So, Are You Counting Calories or Kilometres?

Some people run in the hopes of burning calories. But a study of thousands of runners shows how biological efficiency works against that.


There are so many great reasons why people run to get exercise. Running helps build muscle strength and bone density, and physical activity also gives a boost to mental health.

But many runners are also hoping to burn calories, and biology could be working against that goal.

Jessica Selinger, assistant professor of kinesiology and health studies at Queen’s University, is interested in how people run in the real world. Free from the influence of how an experiment might be set up in the lab, and without the feeling of being observed, she studies how people move when left to their own devices.

Selinger is the first author of a study that tracked the behaviour of more than 4,600 recreational runners over more than 28,000 hours of running during a 14-month time span. Each study participant wore a Lumo Run device around their waist during runs, paired with a GPS-enabled smartphone app to record distance and elevation. The study was published in Current Biology.

Looking at race times of competitive runners, it makes sense why many people expect runners in general to move at a slower pace when they anticipate going on a long run, and speed up for a shorter run.

The study actually found that over the most common running distances — no matter whether it was a short 2 kilometre run or a more substantial 10 kilometre run — recreational runners tend to settle into a single preferred running speed no matter how far they run.

Only at very long distances, like a half-marathon or marathon distance, did people eventually slow down from factors like fatigue or having trouble regulating their temperature.

The authors also looked at data from runners in the lab, where they asked volunteers to run on a treadmill at various controlled speeds. During these tests, they measured how much oxygen their runners were taking in, giving a sense of how much energy they were consuming at each pace.

Just like a car, a runner has a particular pace at which their movement is most energy efficient. Going any faster will cost more fuel, and going at a very slow and nearly idle pace also uses more energy to travel the same distance.

After matching lab runners by age, sex, and body mass index (BMI) to those in the real world data set, they found that the energy-optimal speed found in the lab is indistinguishable from real-world preferred speeds.

In other words, each runner’s preferred running speed in the real world is tuned to save calories. So although many runners have a goal to burn calories, it’s likely that they are moving at speeds that minimize that energy use.

From an evolutionary standpoint, it makes sense that our bodies are tuned to save energy. Even when food isn’t scarce, it takes effort to overcome generations of biology.

Runners can also use various tools to set a faster pace, like running with a faster partner, listening to fast-paced music, or putting conscious effort into going faster.

But people are complex. If a relaxed and comfortable pace helps someone stay on track with an active lifestyle, that’s the most important goal. They could also choose to work with their biology and carve out time for longer or more frequent runs. Over time, the activity will still add up to health benefits and calories burned.

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Karyn Ho is a science animator and engineer who thrives at the interface between science, engineering, medicine, and art. She earned her MScBMC (biomedical communications) and PhD (chemical engineering and biomedical engineering) at the University of Toronto. Karyn is passionate about using cutting edge discoveries to create dynamic stories as a way of supporting innovation, collaboration, education, and informed decision making. By translating knowledge into narratives, her vision is to captivate people, spark their curiosity, and motivate them to share what they learned.