The answer to that question was not exactly what I was expecting. Time has a couple interactive charts that visualize the changes. Here's some interesting things I learned ...
1) We eat more vegetables today than in 1970 (but less than we did in the late 90s and early oughts) and a lot less red meat and dairy. We do eat more sugar, but the change wasn't nearly as large as I was expecting it to be. And I was surprised to see how much more grain we eat now.
2) After potatoes, the three most popular vegetables in America (by a longshot) are celery, lettuce, and ... er ... potato chips. And this has been pretty consistently true for the last 40 years.
3) The average American diet has never involved more high fructose corn syrup than cane sugar. It came really close, around 2000, but the proportion of our sugar intake made up by HFCS has since fallen off again.