More exercise burns fewer calories? You can't be serious?

Thursday, March 15, 2018. Author Dr Haran Sivapalan

More exercise burns fewer calories? You can't be serious?

One of the benefits of getting fitter is that workouts start to feel easier. While the prospect of doing 50 chin-ups or running 10k may sound daunting at first, with consistent training, such feats soon become achievable and even enjoyable. What previously seemed like a mountain to the untrained individual quickly becomes a mole-hill to the seasoned gym-goer.

How and why does this happen? Our bodies adapt to the demands of training: muscles grow, blood vessels sprout, our metabolism improves. Over time and due to these positive adaptations, exercise becomes easier.

But, does this imply that you now burn fewer calories when exercising? Similarly, would a highly trained Olympic Gold medallist such as Haile Gebrselassie use less energy to run exactly the same distance as a newbie runner?

The answer isn't straightforward.

If two people of identical bodyweight were to run the same distance at exactly the same speed, the laws of thermodynamics dictate they would use the same amount of energy – they would each burn exactly the same number of calories.

The trouble is, the calories burned while exercising are only one part of the equation, both figuratively and literally speaking. 

Our Total Energy Expenditure (TEE) i.e. the overall number of calories we use per day, is actually the sum of three component processes:

  • Activity-induced Energy Expenditure (AEE) – the number of calories burned during physical activity and exercise.
  • Resting Energy Expenditure (REE, also known as Resting Metabolic Rate) – the number of calories used for all those internal reactions that maintain our body at rest.
  • Diet-induced Energy Expenditure (DEE) – the number of calories we use to digest and process food.

Conventional wisdom suggests that the more you exercise, the higher your Total Energy Expenditure (because Activity-induced Energy [AEE] increases, while REE and DEE stay the same).  According to this additive model (see fig 1.), the number of calories you burn in a day rises in a linear fashion with how much physical activity you do.

Fig 1. An additive model of energy expenditure

Changes in resting metabolic rate with exercise

Interestingly though, despite leading lives that are far more physically active, studies show that subsistence farmers and hunter-gatherer populations have similar total energy expenditures to us in the West. Apparently, people who need to hunt-down their dinner burn as many calories as us office-dwellers who can simply order a take-out.

How could that be? Is it possible that over time, our bodies adapt and compensate for increased physical activity? Furthermore, is our total energy expenditure regulated to a fixed level, just as we tightly control our body temperature or the pH of our blood?

Some studies do support this hypothesis. One such study, which followed individuals over a 20-week program of combined walking and resistance training, found that total energy expenditure begins to plateau with extensive training. More specifically, although subjects clearly burned more calories during exercise (their Activity-induced Energy Expenditure increased), their resting metabolic rate (or resting energy expenditure) subsided, meaning they were burning fewer calories when not exercising.

What are the reasons for this? Again, it's our body's adaptation to training. A lower resting metabolic rate may be equally viewed as a ‘more efficient' metabolic rate – more of the energy from food is converted for use by cells in the body, while less is wasted as heat. With long-term exercise, our body endeavours to boost this efficiency, wasting less energy as heat and converting a greater proportion for use by muscles.

Likewise, trained muscles have been shown to produce lower levels of ‘uncoupling proteins'  compared to untrained muscles. These proteins, encoded by your UCP genes, serve to ‘uncouple' cell respiration from the production of chemical energy, instead dissipating it as heat. As your muscles become more efficient with training, they express less uncoupling protein (particularly UCP3), thereby making a greater fraction of energy available for muscle contraction and doing mechanical work. 

In light of this compensatory reduction in resting metabolic rate, many researchers posit that total energy expenditure more accurately follows a constrained model -i.e. it does not rise linearly with physical activity, but instead starts to tail off after a point (see Fig 2).

Fig 2. A constrained model of total energy expenditure

Changes in movement activity with exercise

Bear in mind that research findings are mixed regarding this reduction in resting metabolic rate. Several other studies demonstrate either an increase or no change in resting metabolic rate in response to long-term exercise. So, could there be something else limiting total energy expenditure?

One theory is that whenever we have an intense session at the gym, we compensate later in day by moving less. We fidget less, we stand up for shorter durations, we change posture less frequently. Where calories are concerned, all these movements start to add up. 

Indeed, a seminal study tracked the activity of 332 people across Africa and North America using wearable technology. After a certain level of exercise, total energy expenditure started to plateau, as predicted by the constrained model. The authors concluded that increases in purposeful exercise are partly offset by a concomitant decrease in non-purposeful muscle activity like fidgeting.

Take home message

Your total energy expenditure is only one side of the equation - how, what, and when you eat is also of great importance. If your goal is to burn calories and lose weight, none of the studies described above suggest you should suddenly give up exercise after a certain point in training.

Nor is the potential decrement in resting metabolic rate necessarily a bad thing.  A higher, and therefore more inefficient metabolism, can actually impair muscle building and fat loss!

Depending on your genetic make-up, including which versions of the UCP genes you carry, strategies such as HIIT and intermittent fasting could help you to optimize your metabolism.

I hope you enjoyed this article.  Please read my other blogs:  

How muscle memory makes you stronger

FitnessGenes publishes new research on obesity gene

The top 5 fitness trends of 2018

Nutrition tips for exercising in the snow

What do elite athletes think about DNA analysis

Health benefits of Christmas foods

How much coffee should I drink?

Air pollution and exercise:  Is it safe to train outside?

Is struggling with weight your destiny?

How Your Finger Length Influences Overall Strength

Just Say No to Upselling

Unravelling the Secrets of Our Circadian Rhythm

3 Easy Ways You Can Get Started

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