Women who were heavy as girls are at lower risk of developing breast cancer, new research confirms.

The role of body size in breast cancer risk changes throughout life, Dr. Jingmei Li of the Karolinska Institutet in Stockholm, who helped conduct the study, told Reuters Health by email. High birthweight ups breast cancer risk, while being heavy in childhood and during puberty reduces risk; after menopause, a woman’s risk rises with her body mass index or BMI - a measure of weight in relation to height used to gauge whether person is overweight or obese.

Each of these relationships is “well studied and well documented by an abundance of literature,” Li added. In the current study, she and her colleagues sought to gather more detail by looking at whether the effect of childhood body size on breast cancer risk varied by tumor type - specifically, whether a tumor contained receptors for estrogen, progesterone, neither, or both. Breast tumors with hormone receptors will only grow if those hormones are present, so they can be treated with hormone-blocking drugs like tamoxifen; tumors without hormone receptors may be harder to treat.

The researchers looked at 2,818 Swedish women with breast cancer and 3,111 women without the disease, showing them a series of nine figure outlines and asking them to choose one that best represented their body size at age seven.

Women who had larger bodies during childhood were 27 percent less likely to have breast cancer than women who were leaner as seven-year-olds. Being heavier as a girl was protective for all tumor types the researchers studied. But the effect was stronger for tumors that didn’t carry estrogen receptors; larger childhood body size reduced the risk of these tumors by 60 percent, while it reduced the risk of estrogen-receptor positive tumors by 20 percent.

So what do these relative figures mean in absolute terms? According to the National Cancer Institute, the average 60-year-old woman has a 1.8 percent risk of developing breast cancer over the next five years. Based on the new study, if that average woman had a large body size as a child, she would be at 27 percent lower risk of breast cancer; her five-year risk of developing the disease would thus fall to 1.3 percent.

Investigators have proposed that there are “special windows of mammary development” at different stages of life, Li noted, and there’s evidence that the higher levels of estrogen in the bodies of heavier girls may cause permanent, protective changes in their breast tissue. “Being overweight during childhood is certainly not what we are advocating,” Li added. “There is definitely no reason for any health alarms or overfeeding of the kids to get them to reach a particular body size.”

She continued, “To the lay public, the scientific community may appear confused and undecided as to whether being ‘fat’ or ‘thin’ is good or bad for breast cancer, but the main detail is in the timing. One’s destiny does not necessarily lie in the genes.”

SOURCE: Breast Cancer Research, online April 14, 2010.