Does soy cause acne outbreaks?From reactions on forums we can see that soy causes acne outbreaks for some people while improving the conditions of other acne patients.
But what do scientific studies tell us? Not much! There is only one scientific study that mentions a link between soy and acne. A study at The Chinese University of Hong Kong found that "the intake of dairy and soy products was significantly associated with a lower incidence of acne" . In other words, soy reduces acne outbreaks! Another study confirmed that the intake of dairy milk increases acne . They hypothesized that the association of acne with dairy milk may be because of the presence of hormones and bioactive molecules in milk.
While the arguments against soy sound realistic they are actually based on poor evidence. Maybe those who develop acne outbreaks after eating soy belong to the small group of persons (1% of the population) that is allergic to soy. They do not develop real acne outbreaks but conditions that look like acne.
References An investigation of the association between diet and occurrence of acne: a rational approach from a traditional Chinese medicine perspective. Clin Exp Dermatol. 2010 Jan;35(1):31-5.
 High school dietary dairy intake and teenage acne. J Am Acad Dermatol. 2005 Feb;52(2):207-14.