Researchers add, however, that they are not certain if the hormone dip will translate to a lower breast cancer risk later in life.
High-fat diets have been linked with the development of breast cancer, although studies are inconclusive, says Joanne F. Dorgan, an epidemiologist at the Fox Chase Cancer Center in Philadelphia. "Most of the other studies have been done in adults, not girls," Dorgan says.
Dorgan led the study, appearing in today's issue of the Journal of the National Cancer Institute. The team randomly assigned 286 girls, aged 8 to 10, to be in the low-fat dietary intervention group or in the group that received educational materials from the American Heart Association, which calls for limiting fat to 30 percent or less of total calories.
Girls in the intervention group were instructed to limit total fat intake to 28 percent of calories (with less than 8 percent of that saturated), to eat fiber, and to limit cholesterol. All girls had high blood cholesterol levels.
The study was an ancillary study of the Dietary Intervention Study in Children to look at how to improve cholesterol.
The researchers wanted to find out if lower fat intake altered sex hormone levels that, in adults, may be related to the development of breast cancer.
Blood levels of the sex hormones were measured at the study start and at one, three, five and seven years. At the five-year mark, girls eating the low-fat diet had reduced levels of sex hormones compared with girls in the group not instructed to lower dietary fat. They had 29.8 percent lower estradiol, 20.7 percent lower estrone, and 28.7 percent lower estrone sulfate levels during the fist half of their cycles, and 27.2 percent higher testosterone levels during the second half of the cycles. By the seven-year mark, girls in the reduced fat group had half the progesterone levels during the second half of their cycles as did those in the other group.
"The results were stronger than I anticipated," Dorgan says.
"Experts in the field would expect some of these changes," says study co-author Victor Stevens, assistant director for epidemiology and disease prevention at Kaiser Permanente Center for Health Research in Portland, Ore. "But it surprised everyone that the differences were as large as they were."
Still, it's too soon to dispense public health advice for teen and preteen girls, Dorgan says.
The researchers don't think the hormone drop was enough to affect future fertility. "The menstrual cycles appear to remain normal," says Dr. Peter Kwiterovich, a professor of pediatrics and medicine at Johns Hopkins University School of Medicine and another co-author.
The researchers hope to get funding to bring back the subjects studied, who are now in their early 20s, for follow-up, Dorgan says. Findings from a parallel study conducted in boys will be reported separately.
Meanwhile, another expert in the field, Dr. Graham A. Colditz, a professor of medicine at Harvard Medical School, praises the study as "a rigorously conducted intervention. This has really shown quite clearly that differences in hormone levels have been achieved with this intervention. It breaks new ground."
Rather than focus on whether teen and preteen girls should lower fat intake or not, Colditz says, for now, the "real message" should be to maintain a healthy lifestyle, avoiding excess weight and exercising regularly.