Q: As I’ve gotten closer to my 10k run, continuously running for an hour has become a lot easier, and therefore burning calories has become a lot easier. Assuming you did the same exercise routine for a few months, how would the TDEE component due to the calories burned change as your physical fitness goes up and you become more accustomed to the exercise? This would make for a very interesting newsletter.
In short: As your fitness improves, your body becomes more efficient, causing you to burn fewer calories per minute during exercise. However, improved fitness also means you can run faster, longer, or lift heavier, so your overall calorie expenditure during a workout—and therefore total daily energy expenditure (TDEE)—will typically still increase compared to when you weren’t as fit. That being said, this increase won’t be as big as you might think due to the mechanisms in place that ‘constrain’ the total number of calories you can expend.
The deep dive
As your fitness improves, you’ll burn fewer calories performing the same amount of exercise, primarily due to improved movement economy—meaning your body gets better at doing the same activity with less energy.
For example, I haven’t run in a decade. If I lost my mind and decided to go for a run right now, I’d burn more calories covering the same distance than a seasoned runner. Someone who’s been running consistently for years will expend fewer calories thanks to their greater efficiency in running form, gliding along like an elegant gazelle, while I’d be flapping around like a beached whale.
And this isn’t just me showing off that I know how to use similes; the research supports it.
In one study, researchers examined how endurance training affected energy expenditure over eight months in a group of young (~31 years) and middle-aged (~49 years) men. By the four-month mark, there was a significant reduction in energy expenditure of 0.7-1.3 kcal/min. To put this into perspective, what initially might have been around 8 kcal/min might drop to roughly 7 kcal/min after consistent training.
Of note, losing weight further increases this efficiency. Recently, Creasey and colleagues found that walking burns fewer calories in weight loss maintainers than in people who are still obese, even if they used to weigh the same. For example, 30 minutes of brisk walking burned about:
- ~137 kcal in weight loss maintainers
- ~205 kcal in currently obese people
So, the question is, how does this impact your total daily energy expenditure (TDEE)?
This is where it gets a bit tricky.
Firstly, increased activity will increase your overall energy expenditure, and this effect will be greater in people shifting from sedentary to active. However, in practice, the increase in TDEE from added activity tends to be smaller than you’d expect due to two primary factors:
- Energy compensation: If you go for a run, you might feel more tired and unconsciously reduce activity throughout the rest of the day. Perhaps you’re more inclined to sit down, take the lift instead of the stairs, or hop on a bus rather than walk home. Thus, the calories you burned during the run can be partially offset by a reduction in movement during the rest of the day.
- One review paper, for instance, found that about 67% of the studies they looked at showed a decrease in non-exercise physical activity when people began a structured exercise program. This finding was consistent in short-term (80% of studies) and long-term (63% of studies) interventions.
- Flack and colleagues demonstrated this in a 2018 study. Overweight participants who exercised to burn approximately 300 kcal/day compensated by 63%, and those burning 600 kcal/day compensated by roughly 34%. Since neither resting metabolic rate nor caloric intake changed, compensation likely occurred through subtle reductions in non-exercise activity or improved efficiency in everyday movements.
- One review paper, for instance, found that about 67% of the studies they looked at showed a decrease in non-exercise physical activity when people began a structured exercise program. This finding was consistent in short-term (80% of studies) and long-term (63% of studies) interventions.
- Expenditure is constrained: Most people assume that if they do more exercise, they’ll burn more calories. While that’s broadly true, when activity levels get extremely high, your body starts to downregulate other components of your metabolism to keep total daily energy expenditure within a relatively narrow range.
- In a 2019 paper, researchers studied six runners (five men and one woman) who ran six days per week for 20 weeks, completing a ~3100-mile race across the USA. At the start of the race, their total energy expenditure was elevated to approximately 6200 calories, within ~2% of the predicted value. But by week 20, their actual total energy expenditure had significantly decreased by 20%, settling around 4900 calories/day. This notable reduction highlights the body’s constraint mechanisms, limiting total energy expenditure during prolonged periods of high activity.
- Additionally, your energy status will influence the extent of this energy constraint. If you’re at caloric maintenance or in a calorie surplus, more exercise will increase energy expenditure (up to a point). However, when you’re in a calorie deficit and losing weight, increases in exercise won’t have that big of an effect on increasing your total energy burn.
With that being said, there are two things to keep in mind:
- The fitter you are, the harder and longer you can work out: While you may burn fewer calories per minute due to better efficiency, fitter individuals typically perform at higher intensities or durations. For example, a trained runner might burn ~10% fewer calories per mile at the same pace but will likely run faster or further than someone less trained (like me), ultimately resulting in higher total calorie burn.
- There’s a ceiling to energy compensation: Although the body tries to compensate by reducing calories burned elsewhere, this compensation has limits. Elite endurance athletes, with daily activity levels exceeding five times their basal metabolic rate (BMR), still see significant increases in total energy expenditure despite these compensatory mechanisms. But unless you’re engaging in extremely high levels of exercise, it’s unlikely you’ll fully overcome this energy compensation barrier.
In sum
If you keep your exercise routine exactly the same (same intensity and duration), you’ll burn fewer calories per session over time as your body becomes more efficient. To increase your total calorie burn (and thus TDEE), you’ll need to progressively push yourself by running faster or longer. Even then, the overall increase in TDEE will be smaller than you’d expect due to compensatory reductions in non-exercise activity and metabolic constraints.
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