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Eating vs. exercise: Science declares a weight-loss winner


Dr. Yoni Freedhoff

A new study came out last week that further explained why it is that while exercise does burn calories, it doesn’t help with weight management nearly enough to be fair.

The paper, “Constrained Total Energy Expenditure and Metabolic Adaptation to Physical Activity in Adult Humans”, aimed to explore why it is that exercise’s impact on energy expenditure doesn’t appear to be linearly “additive”—meaning that studies on energy expenditure suggest that increases in exercise don’t come with a comparable increase in total daily calories burned. In part this is because with increasing exercise comes increasing exercise efficiency, but the remainder is less clear. Is there an “exercise” thermostat in our bodies that effectively dials down our activity levels outside of our workouts, whereby if you work out hard in the gym in the morning you’ll sit more and fidget less for the rest of the day?

Here the authors put forward their theory of “constrained total energy expenditure” to explain the phenomenon that objectively measured energy expenditures don’t seem to vary much the world over. From the First World to the Third World, as a species we seem to share the same total daily energy expenditures each and every day. This fits with the authors’ constrained hypothesis.

To test their constrained model, they objectively evaluated energy expenditures, in 332, mixed-sex adults drawn from Ghana, South Africa, Seychelles, Jamaica, and the United States. The measures tracked were total energy expenditure by way of doubly labeled water method, resting metabolic rate by way of respirometry, and physical activity by way of wearable tri-axial accelerometers.

What they sought to learn was which model would be best represented by the subjects’ objective measurements: additive or constrained? If additive, you would expect a linear increase in energy expenditure with activity. If constrained, you’d expect adaptations to blunt increasing energy expenditure even as activity levels rose.

additive-constrained-energy-graphThe authors found that the plot of their cross-sectional subjects’ activity vs. energy expenditure wasn’t linear and additive like the first graph, but rather was blunted and constrained like the second.

Also worth noting, the authors point out that regardless of which model you choose to believe in, their findings had physical activity accounting for only 7% to 9% of the variation in total energy expenditure after controlling for anthropometric variables and population location. 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, no one should 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.

(Below is my keynote presentation for PHE Canada where I make the case for rebranding exercise)

Dr. Yoni Freedhoff is a family physician and assistant professor at the University of Ottawa. He is the author of The Diet Fix, and founder of Ottawa’s non-surgical Bariatric Medical Institute, a multi-disciplinary, ethical, evidence-based nutrition and weight management centre. Visit him on Twitter and Facebook, and read his blog, Weighty Matters.




PV Mayer

Dr. Perry Mayer is the Medical Director of The Mayer Institute (TMI), a center of excellence in the treatment of the diabetic foot. He received his undergraduate degree from Queen’s University, Kingston and medical degree from the Royal College of Surgeons in Ireland.

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