Genetic risk factors play only a limited role in causing breast cancer and seem to work independently of lifestyle factors such as weight and breastfeeding, British scientists conclude in new research.

The study, published in The Lancet, suggests that while genes and lifestyle each influence breast cancer risk, they appear to do so independently of one another.

"We looked at whether lifestyle factors for breast cancer… influence the genetic risks. And the answer is that they do not," said the lead author, Dr. Ruth Travis, of the cancer epidemiology unit at the University of Oxford in the United Kingdom, in a news release.

Travis said her team looked at 7,610 women who had been diagnosed with breast cancer, as well as nearly 10,200 women who did not have the disease.

All the women provided blood samples for genetic testing, as well as detailed information about their lifestyles. They analyzed the effect of 12 genetic mutations linked to small increased risk on breast cancer.

(They didn't look at the genes known as BRCA-1 and BRCA-2 which confer a high breast cancer risk among women who carry the genes, but which are relatively rare.)

They then related the genetic risks to 10 known lifestyle risk factors:

  • age at puberty onset
  • number of births
  • age at first birth
  • breastfeeding
  • menopausal status
  • age at menopause
  • use of hormone replacement therapy (HRT)
  • weight
  • height
  • and alcohol consumption

The gene risks added to, but did not multiply, the risks posed by lifestyle factors, they concluded.

"None of the 120 comparisons yielded significant evidence of gene-environment interactions," the authors wrote.

The researchers believe that genes account for only a small proportion of breast cancers and that the main risk factors relate to lifestyle.

"The good news is that some of these are modifiable, so by changing their behavior women can alter their risk of breast cancer," co-author Dr. Jane Green said in a statement.

The authors noted that since they designed their study, another five genetic variants linked to breast cancer have been found.

"Of course, there are always more SNPs [single nucleotide polymorphisms; genetic differences] to study, but to date, none of these seem to contribute to breast cancer in a big way," Dr. Steven Narod, from the Women's College Research Institute in Toronto, in an accompanying editorial.

Perhaps the SNP approach has been "oversold," he suggested.

"Others have come to the conclusion that SNPs, alone or in combination, are not that helpful in predicting cancer risk."