Butter was once a USDA food group, according to The Warmest Room in the House: How the Kitchen Became the Heart of the Twentieth-Century American Home by Steven Gdula, a microhistory of 20th century American food culture. Though I can not seem to find the statement now, I am sure he said that butter was the eighth of eight groups and that the USDA said that you needed to eat some every day (at least until margarine was invented).
There are many other startling statements in the book. On page 84, Gdula quotes FDA Commissioner George Larrick, who in 1956 said, "Our industry will not have done its job until housewives buy most of their meals as packaged, ready-to-serve items." That was government policy in support of corporate agriculture.
Though the author says that the kitchen is the topic of The Warmest Room in the House, the focus seems broader to me. How the kitchen transformed from a hellishly-hot sinkhole to a shiny, comfortable room where you entertain guests is one of the major plots, but there are many others. He chronicles trends in meat eating and vegetarianism, the appliance industry, government food regulation, the restaurant industry, and food habits displayed through motion pictures and television. He also concentrates on the history of cookbooks and the home economics movement, as well as diet crazes through the century.
That is a lot to cover in 209 pages. Gluda's text is economic in that he tells his stories relatively quickly and then tells more. I especially enjoyed reading about the 1950s, 1960s, and 1970s, mentally reliving my childhood through food memories.
Gluda's book is packed with provocative details and would be a good discussion book.
Gdula, Steven. The Warmest Room in the House: How the Kitchen Became the Heart of the Twentieth-Century American Home. Bloomsbury, 2008. ISBN 9781582343556