There is evidence of brain surgery going back thousands of years:

https://neurosurgerycnj.com/craniotomy-through-the-ages-weve-come-a-long-way/#:~:text=Neolithic%20Times,France%20as%20early%20as%201685%C2%B9.

The Ancient Egyptians were the first to document the symptoms of a heart attack (and not-coincidentally, they ate a grain-based diet).

You don't really need a lot of scientific tools to recognize a large tumor that's growing abnormally on the surface of the body.

So it's not like these things weren't diagnosable a long time ago. Before glucose tests doctors would taste the urine of a patient to see if it's sweet or not.

And if nothing else, we have pictures of people before and after industrial food. New York City in 1900 was much slimmer, on average, than in 2023. You used to have to pay a carnival an admission fee to see a really fat person. Now we don't go a single day in public without seeing a few of them.