It's not like an extra gram of carb is going to turn into 2 pounds of body fat. So I don't believe there's any precise switch. But there's a personal treshold above which you can't control your appetite so well and you eat an excess of food hence getting fatter.
But if you eat 400 calories of sugar, 100 calories of fat and 100 calories of protein... you do lose weight, no matter what. Insulin is not the be all end all of gaining excess weight (if that were the case we would have a non-economical body unfit for survival) and calories still matter. It's a YMMV thing.