Although many people state that diet doesn't affect diet there is evidence that dairy products and high glycaemic may worsen acne in SOME people. It's believed that natural hormonal components of milk may be what exacerbates it, but others believe it's exacerbated by high insulin-life-growth factor (IGF) blood levels. Both milk consumption and high-glycaemic foods have been associated with increased levels of IGF in the blood.
A study of 43 males with acne found that low glycaemic diets resulted in less acne counts, but those patients also lost weight which may have been what caused tha reduction in acne.
Refence: UptoDate article: Pathogenesis, clinical manifestations, and diagnosis of acne vulgaris, cited 12/May/2011