All of this shit is stupid. Research has pretty conclusively shown at this point that a high proportion of calories coming from saturated fat specifically will raise LDL cholesterol levels, in people who have neither genetic predisposition toward low or high LDL. If you have one of those, it won't make a difference either way.
Whether that leads to actual heart disease is iffier, but not terribly controversial among cardiologists as far as I can tell. It's only doubted on the Internet where everyone wants to be a galaxy brain with some answer the doctors don't want you to know.
But nobody ever demonized fat in general, and demonizing carbs is just as stupid. Eating enormous amounts of carbs is fine as long as you actually use them. My daily calories right now are around 3,900 with carbs at 650 grams a day, a fair amount being syrups I eat early in the morning while running. If you listened to the Internet, you'd believe I was diabetic already, yet basically every remotely serious endurance athlete eats like this and is fine. Glucose that is continuously and immediately shuttled into muscle cells to power mitochondria and provide energy for movement does no harm whatsoever. It's roughly the entire point of animal metabolism. Glucose that sits around in your blood forever because you're sitting around staring at a screen for 16 hours a day while stuffing your face is what causes problems because of all the oxidizing effects of glucose when it isn't taken apart quickly and turned into ATP.
My BMI is 21.6 for what it's worth. As far as I can tell, the whole "mystery" behind why no diets work is because no diet can magically make people eat less when they spend the overwhelming majority of their time not moving and hunger decouples from energy expenditure. If you're sufficiently active as a lifelong athlete, every diet works. I ate super sized McFlurries, entire boxes of Entenmann's donuts, and Little Debbie's treats as a teenager as staples of my diet. As an adult, I've tried paleo, zone, mediterranean. Right now, I pretty much just eat the standard American food pyramid. None of these has managed to magically poison my brain or destroy my metabolism because metabolic function can be trained just like any other bodily function and it is trained by doing regular athletic activity with a high energy demand. Just like your muscles atrophy if you never do any resistance training, your metabolism atrophies if you never do any aerobic exercise.
I can't claim to know the secret to weight loss but I know how to never get fat in the first place. On every team I was ever on from middle school to college to my time in the Army, whether that be cross country, track, volleyball, basketball, tennis, or general outdoor adventurism and long-haul hiking with a weighted pack, the overweight rate was never 0 but it sure as shit wasn't 70%. And we were all eating the same "poisons" and manufactured foods from evil Nestle that the rest of you were eating.