I posted this originally in the nutrition section but realized it would also be helpful for those who might not read that forum and suffer adult acne. The diet works really well on resistant chronic adult acne that responds poorly to meds. I've suffered acne for decades since my teens and used all kinds of meds and natural methods to control it. Even now I'm on a regime (see signature below). Despite everything I still broke out...maybe not cysts like before but still had small pimples and whiteheads pop up everyday.
Here's the diet and reasons why. It's based on years of research and my interest in nutrition and cultures that do not get acne. It's also based on the work of Dr McDougall who has studied extensively the diets of people who do not get inflammatory deseases, and particularly the effects of vegetable oils on acne. Since following this diet religiously I'm now almost 100% acne free. I might get a tiny whitehead occasionally but they go away in a day...and this is how normal skin is anyway.
Eat only whole foods versus manufactured foods or junk foods (i.e. apple versus apple juice) --- manufactured foods or junk foods are made as cheaply as possible with additives, sugars, poor quality oils, etc. that increase inflammation throughout the body. If you are prone to acne then the inflammation will occur there causing more sebum, more blockage and hence more acne. Also avoid foods made from flours as much as possible...instead eat whole foods like yams, potatoes and rice. Flours are very concentrated and can raise inflammation if eaten in excess.
Avoid all dairy including yogurt and cheeses. Dairy has natural hormones produced by cows as part of lactation. These hormones are made for calfs, not adult humans. If you are sensitive to hormones which most adult acne sufferers are, then they will make your acne worse. Plus cows milk has iodine as part of the milking process which can inflame acne.
Avoid all sugars including fructose, alcoholic beverages, honey, maple sugar, etc. Acne sufferers are sensitive to changes in insulin levels and the resulting inflammation comes out on their skin. I read people posts who are upset because those who don't get acne seem to be able to eat anything...but actually if you talk with them you will find out they have gut problems or rashes or heart disease or diabetes, etc....also inflammatory problems but just not acne. You may find it is helpful to also avoid high sugar fruits (high fructose) and instead eat berries which are low sugar. If you must have pancakes, etc. then make your own without sugar and add a fruit topping made by microwaving or cooking berries into a saucy syrup (yum).
Avoid all caffeine in drinks and foods. Adult acne often has a rosacea component to it...a generalized inflammatory response that caffeine is a known trigger for. Avoiding caffeinated foods and drinks helps the skin stay calmer...not to mention the rest of the body!
Avoid all oils, except EVO for cooking. This means avoiding all salad dressings, margarines, butters, cooking oils and the foods they are in. The only thing that seems to be less inflammatory is extra-virgin olive oil and that may be because humans have been consuming this for thousands of years. However it was a precious commodity so only tiny amounts were traditionally used...not like we do now where we drown our salads and pasta in oil. Vegetable oils (canola, safflower, etc.) only became part of our diet after industrialization when solvents were discovered that could extract the oils cheaply eneough to market at a profit. Our bodies do not know what to do with these manufactured oils and so they cause inflammation...and acne. If you are careful you can find breads made with olive oil...which was the oil that was used for breads by our ancestors.
Eat only wild fish and free-range organic fowl in moderation. Cultures that traditionally have less inflammatory disease also eat much less flesh food...even so-called healthy ones like fowl or fish. They eat these things sparingly because they are hard to hunt. Avoid all red meat which takes a long time to transit the body and can cause gut inflammation which also can inflame acne.
Hope this helps. It has taken me a very long time to figure this all out but it has made a real difference. Most recently I discovered the part about oils and it made the most positive difference...to the point that I will be weaning off some of my oral meds below. My derm is amazed with my progress.