Fat is back, at least in some sense. It has just been replaced with polyunsaturated fats in the minds of nutritionists.
Studies on low-fat, high-carbohydrate diets and the recently popular fad called the Mediterranean diet have shown similar results in weight loss but are mixed when it comes to cholesterol, mostly because nutritional epidemiology relies on people remembering what they ate. To try and augment those, studies are done with animals, and they can provide some guidance but are often misused (what food hasn't been found to be a carcinogen in rats by now?). Still, that is what we have and they can at least provide some guidance on what works best for improved health. Then studies can be done with people.