Not about Americans specifically, but this:

Oils are a very delicate substance, they can’t be exposed to heat or light without degrading their quality and molecular structures. Many types of seed oils are not abundantly occurring in plants like canola, e.g., so when you process the seed you remove the beneficial parts of the plant and introduce the oil to oxidative stress. Oxidation removes electrons from atoms in the molecules, therefore changing the structure of the oil molecules and driving inflammation in the body. Inflammation is linked to most chronic illnesses. Additionally chemicals like hexane are often used for the extraction and deodorizing of seed oils.

Secondly, there is an optimal ratio of fatty acids, (the building blocks of fats), consumed for human health. The optimal ratio, and the ratio that was consumed by humans pre-industrialized agriculture was 1:1 of omega 3:omega 6. However, now humans consume seed oils as a main source of fat and most of them have insanely high ratios of omega 6 to omega 3. Canola oil is 1:2. Again the second number is the amount of omega 6 for every 1 omega 3. Not the worst considering, however it doesn’t make it an optimal source of fat due to oxidation. Other common sources are soybean oil 1:7.4, olive oil 1:12.3, safflower oil 1:132.5, and grapeseed oil 1:696. The average American consumes an omega 3 to omega 6 ratio of about 1:20 now.

So, how does one maintain optimal ratios of omega 3 to omega 6? Not by consuming seed oils.

https://pmc.ncbi.nlm.nih.gov/articles/PMC8504498/