Skin health, and acne in particular, are tied strongly to diet.
Acne is caused by inflammation and oxidative stress.
Traditional indigenous cultures have little acne, but as soon as they adopt an American diet, they see increasing levels of acne.
Sugar raises insulin levels, which promotes the production of testosterone in women, and inflammation in general, causing acne.
Saturated and processed fats increase arachidonic acid levels and compete with omega-3 fats in the body, leading to more inflammation and acne.
Milk and dairy consumption is closely linked with acne (and many other skin and health problems) in part because of the hormones (including growth hormone) in dairy and because of the saturated fats.
High-sugar milk chocolate can increase acne by increasing inflammation, but dark chocolate does the opposite.