I don't have anything more than acne caused by external sources (just Wellbutrin-induced occasional cysts, and mild whiteheads caused by cosmetic products), but I can definitively say that whenever I consume more than a small dosage of Vitamin C (I.E. 2 clementines or 1 standard dosage of Vitamin C in capsule form), I break out in what humans on this board would look at in photo form and call "hormonal acne." It takes form as mild cysts and appears on my jaw and upper neck.
Fortunately the spots go away within two days for me, and then I refrain from consuming Citrus fruits again.
I can imagine that, had I grown up on a diet high in Vitamin C, I'd have fairly bad acne now and would not know the cause until I was tested for food allergies.
"Have you ever wondered why people in third-world countries have relatively clear skin"
Having traveled through Vietnam and rural areas of China (non-industrialized), I saw a lot of people with fairly severe and cystic acne. Their diets were clean.
With that said, the extreme heat/humidity in Vietnam would've most likely exacerbated any skin conditions due to excess moisture.