Soy that is. The average person thinks that soy is something that only those fruity Whole Foods people eat -- no soy milk for them! However, while this person wasn't looking, the soy industry has managed to get its junk into just about everything. Ketchup and mayonnaise -- soybean oil. Microwave burritos -- soy lecithin. Frozen hamburger patties -- soy protein isolate. Seriously, take a look at just about anything in the grocery store the next time and see for yourself.
I won't re-hash the many dangers of including lots of soy in your diet -- the Weston A. Price Foundation has a nice summary here -- but I am curious about when this change took place. Looking at availability data from the Statistical Abstract of the United States, we see that, as high as it was even during the mid-1980s, it was the mid-1990s when things really took off. This is unaffected by how we measure availability.
Unlike other junk food that replaced something good, like margarine edging out butter, soybean oil and soy protein isolate didn't launch an aggressive PR campaign to convince us to change our diet. No, here the processed food industry -- which makes the food that forms the bulk of most Americans' diet -- just started sneaking in all that soy. Given how widespread it is, I wouldn't be surprised if the average lower-class couch potato now consumed more soy than a hoity-toity fitness nut!