A healthy lifestyle can delay the onset of breast cancer even among women at the highest genetic risk of the disease, research finds.
Genes and environment determine breast cancer risk
The New York Breast Cancer Study Group found women who carry mutations in the BRCA1 and BRCA2 genes have an 80% chance of developing breast cancer.
The Science study also showed they have a raised risk of ovarian cancer.
However, onset of the disease was delayed by exercise and healthy weight as an adolescent.
The study also found that many women with these inherited mutations come from families with few if any reports of breast or ovarian cancer.
What the study found
Chance of a women with the BRCA1 or BRCA2 mutation developing breast cancer:
By age 40 - 20%
By age 60 - 55%
By age 80 - 82%
The overall risk for all women is 10%
In most cases, this is because they inherited the mutation from their fathers.
Lead researcher Professor Mary-Claire King, of the University of Washington in Seattle, said: "Women with inherited mutations were at extremely high risk, but exercise and appropriate weight during their adolescent years clearly delayed the onset of breast cancer.
"It was a surprise but a source of hope to learn that factors over which we have some control made a difference in the age at which these highest-risk women developed breast cancer."
Previous studies had produced widely varying estimates of just how likely it is that women carrying the BRCA1 and BRCA2 mutations will develop breast cancer.
Some studies put it as high as 80%, while others concluded it was as low as 25%.
The latest study examined the genetic make-up of more than 2,000 people from families in which at least one woman had developed breast cancer - the most extensive study of its kind.
The researchers also found that women born before 1940 had a 24% chance of developing breast cancer by age 50 if they carried the rogue genes.
However, in women with the same genes born after 1940 the risk of developing the disease by age 50 was 67%.
This strongly suggests that environmental factors play a key role in determining when cancer will strike.
However, regardless of when a women was born, if she exercised during her teenage years, she was less likely to develop cancer later in life than women who did not exercise.
Similarly, women who avoided becoming obese during their teenage years developed cancer at similar rates but later in life.
Dr Larry Norton, of the Memorial Sloan-Kettering Cancer Center, said: "The possibility that lifestyle changes such as increased exercise and weight control could modify the impact of genetic risk has very intriguing implications, not only for BRCA-related cancers but for other breast cancers as well."
Dr Elizabeth Swisher, of the US Breast Cancer Research Foundation, said the study showed that the idea a woman inherited a raised risk of breast cancer solely from her mother was a misconception.
"It's very important that a woman try to get as much information as she can about her father's side of the family when gathering a medical history."
Charlotte Augst, of the charity CancerBACUP, said: "There is a lack of good evidence that focuses on women who have inherited a mutation in one of the known cancer genes.
"So this research is definitely a positive step forward, showing a clear link between exercise and a reduction of breast cancer risk."
The research focused on Ashkenazi Jewish patients. This group was selected because it carries a limited number of different BRCA1 and BRCA2 mutations, making analysis easier.
It is thought that particular mutations of the BRCA1 or BRCA2 genes impair the body's ability to repair the damage to its cells.