Energy expenditure equals BMR plus energy burnt from acivity – as Pontzer calls it the “armchair engineer’s theory of metabolism” – is as intuitive a model as it is inaccurate. In a journey from observational studies on the Hadza tribe to the latest research in biochemistry and nutrition, Pontzer gives a detailed account of the reasons as to why that simplistic understanding does not apply in practice and the main consequences for tackling the obesity crisis: above all that a healthy weight will not come from more exercise alone.

The simplistic approach of summing calories from activities simply doesn’t, allowing the pun, add up. When energy is scarce, our bodies downregulate non-essential functions like inflammation and subliminal movement (fidgeting, gesticulating) to maintain energetic homeostasis. Which applies to exercise as well, more of which just causes the body to shut down even more whilst becoming unusually efficient at extracting calories from the food we ingest. As a consequence, people who differ wildly in extercise volume and overall activity still end up basically consuming the same amount of energy on average – be they sedentary people, athletes or hunter-gatherers. Thus exercise, whilst beneficial for our health in many ways, is not in itself sufficient to cause weight loss and long-term weight maintenance.

Apart from the overall entertaining and witty writing style, it is the selection of facts that the author cites in support of his research that make Burn worth reading cover to cover. Probably the most interesting factoid I learned from this book is that mammalian organs change size during times of drastic caloric deficit, and that most of them shrink (like the liver, and above all the spleen), while others retain their size (brains and testes), whereas the gut actually grows. “The Darwinian manager is making tough decisions, picking winners and losers.” Fascinating stuff and exceptionally well presented. No less informative is Pontzer’s discussion of various adaptations in humans to the agricultural / pastoralist lifestyle (genes like NAT2, FADS1, FADS2) which are part of the puzzle that is the explanation of the vast supply of calories that individuals have access to in modern times. Delightful too Pontzer’s rather fatal application of his research to fad diets like the paleo and raw food diets.

It is somewhat irritating that the author refers to professional road cycling as “Tour de France cycling” but as an American he may be forgiven. Also it is unusual and highly confusing for a science book with graphs, popular or not, to default to imperial units over metric ones. These however are the only apparent flaws of Burn, which among non-fiction books is a rare page-turner.

Herman Pontzer (2021): Burn. The Misunderstood Science of Metabolism, ISBN 978-0141990170