The western diet pre-1960s had a normal amount of fats/carbs. It's not fats per se, but 1) too much fat and 2) eating omega 6s in processed plant oils. If you're going to eat fat, then eat either butter or coconut oil. (Olive oil is obviously ok but this industry has been screwed with so much, that most olive oils are bogus and tampered with. So it's best to stay away).
Canola, peanut, soybean, etc. These are all garbage for you and will do way more damage than any amount of sugar. These oils block your insulin receptors, so that when you DO eat carbs, and when your body releases insulin, it releases too much because the receptors are blocked with oil and can't get the message that "ok, that's enough food". Then you have a tendency to overeat because you're not satisfied. The body releases insulin to tell the digestive system that enough energy has been eaten. If the insulin receptors are blocked off, then the body doesn't get this message and food cravings continue and more insulin is released. Then your system gets stressed out (i.e. insulin sensitivity).
The other problem with omega 6/plant oils is that the body is not designed to eat this much of it. The amount of corn oil in one teaspoon is equivalent to like 30 pieces of corn. But thanks to modern engineering, we're able to "eat" these types of oils, in HUGE amounts that nature never intended.
The body can break down large amouts of butter; God gave us cows and butter in large quantities; we've been eating it for centuries with no obesity issues.
The body can break down fish oils; humans have been eating fish for centuries.
The body can break down coconut oils and olive oils and avocados. These plants have naturally occurring high-fat content. We're meant to eat them like this.
But it's a modern science experiment to be eating plant oils coming from plants which (to the naked eye) don't have a lot of oil. Corn, soybean, peanuts --- eat a handful of these plants. Are they oily? No. Not like an avocado or an olive or a coconut.
But process 1,000s of these plants at once, put them into a blender and extract the oils -- and then add an entire teaspoon of it in the same ratio that we eat a teaspoon of butter. Does that sound normal? No. And the human body can't handle it. It's a huge amount of calories and a huge strain on the system.
Not only are omega 6 oils not the greatest, once you process them, they become even worse.
Processed oils and too much (even good) fats cause diabetes. Not sugar. To get rid of diabetes, switch to a low, low fat diet. Carnivore/butter/meat can also work.
Weighlifters eat low-fat protein and carbs - no problem.
Carnivores eat high fat, no carbs - no problem.
High carb people eat high carbs and no oils - no problem.
The major problem is a mix of carbs/oils.