You can't eat total crap and then bust your butt exercising to magically erase those kilojoules: It's not that simple. If you want to make longer lasting, healthier changes, what and how much you eat is far more important than how much you exercise.
Image by Georgie Pauwels.
Thinking in terms of kilojoules burned through exercise will only hurt your weight loss efforts. The reason is that we tend to grossly underestimate the number of kilojoules we've eaten and overestimate how many kilojoules we've actually burned.
Dr. Yoni Freedhoff, an assistant professor at the University of Ottawa who blogs at Weighty Matters, notes a study in the journal Current Biology, where the authors propose the fascinating idea of "constrained total energy expenditure".
In short, the number of kilojoules you burn doesn't go up in a straight line with increased physical activity. Further, our bodies adapt to long-term increases in activity either by subconsciously moving less in other cases, or by "slashing" energy costs elsewhere in the body.
In other words, after a certain point our bodies try to keep the kilojoules we burn relatively consistent, regardless of how much more general physical activity (such as walking) we do. Sounds counter-intuitive, right? To test the idea, the researchers looked at the total energy expenditures, resting metabolic rates and physical activity levels of 332 adults (with a nearly 50-50 representation of sexes) from five populations: South Africa, Ghana, Jamaica, Seychelles, and the United States. They made sure to control for variables like weight, age, height, body fat and so on.
If we do burn kilojoules strictly according to how much we exercise, the authors would expect to see a linear increase in calories burned with activity. On the contrary, they found that at a certain point daily energy expenditures hit a plateau and were "constrained", with physical activity accounting for only seven to nine per cent of the differences in total kilojoules burned across their cross-sectional population.
Essentially, physical activity has a smaller impact than #fitspiration quotes like "Excuses don't burn calories" would have you believe. The study's not perfect, and there's no excuse for not exercising, but still, the takeaway here is as Dr. Freedhoff says:
Translated this means that when it comes to energy balance, what you eat matters a hell of a lot more than how much you exercise regardless of how exercise contributes to energy balance. It also means that you're not likely to be able to outrun your forks.
Of course you shouldn't take this as a licence not to exercise, as exercise is probably the single most important modifiable determinant of your health. Putting this another way, you lose weight in the kitchen, you gain health in the gym.
Exercise is great for energy, mood, and health, but as far as weight loss goes, "moving more" is only a part of the equation.
"Constrained Energy Expenditures" and Not Outrunning Our Forks [Weighty Matters]