Scientists have found evidence to suggest that women who have a higher body mass index (BMI) have a lower risk of breast cancer before the menopause.
In particular, researchers at the Institute of Cancer Research, London found that 18 to 24-year-olds in the highest BMI category (classed as obese) were 4.2 times less likely to develop breast cancer before the menopause than those in the lowest category (classed as underweight).
However, health experts warned that the results should not lead women to consider gaining weight as a way of preventing breast cancer.
Previous evidence shows that, after menopause has taken place, excess body weight increases breast cancer risk and this is when breast cancer most commonly develops and where obesity remains a leading lifestyle cause of the disease.
The latest research analysed the data of more than 750,000 women from 19 prospective studies around the world, collecting information on women's weights at different ages before following the participants for a median of 9.3 years.
Other lifestyle information was also collected to adjust for other potential breast cancer risk factors, including age at start of menstruation, age at birth of first child and a family history of the disease.
They found a 12%-23% relative breast cancer risk reduction per five BMI unit increase - about a 22lb (10kg) weight gain for women of average height - depending on age.
The researchers observed that 13,082 out of the 758,592 women went on to develop breast cancer, and found a strong inverse association between BMI and the risk of the disease before the menopause. This applied across the entire weight spectrum, not just among those who were overweight or obese.
This effect was found to be strongest for BMI at younger adult ages, with a 23% relative risk reduction per five-unit BMI increase for BMI at ages 18-24 - compared with a 12% relative risk reduction per five-unit BMI increase for BMI at ages 45-54.
Scientists said that if they could find a way to mimic the chemical changes in the body in response to body fat which are causing this protective effect - without weight gain being required - it could ultimately lead to a way of preventing breast cancer.
The research, which they described as the largest global study into the relationship between weight and pre-menopausal breast cancer to date, was carried out alongside teams from the University of North Carolina, the US National Institute of Environmental Health Sciences and other international collaborators.
Around 55,000 women are diagnosed with breast cancer each year in the UK, with nearly 20% of cases developing in women under the age of 50.
Previous research has established a link between increased body weight and a decreased risk of breast cancer before the menopause - but due to the lower rates of breast cancer among younger women, past studies had not been large enough to investigate the link in detail or by type of breast cancer.
Lead author Dr Minouk Schoemaker, of the Institute of Cancer Research, London, said: "Obesity is linked with a higher risk of breast cancer in older women and is one of the leading causes of cancer worldwide.
"But our study shows that the link with breast cancer is more complicated than we thought, and that younger women with higher BMIs are at lower risk of the disease before the menopause.
"After the menopause, obese women have an increased risk of breast cancer, which is likely due to oestrogen hormones produced by fat cells. We now need follow-up research to understand why this effect seems to be reversed in younger women."
Baroness Delyth Morgan, chief executive at Breast Cancer Now, which helped fund the research, said: "This is the strongest evidence yet that having a higher BMI when you are younger lowers your risk of breast cancer before the menopause.
"But we must be really clear that weight gain should not be considered an approach to prevent breast cancer.
"This protective effect is contrary to the situation after the menopause, where excess weight then increases breast cancer risk, where obesity is a leading cause of the disease and where breast cancer is also most common.
"We'd encourage women of all ages to maintain a healthy weight throughout their life to help lower their overall risk of cancer and other health conditions."
The study is published in the journal JAMA Oncology.>