Women in their thirties should be warned that even one glass of wine a night could raise the risk of breast cancer, experts have suggested.
Meanwhile, mid-life women should be especially wary of weight gain – as this is the life stage when cancer-causing fat is likely to pile on.
Around a quarter of breast cancer cases are thought to be preventable through lifestyle changes such as drinking less alcohol and maintaining a healthy weight.
But new research, led by experts at Imperial College London, has discovered that risky behaviours are more prevalent at certain life stages.
While women in their 20s consume more alcohol than any age group, those in their 30s are at greater risk due to the combination of booze and excess weight.
It comes amid a rise in breast cancer among 20 to 49-year-olds, with cases having increased by almost 10 per cent in the last decade.
Professor Marc Gunter, the study’s lead author, said health messaging must target women at the moments in life that their habits typically shift.
‘Adolescence and early adulthood may be a particularly important time to encourage physical activity and help prevent rapid weight gain,’ he explained.
Meanwhile, the 30s to 50s are crucial for tackling creeping body weight and curbing increases in alcohol consumption.
The study – published in the journal Scientific Reports – analysed data from more than 110,000 women taking part in the Breast Cancer Now Generations Study.
Researchers tracked how major breast cancer risk factors – including smoking, alcohol consumption, exercise and weight – changed as women aged.
They found that drinking, smoking and exercise peak in late adolescence, while body fat steadily increases throughout adulthood.
Obesity is already known to raise the risk of at least 13 types of cancer, including bowel, breast, liver and pancreatic cancer.
Fat cells produce an enzyme called aromatase which converts testosterone into oestrogen.
This can prompt cells to divide and multiply, increasing the risk of cancer.
The more fat cells a woman has, the higher their oestrogen levels and greater their risk of breast cancer, research suggests.
This effect is particularly strong after the menopause.
Obesity in post-menopausal women is associated with a 30 per cent higher risk of breast cancer, compared with women who are not obese.
The Imperial team also found that women aged 23-66 tend to have a higher BMI throughout life, and their drinking and smoking peak earlier, compared to women aged over 67.
When alcohol enters the body it is broken down into a toxic chemical called acetaldehyde, which can damage DNA and trigger mutations.
Alcohol also raises oestrogen levels, which can encourage the growth of tumours, and makes other carcinogens – especially those in tobacco – more harmful.
Just one glass of wine a night – the equivalent of one to two units of alcohol – has been shown to increase breast cancer risk by 15 per cent.
This risk increases by a further 10 per cent for each additional daily drink.
Professor Amy Berrington, researcher from The Institute of Cancer Research who was involved in the study, said warning women of alcohol-related risks at specific ages could prevent diseases other than cancer, including diabetes and cardiovascular disease.
‘Public health strategies would have a much broader impact on women’s health.’
There are around 59,000 new cases of breast cancer in women every year in the UK and 420 in men.
Major signs include a lump or swelling in the breast, upper chest or armpit, a change in the skin of the breast, changes in size and shape and discharge from either nipple.
There is no one cause but age, family history, drinking alcohol, smoking, being overweight, taking the contraceptive pill or hormonal replacement therapy (HRT), and breast density are all known to increase the risk.
Women will be invited for screening on the NHS every three years after their 50th birthday.
