One in four breast cancers are potentially preventable, an exhaustive evidence review of a staggering 68 breast cancer risk factors shows.
Evidence is mounting that smoking may increase the risk of breast cancer, and the case for recommending a diet rich in vegetables, dairy and calcium is also strengthening, according to the latest data.
Finding accurate and potentially life-saving information about breast cancer can be a confusing exercise in fear and self-flagellation amid the cacophony of well-meaning advice, research, and alarmist rubbish.
To cut through the noise, Cancer Australia has launched a website that provides the most up-to-date, evidence-based information about 68 personal, family, genetic, lifestyle and environmental risk factors that can raise or lower the chances of developing breast cancer.
Launched on Monday, the Breast Cancer Risk Factors website has distilled a 563-page review of evidence to explain the magnitude of risk for each factor, and the strength of the research supporting them (from "convincing" to "unproven or unlikely").
Some of the most powerful risk factors can’t be changed, such as being a woman, ageing, family history and genetic mutations.
But one in four breast cancers are potentially preventable, Cancer Australia CEO Dr Helen Zorbas said.
For instance, alcohol significantly raises the risk of developing breast cancer.
Just one standard alcoholic drink a day ups the risk of breast cancer by 7 per cent. The more alcohol consumed, the higher the risk.
Alcohol as a risk factor is backed by "convincing" evidence – the highest classification denoting strong, compelling and consistent evidence that either shows causation or is a marker for underlying causes.
Other "convincing" risk factors are: living in major cities, higher socio-economic status, higher breast density, younger age of menarche, older age at menopause, having a first child at an older age and not having children.
Being overweight or obese are also in the "convincing" category. For every 10-centimetre increase in waist circumference, the risk of postmenopausal breast cancer increases by about 6 per cent.
The next tier down are the "probable" risk factors – defined as likely to be linked to breast cancer but the evidence isn’t as strong as the "convincing" category.
Probable protective factors are breastfeeding and being physically active, according to the review of evidence.
Nearly 8 per cent of post-menopausal breast cancers in Australia each year are linked to a lack of physical activity.
Women who do large amounts of physical activity (vigorous, occupational, recreational, walking and household activity) probably have an approximately 13 per cent lower risk of post-menopausal breast cancer than women who do the lowest amounts of physical activity.
For pre-menopausal women, those who exercise vigorously probably have around 17 per cent lower risk of breast cancer than women who do low levels of vigorous physical activity.
"That’s really powerful information to act on," Dr Zorbas said. "The more active you are, the greater the benefits."
The "suggestive" risk factor category – defined as having some evidence of a link but not strong enough to be more certain – include eating processed meats, shift work, and having a previous cancer other than breast cancer.
Until recently the evidence that smoking tobacco increased the risk of breast cancer was inconclusive. But mounting research has bumped up smoking into the "suggestive" category.
The latest science also suggests eating dairy, food containing calcium, and vegetables (particularly red, yellow and orange-coloured) may reduce the risk of breast cancer.
There is a long list of modifiable and protective factors in the "unproven or unlikely" category, such as wearing bras, larger breasts, fruit, caffeine, soy, aspirin, breast injuries, silicone implants, stress, diabetes, polycystic ovary syndrome, working as a hairdresser, plastic bottles or passive smoking.
"We sometimes tend to focus on rare or unproven risks instead of what the evidence is really telling us," Dr Zorbas said.
On the bottom tier of evidence are abortion and miscarriage, which have no association with breast cancer, according to information on the Breast Cancer Risk Factors website.
Dr Zorbas said the website was a powerful tool that will shape our future conversations around breast cancer.
"Women can now readily access the most accurate information, based on the highest quality evidence, on which they can act," she said.
Dr Zorbas said having one or more risk factors doesn’t mean an individual will develop breast cancer. Most people with a number of risk factors won’t.
"In practice I see women who develop breast cancer and there is no obvious or no risk factors," Dr Zorbas said.
"But we should not resile the importance and power of knowledge on which women can act to reduce their risk."
Breast cancer is the most commonly diagnosed cancer in Australia, accounting for over 13 per cent of all new cancers in 2018.
By the end of this year, 18,235 new breast cancers will be diagnosed and 3,157 people will have died of the condition.