To test the widespread idea that chocolate is harmful in instances of acne vulgaris, 65 subjects with moderate acne ate either a bar containing ten times the amount of chocolate in a typical bar, or an identical-appearing bar which contained no chocolate. Counting of all the lesions on one side of the face before and after each ingestion period indicated no difference between the bars. Five normal subjects ingested two enriched chocolate bars daily for one month; this represented a daily addition of the diet of 1,200 calories, of which about half was vegetable fat. This excessive intake of chocolate and fat did not alter the composition or output of sebum. A review of studies purporting to show that diets high in carbohydrate or fat stimulate sebaceous secretion and adversely affect acne vulgaris indicates that these claims are unproved.
Fulton JE, Plewig G, Kligman AM. Effect of Chocolate on Acne Vulgaris. JAMA. 1969;210(11):2071–2074. doi:10.1001/jama.1969.03160370055011