I don't think its necessarily "low carb" that reduces acne; I think its all about eliminating specific types of carbs. For me personally, grains definitely cause my skin to break out. But I am absolutely fine eating dairy and some sugar. I thought that potatoes made my skin break out too but I had a good amount of french fries recently and my skin has been unaffected.
Ever since I went mostly Primal, my acne has only worsened :s I don't blame Primal, but it has maybe set something undesirable in motion?