Burn: New Science Reveals How Metabolism Shapes Your Body, Health, and Longevity by Herman Pontzer
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This book is an extremely interesting exploration of some emerging science on human energy expenditure and metabolism. The book is more or less complementary to The Hungry Brain: The Hungry Brain explored the science of appetite as it relates to overeating, while this book primarily looks at how energy expenditure affects (or in some cases doesn't affect) weight gain. The two books are broadly compatible, and (in relevant chapters) they actually cite some of the same science.
The core idea driving this book is what the author refers to as the "Constrained Energy Expenditure" model of human metabolism. Put simply, our body will only briefly increase our overall energy usage in response to exercise or stressors, and will, over time, return close to our baseline energy expenditure even if the same activity levels are maintained. This is possible because much of our daily energy expenditure is taken up by autonomous tasks such as immune response and reproductive function or extraneous movement (fidgeting). When we exercise regularly, after a short adjustment period, our body reacts by either encouraging us to eat more, or, in the absence of excess calories, reducing energy spent on autonomous or extraneous functions until we're close to our original energy baseline. This, if true, means that exercise is effectively irrelevant to the process of weight loss.
The author provides scads of research from a number of novel directions that all seem to confirm this hypothesis. For example, modern hunter gather cultures who are significantly more active than the average city-dwelling adult, nonetheless burn just about the same number of calories a day. According to the author's theory, the fact that they spend more of their energy in motion means that they would have suppressed some of their autonomous functions, and this is borne out, for example, by the fact that they have lower levels of sex hormones (representing reduced reproductive function). Similarly, the author suggests that while reduced immune function may sound bad, it may be that inflammatory diseases and allergies are a byproduct of too much energy available to the immune system, and that modestly reducing immune function (through exercise) may reduce the incidence of some "diseases of civilization."
The theory seemed (to me) a little too pat because there were surely limits to this kind of auto-cannibalization, and the author, helpfully addresses that too. Obviously, professional athletes, soldiers in the field, and other very active individuals expend more energy than their body could possibly redirect from other functions, and, in some cases, even more than they can effectively replace by eating ad libitum. That threshold of digestion, for most people, the author suggests is about 2.5 times their resting metabolic rate (~5,000 kilocalories on average, but obviously variable). Above that ~2.5x threshold-- burning more than that number of calories-- athletes begin losing weight no matter how much they try to eat. But that kind of energy expenditure is on the order of running a marathon every day or walking for four or five hours, not the kind of activity levels sustainable for most people.
So what good is exercise anyway? The author notes that, weight loss aside, exercise has many obvious health benefits. He also notes that, while exercise is not particularly useful for weight loss it seems to play an important role in preventing weight gain or re-gain after loss. For example, there is some evidence that total sedentariness leads to significant weight gain as compared to even very moderate activity levels. Additionally, the bodies of folks who lose significant quantities of weight don't naturally reduce their energy intake, they continue to seek higher levels of calorie intake suitable to maintain their previous weight. As a result as soon as restrictive diets are eased, the folks begin regaining all of the weight. The author argues that the science suggests that exercising after weight loss diverts those excess calories into exercise instead of rebuilding the weight, and is one effective way to prevent weight re-gain.
Taken together with the information in the The Hungry Brain this creates a very different picture of how obesity works than the "standard model." Rather than exercising more and eating less, The Hungry Brain suggests that food composition and preparation have more to do with overeating than raw calories would suggest (although calories obviously count), and this book suggests that exercise quantity (beyond a minimum threshold of activity) is, essentially, a neutral factor while losing weight, but more exercise is an important lifestyle choice for health and weight maintenance.
And I find it all, more or less, convincing. But, unlike the The Hungry Brain which seemed to address all of the hard issues in its topic area, Burn left me with some questions unanswered. The biggest of these, which the author touches on but doesn't really run to ground, is how muscle mass and body composition figure into it. He acknowledges that lean mass affects resting metabolic rate, in some cases meaningfully, but the studies he relies on are overwhelmingly focused on cardiovascular exercise. He doesn't seem to seriously engage with or refute the idea that strength training (or other exercise that builds lean mass) might actually be effective at weight loss by changing the baseline energy expenditure.
But, quibbles aside, the book is nicely written, exhaustively footnoted, and it definitely added to my understanding of human metabolism. It can be a bit chatty at times, but that's de rigeur for popular science writing. Recommended.