I think this is mainly a holdover from natural hygienist fruitarians or as mentioned in complications with high processed starch/sugar/fat. Most people that experience problems with fat/sugar, especially veg, have some kind of fungal/sugar issue to begin with. Therefore abstaining from either fat or sugar alleviates symptoms. Ironically, as these same theorists propose, it has to do with fat intake overall rather than on some daily changing intake. The advantage I see in doing fruit on days of which no fat is consumed is that there is less chance of fermentation. Other than that it seems that on a diet that is otherwise low carb, there is advantage in slowing sugars with animal and dairy fat, and major digestive issues with actually trying to combine carbs with proteins, other than low sugar fruits veg and herbs.