What Is Health? Allostasis and the Evolution of Human Design

There is valid criticism to be had of medicine and its reductionist approach and What Is Health? sees neurobiologist Peter Sterling offer a critique grounded in physiology. Much medical thinking and education, he writes, revolves around homeostasis: self-correcting negative feedback loops, comparable to the thermostat in your home. These allow the body to regulate physiological processes, e.g. blood pressure, without involving the brain. However, he found that many endocrine cells do have nerve endings terminating on them. Homeostasis is part of the story, but "error-correcting feedback offers no basis for a full model of human design" (p. xxx). Together with Joseph Eyer, he coined the term allostasis in the 1980s: the brain is involved by predicting the body's needs and mobilising resources to meet expected demand, temporarily up- or downregulating processes when an organism's environment changes. In simple terms, homeostasis corrects, allostasis predicts. This also gives us Sterling's definition of what health is: "the capacity to respond optimally to fluctuations in demand" (p. 154). This idea has been criticised as resulting from too narrow a reading of homeostasis that offers nothing new. Do we really need allostasis as a separate concept? Having read the rest of this book I am not sure that disagreements over definitions make much difference to Sterling's point that modern life can break this physiological control system.

Before getting to that discussion, though, he spends the first four chapters on a deep evolutionary history tour to trace the origins of the components of allostasis. This tour encompasses the molecular details of genetics and biochemistry; the evolution of multicellularity, early brains, dopamine reward circuits, and endothermy; and the evolution of Homo sapiens when it left Africa ~150,000 years ago and the changes in brain design that allowed us to oust our Neanderthal cousins. Overall, these chapters are well written and full of fascinating information, though I am not sure all of it is necessary to understand allostasis. Also, there are a few minor points I take issue with. Some phrases are clunky shorthand that could be mistaken for outdated linear thinking about evolution, even though Sterling's writing in the rest of the book suggests no such thing. Furthermore, he strongly argues that evolution has produced optimal structures or optimal trade-offs and concludes that "clear examples of suboptimality are scarce, if they exist at all" (p. 9). I am not sure I agree. Nathan Lents discussed many examples in his book Human Errors, and we know that evolution excels at reusing existing structures for new functions. Perhaps these are just issues of semantics.

Chapters 5 and 6 finally deliver the goods. The mechanism of allostasis breaks down when demand is excessively high for sustained periods. The body responds by shifting its operating range upwards and what used to be exceptional becomes the new normal. In the example of blood pressure you end up with chronic hypertension. Something similar happens with our reward circuits. We evolved as socially living hunter-gatherers that had regular physical exercise, where children of different ages played together unsupervised, where we learned and perfected skills such as hunting over decades, and where the elderly contributed to care of the young. Call them the simple pleasures of life but that is the whole point, they were sources of regular small pulses of dopamine.

Sterling argues that instead we now try to get our dopamine hit from alcohol, nicotine, drugs, food, gambling, pornography, or shopping. And many of these deliver greater surges, with allostasis adapting us to take such surges as the new normal, fostering addiction. By his reasoning, the Western epidemic of "lifestyle diseases", even climate change, are all a consequence of our modern, dopamine-deprived lifestyle.

Many books argue that we are the victim of an evolutionary mismatch and Sterling puts neurological flesh on the bones of that argument. More importantly, he pleads with us not to blame our heritage. This reward circuit "inherited from worms [...] works exactly as it is supposed to – just not for what it was intended. This has been termed a "mismatch" [...] but that euphemism avoids facing squarely that "how we live now" is intolerable to a large fraction of our population" (p. 138). The tragedy is that medical practitioners try to combat it with pharmacotherapy – a pill here, a beta-blocker there – in a vain attempt to correct specific physiological parameters without recognizing that "the underlying biochemical and neural circuits are not actually broken" (p. 165).

Sterling's argument is attractive though some claims do seem rather sweeping. Contrasting our hunter-gatherer lifestyle with "how we live now" raises the question: what of the intervening time? He mentions the situation eroded first gradually with the advent of agriculture, then rapidly with the Industrial Revolution that "accelerated the process and exaggerated it grotesquely" (p. 131). Archaeology and palaeopathology tell us that the shift from foraging to agriculture was a Faustian bargain that took a heavy toll on our health. But did we suffer the same lifestyle diseases that we see now? Or was it the Industrial Revolution and especially The Great Acceleration that pushed us over the edge? In the latter case, it would seem there are more ways than the hunter-gatherer lifestyle to live a fulfilling life.

Are there solutions? Fortunately, Sterling does not advocate we head back to the caves and draws attention to the, I think, underappreciated argument that history has acted as a ratchet: "higher population densities gradually disallowed any return to the wild" (p. 128). One option is to work with the body's mechanism of allostasis, not against it, through what he dubs "system therapy", which is basically what rehab is for drug addicts. It is hard and slow, but the only way to resensitize our body's reward circuit to more modest dopamine pulses. Preventative strategies would involve changes to modern life to restore physical and mental challenges, lifelong learning, and social relationships between the generations. These suggestions are, to my taste, rather generic and I would have loved for him to develop this part of the book fuller.

Given that neurobiology can be a technical topic, Sterling writes accessibly and makes good use of illustrations to clarify principles further. I found the detour into deep evolutionary interesting, even if not all of it was relevant to the central argument. Though I am on the fence regarding some of the material here, What Is Health? is overall a cogently argued book that provides both reason for concern and food for thought.