IGF1 and breast cancer: The strong inverse association of breast cancer risk with early adult body size4 may originate in early life, or in childhood/adolescence.5, 6 It has been hypothesised to be due to greater differentiation of breast tissue during puberty,2, 32 altered oestrogen metabolism,33 lower adult mammographic density34, 35 and/or lower circulating IGF‐I levels36 in heavier girls.