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Why You Don't Always Lose Weight With Exercise

A new study finds that obesity makes it harder to exercise the weight off.

Key points

  • Energy expenditure and physical effort are not linearly related in the human body.
  • Our bodies evolved compensatory mechanisms to balance the energy required to exercise with the energy available for key biological processes.
  • The body automatically compensates during exercise and holds back at least 25 percent of the calories we might expect to expend.
  • Additional body fat makes losing weight even harder due to these evolved compensatory processes.

Once upon a time, we were led to believe that if do some exercise and expect to burn 100 calories, that we would actually burn 100 calories. A new study suggests that this expectation will not be fulfilled. At best, you are going to burn about 25 less than desired.

I know that sounds like a small difference. However, look at the math this way: What if you ate 25 percent fewer calories every day? Scientific studies have shown overwhelmingly that a 25 percent reduction in daily calorie intake will significantly improve your health far more effectively than exercising. Therefore, the results of this new study provide a critical bit of information that should influence our expectations about exercise on general health.

The results of this study, combined with the findings from numerous similar studies, explains why most people who begin to exercise actually lose less weight than would be expected for any given effort expended. Bodies evolved complex compensatory mechanisms to balance the energy necessary to contract muscles with the energy available for other critical biological processes. This new study reported that the body automatically compensates (with some variation from person to person, of course) during exercise and holds back at least a quarter of the calories we might expect to expend.

Scientists have speculated that energy expenditure is not linear. A nine-year-old study of African hunter-gatherers discovered that people who regularly walked or jogged for hours burned about the same number of calories each day as relatively sedentary Westerners. Apparently, the active tribesmen's bodies compensated by reducing the overall rate of calorie consumption in order to avoid starvation while hunting.

Additional studies have also reported that more activity does not necessarily result in greater daily calorie expenditure. Understanding the nature of this biochemical compensation is important because this evolved trait has become maladaptive for modern humans who consistently fail to burn off excess calories with exercise.

The investigation, published in Current Biology, collected data from 1,754 adults who drank doubly labeled water (containing isotopes that allow researchers to determine the number of calories burned per day). The researchers also monitored their body compositions and basal rate of energy expenditure. This basal rate represents the number of calories someone burns by simply being alive. The researchers subtracted the basal energy consumed from total energy expenditure to obtain an approximation of the energy expenditure from exercise as well as other movements, such as standing, sitting, and walking.

As expected, more movement burns more energy. But the data revealed a surprising outcome. As each person exercised, they did not burn as many total calories as expected. In fact, most subjects burned only about 72 percent as many additional calories as would be expected given their level of activity.

The compensation was even greater in obese adults. Their bodies tended to reduce by 50 percent, or more, the actual number of calories burned by exercising.

What This Means

This surprising discovery raised some interesting questions. Is there an association between the body’s compensation and obesity? Are some people genetically predisposed to over-compensate and therefore more likely to accumulate body fat? Future studies will focus on the specific components of cellular or organ functioning that are being slowed in order to allow a reduction in the number of total calories expended during exercising.

Overall, these novel discoveries are consistent with numerous previous studies showing that exercise alone is often not an effective way to lose weight. Usually, people will also have to eat less.

References

Wenk GL (2021) Your Brain on Exercise. Oxford University Press.

Careau et al., (2021) Energy compensation and adiposity in humans, Current Biology, https://doi.org/10.1016/ j.cub.2021.08.016

Pontzer H et al (2012) Hunter-Gatherer Energetics and Human Obesity. PLoS ONE 7(7): e40503. https://doi.org/10.1371/journal.pone.0040503

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