So, in order of how bad the thing is for you and if you should take trying to minimize/cut it out of your diet entirely, the top of the list that even normie doctors and pretty much everyone agrees on is High Fructose Corn Syrup. Sugars in general are terrible for you except in moderation, but unfortunately are extremely difficult to avoid since pretty much all processed foods have a lot of them in it.Ok at the risk of sounding like a normie. How much of these dangers are real and how much of it is fearmongering designed to sell you stuff? As a hypochondriac i've done my fair share of internet health searching. both WebMD and Mayo Clinic should be renamed to "Itsprobablycancer.com" & "YouHaveCancer.com" respectively and at the very bottom they always encourage you to make an appointment with a specialist (Which they just happen to have a list of) and I notice with a lot of these food news sites, there are a lot of organic food & supplement sponsors. You can find a "Study" with just about any result you want if you look long enough. I don't know, my fearmongering senses are tingling.
The Seed Oil stuff is, as you can tell reading this threat, more controversial, but there is a growing consensus that fats are not as bad for you as was once thought, and that most weight gain doesn't come from eating things high in far and protein, but rather high in sugars and carbohydrates.
Basically a healthy diet consists mainly of meat and vegetables, sugars can be had in moderation and then carbohydrates in many respects being the least important to get, and especially if you're American store bought sliced bread (especially white bread) is should be avoided.
The thing is, when it comes to cooking via frying or sauteing, a large part of the pushback labeling seed oils as "unhealthy" is because that they were originally sold as "healthy" alternatives to things like Lard, Tallow, and Butter. Meanwhile things cooked in seed oils taste worse than things cooked in those oils derived from animal products, though this is not because of any taste the seed oils are ADDING, but rather they lack things that enhance the flavor that butter, tallow, and lard all have. Now, something to note, Lard, Tallow, and Butter were literally used in cooking for thousands of years with no issues, with no impact on human health that could be tracked historically. Meanwhile seed oils displaced them in the late 20th century and we've seen a massive rise in numerous health issues that humans didn't have before, especially weight. Though seed oils are not responsible for that, the general attack on animal fats DID play a huge role in it, you see, prior to the 1970s mass produced food was made with things like tallow, lard, and butter, but due to the systemic demonization of animal fats (as well as rising costs) companies phased out their use replacing them with seed oil and corn syrup to make up for the loss of flavor... this means that in the late 20th century we suddenly began eating MASSIVELY increased amounts of highly processed oils and sugars that humans had never before eaten at that scale... and then combine that with the onset of a more sedentary lifestyle, we've seen massive health problems due to weight develop.