obesity increases a man's risk of breast cancer by nearly one-third, research suggests.
Other risk factors include enlarged male breasts, and having an extra X chromosome besides the one men inherit from their mothers.
Around 400 men are diagnosed with breast cancer each year in the UK, but the causes of male breast cancer are poorly understood.
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The Male Breast Cancer Pooling Project found that obesity, and other physical and hormone-related traits, increased breast cancer risk in men.
The results were published in the Journal of the National Cancer Institute.
Having a Body Mass Index of 30 or above increased the chances of a man developing breast cancer by around 30%.
An association was also seen with gynecomastia, enlarged breast tissue in men, which appeared to be separate from the effect of obesity.
Klinefelter syndrome, the presence of an extra X chromosome in men, was another confirmed risk factor.