Explain how corn makes up a variety of food in the supermarket.

First of all, corn is seen as both a food in its own right in its natural form (whether sold fresh, frozen, or canned) and as an ingredient (cornmeal, corn oil, or corn syrup). One issue that Pollan discusses is the way that corn syrup (also known as high-fructose corn syrup) has become a prominent ingredient in prepared foods, functioning as a sweetener. In the 1970s, American farmers were offered government subsidies to grow a selection of crops, most notably corn, wheat, and soybeans, with the idea that the country could become a major producer of food for world markets. American dairy farmers were also offered subsidies. With so much product on the market, prices were not competitive, and the government ended up having to purchase much of the product they helped subsidize. Pollan makes the point that greater diversification of food crops is better for farmers and consumers and can help avoid the problem of soil depletion common on farms where there is monoculture, or only a single crop grown.
In some cases, the food produced as a result of these government subsidies was donated to help needy families—as with the "government-donated commodities" blocks of cheese in the late 1970s. Excess corn was processed into a product with a long shelf life: high-fructose corn syrup. This sweetener is much cheaper to produce than other sweeteners such as cane sugar or beetroot sugar (the forms of actual sugar most commonly used). Because of huge surpluses of corn, large amounts of corn syrup were produced, and so it started to be added to many prepared foods such as cereals, breads, condiments, peanut butter, and so on. The ingredient has been linked to various health problems, including obesity.