That’s not quite right either. You need fat and sugar together. Humans can’t synthesize fat easily, so dietary fat is what gets stored. But insulin is the chemi

That’s not quite right either. You need fat and sugar together. Humans can’t synthesize fat easily, so dietary fat is what gets stored. But insulin is the chemical responsible for storing said fat, and insulin levels are heavily modulated by sugar and other simple carbohydrates. The rub is that humans need some level of dietary fat. Without it we go into “rabbit starvation” and die. What we don’t actually need is carbohydrate, we can live just fine without it for arbitrary periods of time. Since any human in a non-extreme situation will have dietary fat and can control their carbs, it’s colloquially correct to say that “sugar causes weight gain”, even if the technical answer is more complicated.