I've been troubleshooting my acne for about 13 months now. I went paleo in March of 2010, started getting forehead bumps in June 2010, and then cystic acne around my mouth starting in August of 2010. It's been horrible-- up to dozens of cysts at a time sometimes. But I've done a ton of research, learned a lot about acne, and also experienced a ton of n = 1 self experimentation stuff. Only in the past few weeks have I found all (most?) of the answers I needed, so while I know that everyone responds to foods differently, I'll share the most pertinent of my experiences with you to see if it helps.
There are a lot of causes of acne. One apparently common one is face washing. I have found that all the redness that used to be "just a part of my face" is now completely gone that I don't wash with soap. Just water a few times per day, and, incredibly, I'm doing really well with that. Also, re: non-diet treatment: do NOT touch your face, do NOT pick your acne, no matter how horrific it looks. Trust me. Ack. It's not worth it.
Another prominent cause is an inflammatory diet. This just makes sense. If you're inflamed, your body is going to be sending all sort of inflammatory cytokines to sites needing repair, so you'll just keep getting more and more obviously inflamed. Up the omega 3s. I have gone up to eating a pound of salmon per day without ill effect (so far as I can tell-- I know that I probably exhibit some oxidative damage as a result).
Inflammation, so far as I can tell, is why food allergies can contribute to acne. If you're allergic/sensitive to foods, they can upregulate your immune response, and they can contribute to your general inflammation.
The final and, so it seems from my research and my own experience, largest cause of acne is hormone imbalance. Testosterone, particularly in females but also in males, if too prominent in the body relative to other hormones, will cause cystic type acne, mostly around the mouth but also on the cheeks and forehead. Our testosterone can get messed up for a number of reasons, but this is how various foods can affect our testosterone levels:
Dairy: I just read on a link from Seth Roberts blog (I know, helpful... sorry) that pregnant cows produce a protein that binds with another protein in our bodies which is responsible for binding with excesss testosterone and clearing it out of our systems... basically, it steals one of our testosterone "receptors" and this is how dairy makes such a big impact on people (other than gut issues and food sensitivities). This is HUGE for me. If I have just a sip of milk, a bite of cheese, I can immediately feel my face start burning, etc.
Soy: Soy acts as an estrogen in our bodies. If we continue to eat external estrogen, our bodies "forget" how to produce it themselves, and therefore including high amounts of soy in our diet will mess up our testosterone/estrogen balance. Best to leave it out altogether.
Industrial meat, chicken, eggs: Despite what the industry will tell you about "minimal" hormone levels in the animals, it's lies! all lies! Honest, though. I knew that testosterone was my problem from the get-go, since I have poly cystic ovarian syndrome, but I didn't understand why just about every paleo food made my acne worse until I found out that small amounts of hormones over large amounts of time can make a big difference. Eat local, grass fed, etc. I really do experience problems after just one meal of an industrial animal, and it was hugely frustrating to think it was the paleo diet or some macro nutrient ratio or something effecting me when it was in fact testosterone levels. I must confess, however, that I haven't tested grassfed beef. I have no idea if that would give me problems. There is a well-known correlation out there between high testosterone levels and high meat consumption, but that could be for a variety of confounding factors. It is possible that meat has an inherent testosterone increasing component that is outside of grass/grain fed cows. For right now, I'm avoiding all meat, just to be safe.
Oddly, pork in my experience seems to be better than both meat and chicken.
Sugars, high GI foods, etc: Insulin spikes and hyperinsulinemia both lead to increased testosterone production. For this reason, intermittent fasting has helped me, to a degree, mitigate my acne and other hormone problems.
Stress: increases testosterone.
Some more results of my self-experimentation and reading anecdotes:
It takes longer than two weeks to see the full effect of removing a food. I "removed" dairy a number of times to test my sensitivity, but only for a week or so. Because my acne was so bad, it decreased a little bit with removal, but not significantly for months. Loren Cordain recommends waiting 3 months to see real changes in your acne from food changes. Sometimes I get effects overnight, but in general, overall, my experience lines up with Cordain's recommendation.
Dairy includes butter. Don't let yourself just think it's lactose or casein effecting you. It could be an inherent part of the dairy itself, such as the proteins I described briefly above.
Rosacea is different from acne and should be treated more as an autoimmune condition.
If you're going to experiment with macronutrient ratios, give yourself a lot of time and try to set up the cleanest experiment. I've been wondering about whether or not I need more carbs in my diet for a year and kept thinking "maybe" after my experiments. However, it seems from this point (where all I have left is the scars of my acne) that macronutrient ratios aren't important (for me) at all.
Just to wrap it up with my experience:
I have Poly Cystic Ovarian Syndrome, which is a condition in the ovaries caused by male-hormone excess in a female body. I went paleo after my diagnosis but nothing really improved. I added dairy in huge quantities, plus meat and eggs into my diet (I had been a vegetarian). Slowly the acne came on, and then I became experimenting with eliminations with very little success. It took me a very, very long time to figure things out. It sounds obvious in hind sight, but the acne really could have been from anything. Or so I thought. In any case, I know a lot of women who started getting acne once going on a paleo diet, and I think this may be an (the?) answer. My acne got worse and worse over time, and as I eliminating the right foods, from dairy and then butter, then meat, then chicken, then soy, then eggs, it has very slowly abated. Now that I am on a diet that doesn't actively create new cysts and keeps my testosterone in check, the acne is going away, but, again, slowly. The lesson here is that it really can take a long time for our bodies to heal. I imagine that once I "reset" and get everything more in balance in a few months or so I will be able to eat these foods in smaller doses. I really just overdid it when I went paleo, sometimes eating 75 percent of my calories in a day from dairy.
Today, I eat vegetables (all vegetables, including nightshades, which I had feared for a long time for apparently no good reason), macadamia oil, coconut oil, avocadoes, and seafood. It's not much, but it's the "cleanest" diet I have found and, for me, it works. I know that this diet and these problems aren't quite the problem discussed in this thread, re: the rosacea, but it seems as though some commenters may be able to relate to my experience and others who read it.
My thoughts and opinions about acne are informed by my experience, as well as the vast volume of acne literature out there. There's not a lot, but if you dig around you can find SOME good stuff. There's a lot of appealing ideas out there that may in fact not have any merit, such as attributing acne to "unclean" proteins, improperly cooked foods, nightshades (the only anti-nightshade information I've ever found is anecdotal), lack of sleep.... it's important, I believe, to think about all of the things that cause acne on that particular spot of your face (it's true that different areas have different causes, though hormones play a role in all of them, and usually testosterone), and then think about how you might be affecting those causes in your life.