This is a bit of an exaggeration. There weren't boxed and bagged foods in 1776. McDonald's and other fast food did not exist in 1776. Diets in 1776-1899 were likely healthier. More convenience type foods started coming around in the 1900s.
Upton Sinclair's released in 1906 the Jungle uncovered a lot of problems and unsanitary processes in our food:
1). Contaminated Meat: The novel depicted the use of contaminated and spoiled meat in the meatpacking plants. Meat that was clearly unfit for human consumption, including rotten or putrid meat, was processed and sold. Workers often failed to differentiate between good and spoiled meat, resulting in dangerous and unsanitary conditions.
2). Adulteration: The use of additives and preservatives to mask the poor quality of meat was common. These additives included chemicals such as borax, formaldehyde, and salicylic acid, which were used to preserve meat or make it appear fresher than it actually was.
Plus America is the birthplace of TV dinners(popularlized in the 50s), canned and processed foods for long storage like Campbells, General Mills, Kraft Heinz..None of these companies just sprung up in the 1970s. In addition, the American diet took a hit during the great depression for obvious reasons.
I think we are arguing for the sake of arguing to claim that the American diet and food standards magically went from pristine to unhealthy in the year 1970. As a southern American that can trace his ancestors back to colonial North Carolina, the southern diet has always been "unhealthy" lol.
In defense, food preservation is a necessity for human survival, so that is how it is.
