For a megadose of zinc to affect acne dramatically, a good bet was that zinc is a cofactor in a reaction that affects acne. If you have a chemical reaction in the body like Zinc + X -> Y, then flooding the area with zinc will at least modestly increase the production of Y, since it makes it more likely that all the available "X" will get used up. After much study, I concluded that "Y" is actually zinc superoxide dismutase, or ZSOD. ZSOD comes with the usual labels people grope for in acne cures: anti-oxidant, anti-inflammatory, etc.
But then, what is the "X" that must be combined with zinc to make this reaction? If I have to overdose on zinc to get enough "Y", the implication is that what I'm really deficient in is "X". Like most people in America who eat meat, it's highly unlikely that there is any lack of zinc in my diet. If I could remedy my deficiency in "X", then I should be able to be acne-free without taking any zinc.
More study led me to conclude that "X" is melatonin. Melatonin slows cell division. It may decrease the production of androgens right in the skin. And perhaps most importantly, melatonin crosses the cell membrane and directly stimulates your DNA to produce the precursor to ZSOD, the molecule that zinc must combine with in order to create ZSOD.
Experiments with melatonin were immediately fruitful. By tending to my sleep cycle, I was soon able to be acne-free on less zinc, but still could not be acne-free reliably for long periods without any zinc supplement. Something was still missing.
The final piece of the puzzle was finding the fairly recent discoveries that show that, in modern life, we fail to effectively suppress daytime melatonin because we live in relatively dim indoor light. When you don't effectively suppress daytime melatonin by having your eyes in outdoor light all day long, two bad things happen. First, your gut thinks it's nighttime and you get carbohydrate malabsorption that keeps it from effectively digesting tryptophan (the fuel your body needs to make melatonin) and (tada!) zinc! Second, you get a "flattened" melatonin curve when you sleep at night -- your body simply doesn't produce the giant burst of melatonin at night that nature intended. The data fits this hypothesis nicely, including the most obvious points: Do low zinc levels correlate with acne? Yes.
Do low tryptophan levels correlate with acne? Yes.
Do low ZSOD levels correlate with acne? Yes.
Can this explain why primitive tribes are acne-free? Yes.
This effect of daytime light is simply astounding. For example, I have long struggled with the ability to consume legumes. I bought into the standard advice that it's a problem of gut flora, if you eat them long enough your gut will adjust and digest them better without gas, etc. If I had a large Coke and a large burrito, the result was 100% predictable: great intestinal discomfort. However, I now know that was simply another problem of failing to suppress daytime melatonin. By living in outdoor light all day, I can slam down a Coke+burrito with zero intestinal discomfort, hardly any gas at all. I've repeated this experiment reliably several times, and outdoor light exposure is like a light switch (heh!) on my ability to digest legumes. I speculate that the growth in acid reflux disease (and the esophageal cancer it can lead to) is probably another result of living in dim light during the day that produces carbohydrate malabsorption.
The unfortunate thing is, although the pill-free cure for my acne is conceptually very simple, it's also very hard for modern people to accomplish. I had to buy a laptop with an extra-bright screen so...