Try to ignore the diety subtitle to the book a moment and strongly consider giving it a read anyway. Dr. Ponzter knows that the majority of his audience is going to be steeped in diet culture and what will motivate them. But this is not a diet book; it’s a science book, explaining the latest understandings (and some best guesses) about metabolic research and energy expenditure. He draws from human evolution, anthropology, and mammal physiology to paint a picture of human metabolism in a very broad and fascinating context.
At the heart of his research is a new model for understanding how human metabolism works, what he calls the constrained energy model, which gives scientific backing for the idea that we can’t just “work off” excessive calories and explores the health ramifications of this idea. To put it as simply as I can, he uses research, including extensive doubly-labeled water experiments, to conclude that human metabolism doesn’t simply use up more energy ad infinitum as we do more activity. Rather, the amount of energy we use in a day or week reaches a sort of equilibrium, and as we put more energy towards activities, we partition less energy towards other tasks within the body. Two-thirds of the book is Dr. Pontzer creating context for this conclusion and the evidence he drew upon and scientific exploration he went through to come to it.
When he does get around to the health benefits of exercise, Pontzer points out that this notion that energy expenditure is constrained leads to a natural conclusion, that habitual exercise and activity has very little impact on the number of calories we expend in a day. However, “exercise doesn’t change the number of calories you burn each day, but it does change how you spend them–and that makes all the difference.” (p237) It “has wide-ranging effects on how our metabolism is managed and where our calories are spent. . . ” (p 241) He then goes into some of the implications for this energy partitioning and why it may be that folks who exercise regularly and vigorously are at lower risk for heart disease, diabetes and other disease states common to industrialized societies. For example, it gives context to why we may have more moderated stress and inflammation responses when we are more active–we’re allocating our confined energies towards physical activity, which means it’s not going towards an elevated stress or inflammatory response.
I mentioned at the start that this isn’t a diet book, and it isn’t. There’s some information about how recent metabolic research might apply to folks hoping to lose weight, but mostly this is a book about the research, not about applying it. I respect that. Dr. Pontzer isn’t a nutrition coach or a trainer, he’s a researcher and academic. He’s staying in his lane.
In one foray into applications of this research, Pontzer explains why exercise alone as a fat loss intervention rarely helps people. I think I understand his point, however I feel that like with any complex system, it isn’t enough to just point to one element (exercise) and one outcome (fat loss) and draw a straight line. I know many people who started with physical activity and over time, their thinking about themselves, their bodies, and food changed and evolved, and ultimately, they developed healthier relationships with their bodies that often included lower amounts of body fat. Maybe, as the constrained energy model suggests, this has little to do with the calories burned, but it doesn’t mean that the physical activity wasn’t playing a role in the eventual changes in those people’s bodies.
I think that as his book gets more widely read, and as this research is slowly digested into the evidence-based fitness community, we are likely to see it influence the practices of coaches and trainers. Endurance athletes have a lot to take from this research, and Dr. Pontzer draws the most lines between high-level endurance sport and the constrained energy model. For physique athletes, I think there’s wisdom to glean here for the limitations of using extended bouts of exercise to meet caloric deficits necessary for their sport.
Maybe the most powerful application, though, in terms of the numbers of lives it could impact, is how this might change the efforts of chronic dieters and others who abuse exercise in order to pursue unrealistic body size. If Dr. Pontzer is correct and our bodies inevitably adjust our energy expenditures to conserve energy when we use more energy towards activity, then folks who believe they must do endless exercise to achieve or maintain a smaller body have a scientific tool that directs them towards a more healthful, balanced option. They can spend less time and energy on intentional activities and their bodies will adjust in order to stay within a certain range of energy expenditure. In simple terms, someone who abuses the stairmaster could quit tomorrow and (assuming that is the only change in their behaviors) shouldn’t see a dramatic change in their body size as a result.(1) I would hope this could be a liberating truth for folks who are working to develop a healthier relationship with movement and their bodies.
Exercise has many important positive outcomes for human bodies, regardless of its impacts on body size or composition. We were evolved to move, and we work best when we move regularly. I think this research may provide an important piece to that puzzle, helping us understand why exercise and daily activity plays such an important role in our health and longevity. Like anything in the realm of science, I’m sure our understandings of these issues will continue to evolve with new evidence, and I look forward to learning more as the conversation continues. I enjoyed reading Burn, and I recommend it to anyone interested in having a more in-depth understanding of human metabolism.
Have you read Burn? Have thoughts? I’d love to hear them! Please keep in mind, we do have a comment policy.
(1) Of course, if the fat-phobia motivating the stairmaster abuse isn’t addressed, it is likely the person will continue to make unhealthful decisions in order to attempt to manipulate their body mass. And if the stairmaster is the only resistance training, there would be implications for the strength of the person’s legs over time. But this is just an illustration, and this is just a blog post, and it’s not meant to be a full analysis of all the ins-and-outs of disordered relationships with exercise.