It is the male hormone, testosterone, which stimulates the production of the sebaceous glands in times of stress. This is why people often develop acne when under stress. Meat (including poultry) contains hormones and hormone-like substances which can affect the hormonal balance in the body. Beauticians have reported that women who regularly eat meat are more likely to suffer from acne and hirsuitism which they put down to the steroids and hormone levels in the meat.

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(2) J Rasmussen - Review - ‘Diet & Acne’ Int J Derm 16 (1977);488
J Fulton, ‘The effect of chocolate on acne vulgaris’ JAMA 210 (1969);2071