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Exercise Burns Fewer Calories Than Previously Thought


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#1 Steve H

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Posted 05 October 2021 - 03:17 PM


Researchers have proven that our bodies partially compensate for calories burned during exercise by cutting energy expenditure on vital functions. Their discovery might explain why it can be so hard to lose weight by working out [1].

The three components of energy expenditure

As much as we love covering mind-blowing scientific breakthroughs in the longevity field, we have said time and again that for now, exercise and weight loss remains one of the most effective life-prolonging interventions available [2][3]. This new study, conducted by an international group of scientists, adds a lot to our understanding of the relationship between exercise and energy expenditure and might help us develop better weight loss strategies.

Our total energy expenditure (TEE) consists largely of basal energy expenditure (BEE), which powers digestive, immune, and cardiovascular systems as well as other basic organismal functions, and of activity energy expenditure (AEE). Scientists have proposed three theoretical models of the relationship between TEE, BEE, and AEE.

One, called the additive model, suggests that BEE hardly changes as activity level goes up, and all the additional AEE just piles up on top, increasing TEE by the same amount. This model is still the one widely used to calculate the caloric impact of exercise.

The performance model says that increases in AEE cause increases in BEE – basically, that exercise increases not just energy expenditure but also overall metabolic rate.

Finally, the third model, called the compensatory model, postulates that increases in AEE cause BEE to decline: our body compensates for the energy spent on physical activity by cutting down the energy allowance on other processes.

Compensation is due

There has been some evidence that the compensatory model is the correct one [4], but in this new study, the scientists were able not just to remove the question mark but also to quantify the compensatory effect.

By using a large dataset generated from more than a thousand subjects along with doubly-labeled water, the ultimate method of calculating metabolic rate, the researchers were able to prove that the median energy compensation stands at 28%. That means that our bodies compensate for more than a quarter of the calories that we burn via exercise.

Among the many variables that the scientists corrected for, such as age and sex, one stood out: the body mass index (BMI). It turns out that obese people experience significantly more energy compensation – up to 50%.

It is hard to overestimate the importance of these findings. They can explain why it can be so difficult and frustrating to lose weight by working out. For instance, while every source in the world tells us that we should have lost a thousand calories during a specific workout, the actual calorie loss might be half of that! This gap between expectation and reality can confuse people and even cause them to give up exercise altogether.

The researchers propose three possible reasons for why obese people experience more energy compensation. First, they might be genetically predisposed to this, which might have contributed to the weight gain in the first place. Second, compensation levels might grow as a function of BMI via a still unknown mechanism. Third, since obese people often accompany exercise with strict diets, their bodies might increase compensation to make up for the combination of more energy expenditure and less energy intake.

Preventing weight loss might not be the only detrimental effect of energy compensation. BEE involves many essential functions that energy compensation might compromise. For instance, the researchers suggest that winding down energy expenditure on the immune system can harm immunocompromised people. Obese people might be in double jeopardy since obesity is associated both with numerous disorders and a higher energy compensation rate. These aspects of energy compensation must be thoroughly studied.

Are other factors involved?

The researchers explore several other possible factors that could contribute to energy compensation. One of them is non-exercise activity known informally as “fidgeting” – the myriad of small conscious and unconscious body movements that we make during the day. The hypothesis was that fidgeting decreases with more exercise (literally, being so tired that you can’t move a muscle). Yet, the scientists conclude that it is very unlikely that fidgeting – or any other factor they explored – has a major contribution to the dynamics of energy compensation.

Finally, in their paper, the authors briefly ruminate on the possible evolutionary origins of energy compensation. They suggest that in the wild, where conditions of scarcity prevail, animals ramp up their physical activity mainly when their energy resources are low, in order to look for food. It is only logical that their bodies are trying to conserve energy while actively foraging or hunting.

Conclusion

Although more research is needed to elucidate the mechanisms of energy compensation, its quantification is a major breakthrough in itself. This will allow scientists, doctors, and fitness trainers to account for energy compensation in their work, and to build weight loss programs that better match expectations while causing less confusion, frustration, and harm.

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Literature

[1] Careau, V., Halsey, L. G., Pontzer, H., Ainslie, P. N., Andersen, L. F., Anderson, L. J., … & Speakman, J. R. (2021). Energy compensation and adiposity in humans. Current Biology.

[2] Mok, A., Khaw, K. T., Luben, R., Wareham, N., & Brage, S. (2019). Physical activity trajectories and mortality: population based cohort study. Bmj, 365.

[3] Peeters, A., Barendregt, J. J., Willekens, F., Mackenbach, J. P., Mamun, A. A., & Bonneux, L. (2003). Obesity in adulthood and its consequences for life expectancy: a life-table analysis. Annals of internal medicine, 138(1), 24-32.

[4] Pontzer, H., Durazo-Arvizu, R., Dugas, L. R., Plange-Rhule, J., Bovet, P., Forrester, T. E., … & Luke, A. (2016). Constrained total energy expenditure and metabolic adaptation to physical activity in adult humans. Current Biology, 26(3), 410-417.


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