An article in the American Journal of Clinical Nutrition looks at food trends from 1909 to 2007 (abstract, full text). Table 1 in the full text version has a lot of very interesting statistics. Butter consumption has drastically declined, but consumption of seed oils has increased substantially (read this page about why seed oils are not good for us and this page about why butter and cream are good for us). Meat consumption has gone up, but beef consumption levels remained constant. The increase in the last few decades is from chicken (probably skinless chicken breasts). People are eating a lot more ice cream and sweeteners (sugars, high fructose corn syrup), especially in sugared soft drinks.
Though the author is a vegan activist promoting a low fat, animal free diet (see the last statement in the full text version of the article, just before the reference section), this tells me a lot of our obesity/diabetes problems could be reversed by eating like we did one hundred or more years ago.