Burn 🔥

I recently finished reading Burn by Herman Pontzer, and I’m going to do a mini-report on it here, because wow — 11/10 recommend. It’s an approachably-written deep dive into the evolution of WHY our bodies and metabolisms work the way they do in the 21st century, with lots of modern science to back it up (and Pontzer gets bonus points for being from Duke! 👊🏼). This book confirmed the tenets I use every day as a medically-educated nutrition coach, but gave me a broader and deeper context than I had ever previously considered.

The biggest 💡 takeaway for me was the constrained model of energy expenditure. This is explored in a large body of scientific literature beyond just this book, but this was my first time reading so many details of WHY our simplistic “additive” idea of calorie burn is wrong. We’ve been taught that we can add up a bunch of numbers (basal metabolic rate plus digestion plus NEAT plus exercise) and calculate someone’s total energy output, when it’s far more complicated than that. (This is also why your fitness tracker has no idea how many calories you’re burning. Would you believe that hunter-gatherers who walk 5-10 miles a day don’t actually burn any more calories than we Western folks do?!)

You can see how this has a bunch of implications. 🤯 One is that exercise is not a “stimulant,” it is a SUPPRESSANT. When we exercise regularly (even just walking), our bodies “turn down” other processes to make room for the exercise within our expenditure budget. And we want some of this “turning down” to happen — because, at the levels that most of us experience, the suppressive effects are good for us. They keep a lid on our inflammation, stress responses, immune function, and reproductive hormones (which, by the way, are higher in our industrialized population than they are in hunter-gatherers — which probably explains some of the reproductive cancer risk reduction in those who exercise regularly. On the flip side, it’s common knowledge that overtraining leads to too much hormonal suppression. Dose matters.)

In a nutshell, we have evolved to have a much higher metabolism than our chimpanzee brothers who lie around eating leaves and bugs all day — and, just like a puppy or a toddler, that high-energy metabolism needs something to DO. It gets up to nefarious tricks if we don’t exercise it (see what I did there? 😉).

The reason I emphasized that part first is because — as I alluded to above — another of the implications of a constrained energy system is that exercise doesn’t cause weight loss, at least not in the medium or long term. And, yeah… this has been studied over and over again, and is completely true. (Sorry. 🤷‍♀️) We’ve been taught that if we burn more calories than we’re eating, we’ll lose weight — when really, the emphasis should be on the other side of the equation: we need to EAT fewer calories than we BURN. A calorie deficit resulting in fat loss cannot usually be achieved by simply exercising more; it must be achieved by EATING LESS than is being expended. (More on that below.)

HOWEVER, as above, exercise DOES do a whole host of other metabolic things that are crucial for human health — including helping the brain match appetite to caloric needs — all of which helps us MAINTAIN weight loss. (And THAT side of the coin has been studied over and over again as well. Once people have successfully lost weight, it ONLY stays off if they are moving their bodies regularly.)

As far as eating less than we burn… Pontzer has with his readers a conversation that I have nearly daily with clients. Metabolism doesn’t “cause” low energy availability; it RESPONDS to low energy availability. The person who “gains weight if they go above 1400 calories” is a person who has been eating so low for so long that their metabolism doesn’t trust that there will ever be anything more. For that person to lose body fat, we have to gradually reverse-diet FIRST, thereby reassuring the metabolism that it’s okay to get out of the famine mentality and actually ALLOW fat loss to happen. Because, in actual fact, that person can probably maintain their weight on 400-500+ calories more than they’re currently eating (and their minds are inevitably blown when they realize this 😎).

💡 Constraints on our energy systems are also what invokes a “metabolic ceiling” to human performance. This is displayed in different ways in different sports, but the overarching idea is that our digestive systems (=how much energy we are able to effectively ABSORB) probably have just as much to do with high athletic performance as strong muscles do. It’s false that ALL overtraining is simply due to undereating; each individual has a metabolic/digestive set point beyond which we are not capable of compensating. The book includes a fairly comprehensive mathematical analysis, but on average, most of us cannot absorb more energy per day than about 2.5 times our BMR. (The author theorizes that Michael Phelps, along with being anthropomorphically blessed, is probably also a digestive outlier, absorbing closer to 3x his BMR and hence having a major recovery advantage.)

Using myself as an example, the highest I’ve ever consistently eaten (during a super-high-volume training period) was about 3200 calories/day, and while that was fine, I learned from this book that my body likely wouldn’t be able to absorb nutrition from beyond about 3500 calories no matter what my activity level was. And imagine someone smaller and lighter with less muscle mass — they might not be able to reap dividends much beyond 2800. This is why extreme endurance folks are inevitably somewhat malnourished and underweight by the end of multi-week events, even with excellent fueling strategies. The constrained model of energy expenditure means there is a point beyond which the in/out equation no longer works. In studies, giving overtrained athletes more food does not solve the problem (unless there’s an underlying eating disorder/RED-S, which there often is, but that’s a post for another day).

Pontzer ends the book with a sobering chapter on our current food and environmental situation. These are inextricably linked, because one of the ways human bodies have become so efficient at extracting energy from food is that we have evolved to be reliant on external energy (=fire!). The human body doesn’t run very well on a raw food diet (leads to malnourishment). Mathematically speaking, our current food system violates the fundamental laws of ecology by burning 8 calories for every calorie of food we produce. We have maybe 50 years’ worth of oil and natural gas left, and perhaps 110 years’ worth of coal — so we’re running out of time from a resources standpoint — but we’re screwed either way, because if we continue as we are and slowly put a few greener systems together while just burning through everything that remains, half of our population will be dead within 200 years due to sea levels rising by 100+ meters. Evolution’s decisions are always driven by what is going to result in (1) more food availability and (2) more babies being born… and we have over-optimized both those systems to the point that we are now “evolving” ourselves right into extinction.

There is so much more in Burn that I haven’t touched on here, and if you are a human who eats food, I highly recommend it. My one critique is that I would’ve liked to see more discussion of the metabolic factors around muscle mass, strength training, and variable exercise like CrossFit, as opposed to straight endurance work. Muscle mass was alluded to once or twice as a contributor to the wide variation in BMR, but discussions of exercise (and the scientific study of it) were limited to endurance activities. However, I’m hoping that that’s simply because there’s such a tremendous amount to say on the topic that it merits its own book altogether (might I suggest it be called Flex?). 😉