Studies link fatty diet, breast cancer
British report targets food diary; nurses quizzed for U.S. finding
From Wire Reports
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A new study from Britain, and separate data from the Nurses Health Study, may add to the debate over whether women who eat high-fat diets increase their risk of breast cancer.
The British study, published in last week's Lancet medical journal, found that women who average more than 90 grams of fat a day have roughly double the risk of those who eat just 37 grams. The study was conducted at Cambridge University in England and involved 13,070 women who kept diet records from 1993-97.
The finding is likely to be controversial – it contradicts many large, careful studies that found no link between what women eat and their risk of this common cancer.
Researchers who conducted the Cambridge study argue that theirs is better than previous work, because it used a more precise method of measuring women's typical diets. But others said the study is too small to overturn the research suggesting diet plays little or no role in breast cancer risk.
In the separate U.S. study, heavy animal-fat consumption during young adulthood was associated with an increased risk of breast cancer.
The findings from the Nurses Health Study, a long-term look at the lifestyles of more than 90,000 female nurses, showed that those women who ate the most red meat and high-fat dairy products were 33 percent more likely to develop breast cancer than the women who ate the least of such foods.
"What this study says is: diet counts," says Dr. Len Lichtenfeld, deputy chief medical officer for the American Cancer Society.
The 33 percent increased risk means that about one in every 20 cases of breast cancer is caused by animal-fat intake in young adult women, says Dr. Patrick Remington, a public health professor at the University of Wisconsin Medical School. That could be up to 10,000 women this year, based on the cancer society's estimate of breast cancer diagnoses in U.S. women.
"In an area of breast cancer research that has yielded often starkly different findings, we have illustrated that there may be stronger support for lowering overall animal fat intake, especially during a woman's early adult life," says Eunyoung Cho, lead author of the study and an instructor at Brigham and Women's Hospital in Boston and Harvard Medical School.
The new nurses study results, appearing last week in the Journal of the National Cancer Institute, are the first based on premenopausal women.
In the British study, the researchers set out to discover whether the reason the previous follow-up studies found no link between diet and breast cancer was that the method they used to examine dietary habits – a food frequency questionnaire – was inaccurate. They also had the women keep a daily diary in which they recorded everything they ate.
By 2002, 168 of the women had developed breast cancer. Each of those cases was matched with four healthy women of the same age who had filled out the questionnaires and diaries around the same time as the women who developed breast cancer had.
The total group was divided into five equal categories of about 170, according to how much fat they ate each day. Two methods were used to place the women in one of the five categories – one based on the questionnaire and one on the daily diary.
The researchers calculated separately for both methods the difference in breast cancer risk between the women who ate the least fat and those who ate the most fat.
"The effects just weren't seen with food frequency questionnaires," says investigator Sheila Bingham, deputy director of the human nutrition unit at Cambridge. She called the questionnaire a "very crude method" that was not reliable.
However, when the food diaries were used to categorize the women, those who ate the diet highest in saturated fat were more likely to develop breast cancer as those who ate the least.
Of those in the lowest category, 14 percent developed breast cancer, compared with 20 percent in the highest class. The more fat that was consumed, the higher the risk of breast cancer.
Women who ate a higher-fat diet were not necessarily fatter; but once the researchers adjusted the results to eliminate skewing by other factors promoting breast cancer, such as body weight and total calories eaten, the women who ate the most saturated fat had twice the breast cancer risk as those who ate the least.
Most of the fat in the women's diets was saturated fat, so findings for total fat intake were similar.
Marji McCullough, an epidemiologist with the American Cancer Society, says researchers disagree over whether a questionnaire or food diary is more accurate.
Furthermore, the number of cancer cases in the latest study is small as such research goes. A recent analysis combined over 7,000 cancer cases in eight studies and found no risk from fat.
"If you consider all the evidence right now, you would assume there is a very small or no effect of fat on breast cancer," she says.
However, Dr. Elio Riboli, a nutrition and cancer expert, says, "These results reopen entirely the issue of the importance of investigating more, and with better data, the saturated fat-breast cancer hypothesis."
The chance of a woman developing breast cancer sometime during her life is between 8 percent and 11 percent, according to the World Health Organization.
"This article is a major step in the tortuous process of identifying the dietary determinants of breast cancer," says Dr. Riboli, who works with the U.N.'s International Agency for Research on Cancer. Doubling or reducing the risk "by 50 percent would make a huge impact on the suffering of tens of thousands of women each year."
Many early studies that looked back at the diets of breast cancer patients and compared them with the eating habits of healthy women of the same age found that a diet high in fat, or saturated fat – fat that comes from animal-based food such as meat, fish and dairy products – was weakly associated with a modest increase in breast cancer risk.
Experiments in lab animals also indicated that high fat intake could increase the likelihood of breast cancer. However, most of the recent studies, which followed groups of healthy women over time, failed to find a link.
Unlike many of the known risk factors for breast cancer, such as age, race or having relatives who have had the disease, women can control their dietary fat.
In the U.S. study, Dr. Cho and her colleagues questioned the nurses, ages 26-46, about their diet every two years from 1991-99. In those eight years, 714 of the women developed breast cancer.
The researchers divided the nurses into five groups based on their consumption of foods rich in meats and high-fat dairy products such as butter, cheese and whole milk. They observed more cases of breast cancer in the group that reported eating the most of such foods. Since that relationship has not been consistently observed in postmenopausal women, the researchers suggest that the age at which high levels of animal fat are eaten might be key.
The Associated Press and Knight Ridder Tribune contributed to this report.