"While industrial agriculture has made tremendous strides in coaxing macronutrients -- calories -- from the land, it is becoming increasingly clear that these gains in food quantity have come at a cost to its quality. This probably shouldn't surprise us. Our food system has long devoted its energies to increasing yields and selling foods as cheaply as possible. It would be too much to hope those goals could be achieved sacrificing at least some of the nutritional quality of our food. As mentioned earlier, USDA figures show a decrease in the nutrient content of the 43 crops it has tracked since the 1950s. In one recent analysis, Vitamin C declined by 20%, iron by 15%, riboflavin by 38%, calcium by 16%. Government figures from England tell a similar story. Declines since the 50's of 10% or more in levels of iron, zinc, calcium, and selenium across a range of food crops. To put this in more concrete terms, you now have to eat three apples to get the same amount of iron as you would have gotten from a single 1940 apple, and you have to eat several more slices of bread to get your recommended daily allowance of zinc than you would have a century ago. These examples come from a recent report called Still No Free Lunch written by Brian Halweil, a researcher for World Watch, and published by The Organic Center, a research institute established by the organic food industry."
-- Michael Pollen, In Defense of Food