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 per cent 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.
For women on the low-fat diet who developed breast cancer, this translated into a 22 per cent 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 randomised 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.
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 tumours.