No one knows exactly what causes acne. Hormone changes, such as those during the teenage years and pregnancy, probably play a role. There are many myths about what causes acne. Chocolate and greasy foods are often blamed, but there is little evidence that foods have much effect on acne in most people. Another common myth is that dirty skin causes acne; however, blackheads and pimples are not caused by dirt. Stress doesn't cause acne, but stress can make it worse.
Acne vulgaris is a chronic skin disease of the pilosebaceous unit and develops due to blockages in the skin's hair follicles. These blockages are thought to occur as a result of the following four abnormal processes: a higher than normal amount of oily sebum production (influenced by androgens), excessive deposition of the protein keratin leading to comedo formation, colonization of the follicle by Cutibacterium acnes (C. acnes) bacteria, and the local release of pro-inflammatory chemicals in the skin.
Diet. Studies indicate that certain dietary factors, including skim milk and carbohydrate-rich foods — such as bread, bagels and chips — may worsen acne. Chocolate has long been suspected of making acne worse. A small study of 14 men with acne showed that eating chocolate was related to a worsening of symptoms. Further study is needed to examine why this happens and whether people with acne would benefit from following specific dietary restrictions.