, you start workouts and races with about 400 grams (1600 calories) of stored carbohydrate energy on board. This is enough to fuel a high-quality 60- to 90-minute workout or race without consuming any exogenous energy. For these workouts and races, focus on consuming fluids.

You don’t always need to maximize consumption

Generally speaking, you can digest and deliver about 1 gram of carbohydrate per minute, or 60 grams per hour (240 calories). That can increase to 90 g/hr (360 calories) through training and by ingesting multiple types of sugar. However, you don’t always need to consume the maximum you can process. When cyclists see 600 kJ/hr on a power meter display, they mistakenly think they need to replenish all that fuel each hour.

With the stored carbohydrate and fat you have on board, while you’re on the bike you only need to replenish 20-30% of the calories you burn. At an intensity of 600 kJ/hr, that means you’d only need 120-180 Calories an hour. At 4 calories/gram, that means 30-45 grams of carbohydrate. Even during high-intensity workouts and races when hourly expenditure can be 800-1000 kJ, aiming to replenish 20-30% keeps athletes close to the 60-90g/hr range for maximum carbohydrate absorption.

Keep in mind, it is better to underestimate carbohydrate intake during exercise. Overconsumption can lead to gastric distress, which is a leading reason athletes drop out of races. If you’re too conservative with caloric intake and start to feel the effects of hypoglycemia, you can fix the problem quickly by consuming a small amount of food. For more information, here’s a guide to eating and drinking for rides of any length.

Beware of perceived exertion bias

When a workout or race feels either very hard or very long, athletes sometimes equate high perceived exertion with the need for high calorie replenishment. You rode hard, you must need a big meal afterward. Or, you rode hard so you deserve or have earned a big meal. It is important to respond to your body’s hunger cues and cravings. Just be aware that workouts that feel strenuous don’t always burn a lot of energy.

A high-intensity interval workout is exhausting. However, the hard efforts are short and separated by easy recovery periods. As a result, the total energy expenditure from the workout may be relatively low. In comparison, an aerobic tempo or sweet spot workout of similar duration may have a higher energy expenditure, despite a lower perceived exertion.

You should NOT attempt to match post-workout meals to energy expenditure from the ride. That’s not the point and is much too granular a view of energy balance. It also sets the stage for an unhealthy relationship with food; you do not earn food through exercise, nor should you use food as a reward for exercising. However, being aware of perceive exertion bias can help you keep your post-workout and daily eating habits in perspective.

By Chris Carmichael,
Founder and Head Coach of CTS

References

Fuller D, Colwell E, Low J, Orychock K, Tobin MA, Simango B, Buote R, Van Heerden D, Luan H, Cullen K, Slade L, Taylor NGA. Reliability and Validity of Commercially Available Wearable Devices for Measuring Steps, Energy Expenditure, and Heart Rate: Systematic Review. JMIR Mhealth Uhealth. 2020 Sep 8;8(9):e18694. doi: 10.2196/18694. PMID: 32897239; PMCID: PMC7509623.

Germini F, Noronha N, Borg Debono V, Abraham Philip B, Pete D, Navarro T, Keepanasseril A, Parpia S, de Wit K, Iorio A. Accuracy and Acceptability of Wrist-Wearable Activity-Tracking Devices: Systematic Review of the Literature. J Med Internet Res. 2022 Jan 21;24(1):e30791. doi: 10.2196/30791. PMID: 35060915; PMCID: PMC8817215.