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Overweight fathers may be putting their daughters at risk of breast cancer, new research suggests.

Scientists say daughters of fat fathers but slim mothers are overweight when born and throughout childhood.

They also have delayed development of their breast tissue as well as increased rates of breast cancer.

The study was the first to examine the effect of a man being obese on his offspring's future cancer risk.

And it suggests carrying extra pounds changes the DNA of a man's sperm, which has health consequences for his unborn daughter.

Obesity changes the microRNA (miRNA) signature - epigenetic regulators of gene expression - in both the dad's sperm and the daughter's breast tissue.

Epigenetics are heritable chemical changes that can switch certain genes on or off.

They are increasingly becoming recognised as a mechanism that allows health effects of the environment or lifestyle to be passed on to future generations.

They work either by altering the protein that wraps DNA, adding or removing chemical 'tags' that change the structure of cells, or via genetic molecules.

Obesity seems to sometimes run in families, as do some breast cancers and obese human mums are believed to influence both conditions.

Overweight woman in pregnancy can produce larger babies, who may have increased risk of breast cancer later in life.

Most research has focused on the maternal side, but few if any studies have looked at the influence of a father being overweight or obese on his offspring future health.

The study by Georgetown University Medical Centre was carried out in lab mice but scientists said the findings applied to humans.

Assistant professor Dr Sonia de Assis said: 'This study provides evidence that, in animals, a fathers' body weight at the time of conception affects both their daughters' body weight both at birth and in childhood - as well as their risk of breast cancer later in life.

'Of course our study was done in mice, but it recapitulates recent findings in humans which show that obese men have significant epigenetic alterations in their sperm compared to lean men.

'Our animal study suggests that those epigenetic alterations in sperm may have consequences for next generation cancer risk.' 

The study, published in Scientific Reports, suggested miRNAs may carry the epigenetic information from obese dads to their daughters.

The miRNAs identified regulate insulin receptor signalling, which is linked to alterations in body weight, and other molecular pathways that are associated with cancer development such as the hypoxia signaling pathway.

Future research will explore if the same associations regarding breast cancer risk hold for daughters of human fathers who are overweight around the time of conception.