Entering puberty early increases the risk of developing cancer in later life, scientists said today.
They found new evidence linking the timing of puberty with a higher risk of cancers that are sensitive to sex hormones, including breast, ovary and endometrial cancers in women, and prostate cancer in men.
Early puberty in boys is classed as aged eight. For girls it is classed as starting periods aged eight, nine or 10.
They found 389 genetic signals associated with the onset of puberty - four times the number previously known.
This was after allowing for body weight, which was already known to influence the timing of puberty and the risk of some cancers.
Senior investigator Dr John Perry said: “Previous studies suggested that the timing of puberty in childhood was associated with risks of disease decades later, but until now it was unclear if those were circumstantial observations, for example secondary to other factors such as body weight.
"Our current study identifies direct causal links between earlier puberty timing itself and increased cancer risk.
“This link could possibly be explained by higher levels of sex hormones throughout life, but we need to do more work to understand the exact mechanisms involved.
"We aim to understand these disease links and thereby contribute to the prevention of diseases in later life.”
The timing of puberty varies between individuals but tends to run closely within families.
Earlier puberty timing may have advantages for some adolescents, such as boys involved in sports.
But it appears to have largely negative effects on later health, such as higher risks of heart disease and some cancers.
The study, in Nature Genetics, sheds light on the mechanisms that regulate puberty timing, including a quarter of those that are inherited from parents.
Dr Perry said: “Our findings highlight the remarkable biological complexity of puberty timing, with likely thousands of genetic factors in combination with numerous environmental triggers.”
Dr Ken Ong, joint senior author, said: “One of the more remarkable findings concerns the role of certain types of genes called imprinted genes, which are only active in your body when inherited specifically from one parent but not the other.
"We identified rare variants in two genes, which both lower the age of puberty when inherited from your father, but have no effect when inherited from your mother.”