The Omnivore's Dilemma - Michael Pollan [92]
The recent discovery of these secondary metabolites in plants has brought our understanding of the biological and chemical complexity of foods to a deeper level of refinement; history suggests we haven’t gotten anywhere near the bottom of this question, either. The first level was reached early in the nineteenth century with the identification of the macronutrients—protein, carbohydrate, and fat. Having isolated these compounds, chemists thought they’d unlocked the key to human nutrition. Yet some people (such as sailors) living on diets rich in macronutrients nevertheless got sick. The mystery was solved when scientists discovered the major vitamins—a second key to human nutrition. Now it’s the polyphenols in plants that we’re learning play a critical role in keeping us healthy. (And which might explain why diets heavy in processed food fortified with vitamins still aren’t as nutritious as fresh foods.) You wonder what else is going on in these plants, what other undiscovered qualities in them we’ve evolved to depend on.
In many ways the mysteries of nutrition at the eating end of the food chain closely mirror the mysteries of fertility at the growing end: The two realms are like wildernesses that we keep convincing ourselves our chemistry has mapped, at least until the next level of complexity comes into view. Curiously, Justus von Liebig, the nineteenth-century German chemist with the spectacularly ironic surname, bears responsibility for science’s overly reductive understanding of both ends of the food chain. It was Liebig, you’ll recall, who thought he had found the chemical key to soil fertility with the discovery of NPK, and it was the same Liebig who thought he had found the key to human nutrition when he identified the macronutrients in food. Liebig wasn’t wrong on either count, yet in both instances he made the fatal mistake of thinking that what we knew about nourishing plants and people was all we needed to know to keep them healthy. It’s a mistake we’ll probably keep repeating until we develop a deeper respect for the complexity of food and soil and, perhaps, the links between the two.
But back to the polyphenols, which may hint at the nature of that link. Why in the world should organically grown blackberries or corn contain significantly more of these compounds? The authors of the Davis study haven’t settled the question, but they offer two suggestive theories. The reason plants produce these compounds in the first place is to defend themselves against pests and diseases; the more pressure from pathogens, the more polyphenols a plant will produce. These compounds, then, are the products of natural selection and, more specifically, the coevolutionary relationship between plants and the species that prey on them. Who would have guessed that humans evolved to profit from a diet of these plant pesticides? Or that we would invent an agriculture that then deprived us of them? The Davis authors hypothesize that plants being defended by man-made pesticides don’t need to work as hard to make their own polyphenol pesticides. Coddled by us and our chemicals, the plants see no reason to invest their resources in mounting a strong defense. (Sort of like European nations during the cold war.)
A second explanation (one that subsequent research seems to support) may be that the radically simplified soils in which chemically fertilized plants grow don’t supply all the raw ingredients needed to synthesize these compounds,