It’s undeniable that fructose can promote weight gain, but that might only be because it’s easy to overeat sugary foods. After I’m full from a meal, I can usually eat dessert without a problem, because the sweet taste “wakes up” my appetite.
It’s also true that fructose is metabolized differently from glucose, and maybe that matters too. I would think this would be most significant on a hypercaloric diet. If liver glycogen stores are already full, and you pour a bunch of extra fructose on the liver, maybe that does contribute to fatty liver.
On the other hand, people who exercise a lot (or otherwise stay in calorie balance) don’t seem to have a problem. The Kenyan marathon runners eat about 20% of their calories as pure sugar, and clearly they’re able to stay fit and athletic. In fact, the sugar should restore their liver glycogen faster than other carbs (because fructose goes straight to the liver), so I think sugar might provide an advantage for athletes.
I usually drink a soda every day and have some other sweet foods, and I’m still gradually losing weight and improving my fitness. However, I’m also exercising and watching my calorie totals, which I think is much more important than what kind of food you eat. So my own opinion is that sugar is perfectly fine unless you’re overeating and gaining weight, in which case it might be as damaging as Doc Lustig claims.