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The 2010 study found an association between cow’s milk and acne. Scientists aren’t yet sure why this may be, but there are several theories. Cow’s milk spikes blood sugar, which can increase inflammation (leading to pimples). It also increases insulin levels, which encourage the production of skin oils (sebum). A lot of the commercial milk we buy comes from pregnant cows, and thus contains other hormones that can trigger the production of sebum. Milk also has growth hormones that can encourage the overgrowth of skin cells, potentially blocking pores. In 2005, researchers studied data from the famous Nurses Health Study II, and found that participants who drank more milk as teens had much higher rates of severe acne than those who had little or no milk as teens.