A lot changed in the 70s and a lot of problems, coincidentally, came with them. Just completely swapping "vegetable" oils with something like lard/butter is kinda crazy the amount of change and improvement most people I've commit to it have experienced. There are plenty of other things as well, but that is definitely a big one in my experience.
This was one of the very first things we did when we started our journey about 15 years ago now.
I started cutting out “vegetable” oils a few years ago replaced with butter and animal fat. I feel much better since making the switch. I had a “cold” for the first time a few weeks ago after a particularly stressful week where I didn’t get much sleep. Even then, it was pretty mild compared to what I would have experienced back when I ate standard American diet foods.