Experts were quick to note that a low-fat
diet is still good for the heart and other
aspects of health. They said the study
indicates a need to look more carefully at
how diet may affect the risk of breast
cancer.
``We should just accept that good
scientists can't tell you yet what to eat to
minimize your breast-cancer risk,'' said Dr.
John A. Glaspy of the University of California
at Los Angeles, who was not involved in the
research.
The Harvard study was published in
Wednesday's Journal of the American
Medical Association.
Doctors have theorized that eating lots of
fat increases breast-cancer risk. They have
based their thinking on animal studies,
international comparisons and studies of
women who developed breast cancer and
women who didn't.
Others have suggested that the key is the
type of fat consumed, rather than the
amount; that fish oil offers some protection
from breast cancer; or that contaminants
stored in fat trigger breast cancer.
The new study tracked 88,795 women in the
continuing Nurses' Health Study. The
women, ages 30 to 55, completed detailed
questionnaires about their eating habits
every four years from 1980 to 1994.
Researchers compared the diets of the
women without breast cancer and the 2,956
women whose breast cancer was discovered
during the course of the study.
Breast cancer was found to be no more
common among women who ate lots of fat,
or among those who ate a large proportion
of animal fat, polyunsaturated fat (vegetable
fat) or trans-unsaturated fat (partially
hydrogenated oils, such as those used in
margarine and to cook doughnuts and french
fries).
Nor was breast cancer any less common in women who got a high proportion of their fats from fish oil or who got less than 20 percent of their total calories from fat.
``Our research indicates it's highly unlikely
that women who consume a low-fat diet are
protected against breast cancer,'' said the
study's lead author, Dr. Michelle D. Holmes,
an instructor of medicine at Harvard Medical
School and Brigham and Women's Hospital in
Boston. ``Equally, it appears a high-fat diet
also poses no increased risk for the disease.''
Most surprisingly, women who ate the least
fat appeared to have a 15 percent higher
rate of breast cancer, the researchers said.
But Holmes said she's not ready to conclude
that a low-fat diet increases breast cancer
risk, because the finding was based on fewer
than 1,000 women who ate less than 20
percent of calories as fat.
UCLA's Glaspy said the new research ``is
something that we're going to need to
explain.''
He said fat intake may need to be very low _
as little as 10 percent of total calories _ to
reduce breast-cancer risk. He also noted
that only some kinds of fish oil have
appeared beneficial in previous studies. The
new study did not distinguish between
types.
Nor did it run long enough to explore the possibility that dietary effects may take longer than 14 years to emerge, he said. Glaspy believes that diet interacts with environmental factors in subtler ways than scientists have been able to identify.
The theory that dietary fat is linked to breast cancer arose from the observation that breast-cancer rates are far lower in traditional Asian cultures, where diets are generally low in fat.
Compared with women in Western cultures, traditional Asian women start menstruating later, give birth at a younger age and gain far less weight in adulthood _ all factors that decrease breast-cancer risk, Holmes noted