One point made by Gary Taubes early in Good Calories, Bad Calories is that mid-20th century axe-grinders like Ancel Keys were able to convince everyone that meat/fat consumption had increased rapidly in the early 20th century. Taubes says this is difficult to dispute due to poor records of the “standard” diet 100 years earlier (or something much to that effect).
One area in which I would think there might be fairly good records of what constituted a normal diet would be in US military procurement. What were the cadets at Annapolis or West Point fed? What did the Officers’ Mess at US Army or Navy bases purchase? Are there really no existing records of procurement for these purposes from, say, the 1870s or 1880s?