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 Researchers studied 19,541 participants in the federally funded
Women's Health Initiative (WHI) who were randomly selected to join a
dietary experiment focused on limiting fat intake to 20 percent of
calories. Researchers also looked at data for a control group of
29,294 women in the WHI study who didn't alter their diets.
By the time the researchers had been tracking half the women for at
least 8.5 years, 1,764 participants had been diagnosed with breast
cancer.
A decade after their diagnosis, 82 percent of the breast cancer
patients on the low-fat diet were still alive, compared with 78
percent in the control group.
For women on the low-fat diet who developed breast cancer, this
translated into a 22 percent lower risk of death during the study,
and these women typically didn't succeed at reducing fat consumption
by the amount suggested in the diet experiment.
"Decades ago, comparison of country-to-country differences in fat
intake found countries with higher fat intake like the U.S. and most
of Western Europe had higher breast cancer mortality, but subsequent
observational studies have had inconsistent results," said lead
study author Dr. Rowan Chlebowski of City of Hope National Medical
Center in Duarte, California.
The WHI dietary modification trial is the only full-scale randomized
trial addressing this issue," Chlebowski said by email.
The main goal of the diet experiment was to get women to change
their eating habits, not to count calories or lose weight.
Women assigned to change their diets had a series of group and
individual counseling sessions with certified nutritionists over the
first year of the program, followed by group sessions four times a
year for the remainder of the experiment.
After one year, women in the diet group got about 24 percent of
their calories from fat compared with 35 percent fat in other
participants' diets. While weight loss wasn't a goal, women in the
diet group weighted about 2.2 kilograms (4.9 pounds) less than other
participants.
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While the diet experiment was ongoing, 671 women in the diet group
and 1,093 who didn't alter their eating habits developed breast
cancer. This difference was too small to rule out the possibility
that it was due to chance.
But women on the low-fat diet were less likely to develop certain
hard-to-treat tumors.
One limitation of the study is that researchers relied on women to
accurately describe their eating habits in questionnaires, the
researchers note in JAMA Oncology. Another drawback is that women in
the diet group managed only minimal increases in their consumption
of fruits, vegetables and whole grains.
Because women in the low-fat diet group did lose weight relative to
other participants, it's also possible that weight rather than the
fat content of the diet might explain the differences in cancer
survival odds, said Dr. Graham Colditz, a researcher at Washington
University School of Medicine in St. Louis, Missouri, who wasn't
involved in the study.
"For lowering breast cancer risk - and cancer risk overall - the
most important part of diet is to keep calories in check," Colditz
said by email. "Weight gain and obesity is an important risk factor
for postmenopausal breast cancer - and 12 other cancers."
Not all fat is created equal, either.
"There is no evidence that total fat intake affects the risk of
breast cancer," Colditz said. "There is growing evidence, however,
suggesting that type of fat could be important, with diets rich in
polyunsaturated and monounsaturated fats possibly lowering risk, and
those higher in saturated and animal fats possibly increasing risk."
SOURCE: http://bit.ly/2LYypwv JAMA Oncology, online May 24, 2018.
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