Weight Gain, Not Fat Intake, Increases Breast Cancer Risk

September 1, 1996

BETHESDA, Md--The latest major study of the relationship between diet and breast cancer risk has introduced an intriguing but somewhat complex element into this enduring controversy.

BETHESDA, Md--The latest major study of the relationship betweendiet and breast cancer risk has introduced an intriguing but somewhatcomplex element into this enduring controversy.

After analyzing data gathered from women enrolled in the Nurses'Health Study, including 2,500 with breast cancer, a team of HarvardUniversity researchers has concluded that weight gain after age18 increases a woman's risk of developing mammary tumors followingmenopause.

"There is a very strong relationship between weight gainand breast cancer," said Walter C. Willett, MD, DrPH, ofHarvard's School of Public Health. His group found scant support,however, for the notion that high levels of dietary fat by themselvesboost a woman's risk. "It's not fat per se that seems tobe important; it's calories," he said.

The study also confirmed earlier studies showing that alcoholuse--as few as one or two drinks a day--and taller stature increasebreast cancer risk, Dr. Willett said at the General Motors CancerResearch Foundation annual conference.

A weight gain of 20 kg (44 lb) beginning in early adulthood doubledthe likelihood of a postmenopausal diagnosis of breast cancer,the Harvard team found. An increase of 10 kg (22 lb) boosted awoman's risk by 70%.

Gaining weight increases estrogen in the body, Dr. Willett noted.Thus, the pounds added by women in the study probably heightenedtheir risk of post-menopausal breast tumors by contributing tohigher endogenous estrogen levels, he said. However, the riskprofile proved complicated.

Increased weight after age 18 slightly lowered a woman's riskof developing breast cancer before menopause, perhaps, Dr. Willettspeculated, because overweight women have more anovulatory menstrualcycles. But this lower breast cancer risk did not translate intoa decreased risk of dying from the disease.

"Prior to menopause, being a bit heavier actually may reducerisk of breast cancer," Dr. Willett said. But it doesn'treduce mortality for breast cancer because more of these womenhave tumors detected at a later stage, and these are more likelyto be fatal, he added.

"There is something besides fat intake that explains thebreast cancer risk associated with increased weight," Dr.Willett said. Rather, the risk appears to be determined by variousaspects of total calorie intake in relation to physical activity."The one thing we can say now is that staying lean and physicallyactive throughout life is an important way to reduce breast cancerrisk," he said.

That alcohol increases a woman's risk makes sense in light ofseveral studies showing that alcohol increases estrogen levels,Dr. Willett said. The role of height as a possible risk factorappears more complex and puzzling, however, because "heightdoes reflect genetics as well as nutritional status during theyears of growth and development."

Some evidence indicates that the breast cancer risk attached toheight reflects an ample diet during early childhood, which isassociated with early menarche, itself a risk factor for breastcancer, he said. "It's probably the nutritional componentof shortness that is important," Dr. Willett said, addingthat "we have to be careful about using height as a riskfactor for breast cancer."

Graham A. Colditz, MD, DrPH, of Harvard Medical School, suggestedthat a higher energy intake allows youngsters to grow faster.This shortens the cell cycle, and thus cells spend less time inthe repair phase, resulting in more accumulated DNA damage onceadult height is reached.

"This effect of height is not limited to breast cancer. It'sseen in prostate cancer and colon cancer," Dr. Colditz said.