Another pillar of unquestioned conventional wisdom is the dogma that the nutrient content of our fruit and vegetables has declined over the past half century or so -- with the culprit cast as greedy farmers increasing productivity at the expense of nutritional quality. This idea gained currency in Michael Pollan's book In Defense of Food, in which he argued that when we eat fresh produce, "we're getting substantially less nutrition per calorie than we used to." Let's take a look at the evidence that might justify such a sweeping claim.
When Pollan says "it has become increasingly clear" that produce nutrition has been sacrificed for productivity, he seems to suggest that mounting evidence -- e.g., study upon study -- points to this conclusion. Yet, if anything, this area of study is more remarkable for how little research has been done comparing "now and then" nutrient contents of produce, in part because of the lack of reliable measures from 50 years ago, and because of how many other variables of farming have changed.
Moreover, when you do look at the USDA figures Pollan cites, they present a mixed -- rather than consistent -- picture of nutrient changes. Here are five examples from the 43 crops reviewed. In some, certain nutrients have gone down -- in others, they've gone up, with a large degree of variation among nutrients due to measurement differences across the decades.
Aggregate all the plants and all the nutrients together and there was a 20% overall nutrient decline -- but that's only when samples are measured and compared on a gram-per-gram basis. It doesn't account for the fact that most of these commodities are now bred to be bigger and juicier (e.g., higher water content), in order to get consumers to eat more fruit and vegetables. A 2011 apple will roughly have the same nutrient content as a 1950 apple -- but because it's bigger, a gram-per-gram comparison will show nutrient decline. Most consumers go to the store and buy 5 apples -- not a gram quantity -- so it's doubtful that they're significantly missing out to the extent Pollan suggests.
At the end of the day, nutrition deficiencies and obesity are due to all the junk people eat -- not to any changes in the nutrient quality of produce. If anything, efforts by agricultural companies to entice consumers -- catering to tastes for juicier, bigger produce and improving access to fresh produce by selling it "as cheaply as possible," as Pollan puts it, would seem worthy of commendation, not censure, if promoting a plant-based diet is our common goal.