I learned a while back but don't have time to dig up sources now that eating simple carbs and fats together encourages your body to take the fast calories and store them as fat, moreso than eating simple carbs alone (fast energy, relatively clean-burning) or fats alone. These kinds of interactions are historically very important for explaining particular quirks of the effects of diets and I wouldn't be surprised at all if microplastics had some sort of catalytic effect, e.g. by being nucleation points for buildup of something (arterial plaque or whatever else).