Preface — The Fire and the Fuel

Preface — The Fire and the Fuel

"La destinée des nations dépend de la manière dont elles se nourrissent." "The destiny of nations depends upon the manner in which they nourish themselves." — Jean Anthelme Brillat-Savarin, Physiologie du Goût (1825), Aphorism III

I n the first volume of this compendium the naturalist knelt, so to speak, at the table, and asked only why: why the tongue is seduced by salt and fat and sugar, why a ripe peach undoes us, why the whole elaborate theatre of flavour exists at all. It was, we hope, a delightful inquiry, and delight was its proper end. But a naturalist who stops at delight has told only half the story of the meal. The peach does not vanish once swallowed. It is dismantled, molecule by molecule, in the dark laboratories of the gut; its sugars are ferried into the blood; its carbon, borrowed a season ago from the sunlit air by the tree that bore it, is burned in some thirty trillion small furnaces and breathed back out into the room. Volume II follows the meal past the palate and into the body, to ask the second, sterner question: not why food delights, but what food does.

The governing image of this volume, encountered already on its title page, is the body as a slow fire. It is not a poet's fancy but a chemist's discovery, and a fairly recent one at that — barely two and a half centuries old, dated with some precision to a Parisian laboratory and a shivering guinea pig, as Chapter One will relate. Before that discovery, digestion was a mystery bordering on the alchemical: food went in, and by some unexamined magic, warmth and motion came out. Antoine Lavoisier was the first to demonstrate that respiration and combustion are the same chemical event conducted at different speeds — that the candle on the table and the diner beside it are, in the coldest and most wonderful sense, doing the same thing. Every calorie printed on every packet in every larder in the land is a distant echo of that demonstration. To eat is to tend a fire that does not go out until we do.

This is, admittedly, a less immediately lovable idea than the one that governed Volume I. A flame is not a poem. But the naturalist who follows it faithfully will find it opens onto marvels of its own: a bloodstream that shuttles glucose like coal to some thirty trillion small engines; a liver that hoards sugar against the night the way a squire hoards firewood against the winter; a molecule called adenosine triphosphate, unglamorous by name, that is quite literally the currency in which every muscle twitch, every thought, and every heartbeat is paid for. In the chapters ahead we will meet the vitamin hunters who chased invisible deficiencies across continents and ships' galleys; the microbial garden coiled in twenty-odd feet of gut, performing chemistry no human enzyme can manage alone; the pigments that colour a plate and, not coincidentally, appear to do it some good. It is, in short, no less wondrous a subject than flavour was — only sterner in its standards of proof.

And on that matter of proof, this volume must state its rules plainly, for nutrition science is, of all the sciences that touch the kitchen, the one most prone to overreach. Two commitments govern everything that follows.

The first we call, only half in jest, the Matrix Mandate. It is the gravest and most persistent error of popular nutrition writing to seize upon a single isolated nutrient — a vitamin, an acid, a pigment — and credit it with the whole virtue of the food that carries it, as though an orange were merely a wrapper for ascorbic acid, or a bowl of porridge merely a delivery vehicle for a species of fibre. It is not so. A nutrient dissolved in a pill and the same nutrient nested in the intact architecture of a whole food — its fibre, its water, its thousand accompanying compounds, its very cellular walls — behave differently once inside the body, and often behave quite differently indeed. Wherever this volume examines a single molecule, therefore, it undertakes to walk the reader back to the food that contains it: to the orange, not merely the vitamin C; to the oat, not merely the beta-glucan. The plate, not the pill, is the true unit of nutrition, and we shall return to it as faithfully as a compass returns to north.

The second commitment is honesty about the limits of what is presently known. Nutritional science is conducted, for the most part, not in Lavoisier's sealed calorimeter but in the far noisier laboratory of ordinary human life, where subjects misremember what they had for luncheon, where confounding variables multiply like weeds in an untended border, and where a single provocative study can command headlines for a decade before a larger and duller one quietly overturns it. The methodologist John Ioannidis has done the field a great service by reminding it, repeatedly and with some rigour, how often its published findings fail to replicate. This volume will not disguise that difficulty behind false confidence. Where the evidence is strong — built upon randomised trials and large, consistent bodies of research — we shall say so plainly. Where it is merely suggestive, drawn from a handful of observational studies or a single elegant experiment awaiting confirmation, we shall say that too, and resist the temptation to dress a hypothesis in the costume of a settled fact. Where genuine scientific argument persists — as it does, honestly, over salt, over saturated fat, over the very question of how much exercise "moves the needle" on the body's fire — we shall report the argument rather than pretend it has been settled by one side's press release. The reader deserves an honest map of this territory, including its unmapped stretches, rather than a confident wrong one.

Neither commitment should be mistaken for gloom. A slow fire, tended honestly and without superstition, is still a marvellous thing to watch and a deeply satisfying thing to feed. What follows is an attempt to do both at once: to trace the chemistry of nourishment with the same delight this compendium brought to the chemistry of flavour, and to leave the reader better equipped — not with rules to obey, but with an understanding to carry to the table. Read the chapters in order if you like your knowledge built storey by storey, from the calorie up through the macronutrients, the micronutrients, the gut, the clock, the rainbow of plant chemistry, the body's water and salt, the tables of the long-lived, and finally the honest, unfashionable business of eating well without dogma; or dip in wherever curiosity happens to strike, for each chapter is built to stand on its own legs. Either way, we begin as any sound account of a fire must: with the flame itself — the calorie, the candle, and the astonishing claim, first proved in a Paris winter, that we are all of us, in our unhurried way, quietly burning.

The fire, once discovered, demands to be measured — and its measurement occupied some of the finest chemical minds of two centuries. Chapter One takes up the flame.