Too little exercise and a stressful lifestyle are the most common risk factors for developing breast cancer, scientists have warned.
Most patients aged under 40 exercise less than three hours a week and suffer from high stress levels, according to research by the Hong Kong Breast Cancer Registry.
Breast tumours in this age group are usually more aggressive and toxic than in older patients, it found.
"Young patients are more likely to have breast cancer with more aggressive biological features, but the reason is not known," said Professor Winnie Yeo, a member of the organisation's steering committee. "Genetic factors may be one of the reasons."
There are about 3,000 new cases of breast cancer every year in Hong Kong. The registry analysed 7,241 local cases, assessing lifestyle-related factors.
Lack of exercise topped the list for most patients, and was more significant for younger women - 85 per cent reported doing less than three hours of physical activity per week.
High stress levels were identified in 46 per cent of younger patients. Nine per cent said they worked night shifts, which is strongly associated with stress.
"Hong Kong has the highest breast cancer rate in Asia, and the metropolitan lifestyle here - including little exercise and stressful lives - is the major contributing risk factor for the disease", said committee chairwoman Dr Polly Cheung Suk-yee.
Among patients over 40, being overweight and obese are more common factors, the research showed. Breast cancer has been the main cancer affecting Hong Kong women since 1993, causing about 500 deaths a year.
Eighty per cent of local women diagnosed with breast cancer are 40 or older, but Cheung said there is a trend for younger women to be affected. The youngest patient recorded was 18 years old.
"The risk of getting the disease is greater for older people, and the rate of women screening themselves in the city is still quite low," said Cheung. "We urge young women to raise their awareness of breast problems, and to perform screening regularly."
Meanwhile, a University of Hong Kong study found that almost half of breast cancer patients suffer mental or physical distress after having breasts surgically removed. Some women may develop chronic psychological problems, experts said.
The study, released this week, looked at the symptoms of 246 breast cancer patients since 2010, six weeks after their breast removal surgery. More than 40 per cent suffered insomnia and psychological problems like anxiety and lack of energy, it found.
"Effective control of the symptoms can reduce the distress caused by them," said researcher Wendy Lam Wing-tak, assistant professor in community medicine at the university.