“carb type”
LOL, fiber is a carbohydrate you can’t digest.
The diet requires a “majority” of your diet’s 70% “carbs” be carbs you can’t pull calories from.
Of course you lose weight on that, because you just lost more than 35% of your total carbs (a digestible carb has 4.5 calories/gram).
Protein was 10% and all 30+ diets tested had a fixed 20% fat (fat is 9 calories per gram).