A new study raises concerns about high-carb diets beyond how they affect weight.
Breast cancer was more than twice as common in women in Mexico who ate a lot of carbohydrates as it was in those who limited carbs to roughly half of all calories they consumed, the research found.
The typical American diet is half carbohydrates, and the study was launched in 1995, when people were being urged to eat carbs and avoid fats. Now, many are following low-carb, Atkins-style diets, which have generated lots of argument about how they affect cholesterol and heart disease.
This is one of the few studies to look at how sugar and starch intake might affect the risk of cancer.
"The findings do raise concern about the possible adverse effects of eating lots of carbohydrates," said Dr. Walter Willett, chief of nutrition at the Harvard School of Public Health.
He did the study with doctors at Instituto Nacional de Salud Publica in Cuernavaca, Mexico. It was funded by the U.S. Centers for Disease Control and Prevention, the Ministry of Health of Mexico, and the American Institute for Cancer Research. Results were published Friday in the journal Cancer Epidemiology, Biomarkers & Prevention.
Fats, fiber and specific foods have long been studied for their effects on various types of cancer, but few firm links have emerged. Being overweight is known to raise risk, but the new study took that into account and still found greater risk from high carbohydrate consumption.
Scientists think carbs may increase cancer risk by rapidly raising sugar in the blood, which prompts a surge of insulin to be secreted. This causes cells to divide and leads to higher levels of estrogen in the blood, both of which can encourage cancer.
A study earlier this year suggested that high-carb diets modestly raised the risk of colon cancer. Little research has been done on their effect on breast cancer, and results have been mixed. One study last year found greater risk among young women who ate a lot of sweets, especially sodas and desserts.
For this study, researchers enrolled 475 women newly diagnosed with breast cancer and a comparison group of 1,391 healthy women in Mexico City who were matched for age, weight, childbirth trends and other factors that affect the odds of getting the disease.
Women filled out a lengthy food questionnaire developed by Willett and widely used in nutrition studies, and were divided into four categories based on how much of their total calories came from carbohydrates.
Those in the top category - who got 62 percent or more of their calories from carbs - were 2.22 times more likely to have breast cancer than those in the lowest category, whose carb intake was 52 percent or less of their diet.
"It adds to the information that diet's important" with respect to cancer risk, said John Milner, the National Cancer Institute's chief of nutrition.
How applicable the results are to American women is debatable.
In Mexico, "the main carbohydrates these women ate were corn-derived, including tortillas, and soft drinks and bread," said Dr. Eduardo Lazcano-Ponce, one of the Mexican physicians who did the study.
Corn isn't fortified with folate and other nutrients as are many grains, cereals and other sources of carbohydrates eaten in the United States, and those nutrients might help prevent cancer, noted Sandra Schlicker, executive director of the American Society for Clinical Nutrition.
Breast cancer rates in the United States are among the highest in the world. Nearly 132 cases are diagnosed for every 100,000 women. In Mexico, incidence is rising and is currently estimated at 38 cases per 100,000 women. But Willett cautioned that those rates are not adjusted for age differences and that the U.S. population is considerably older than Mexico's and therefore more at risk of cancer.
In the study, women who ate a lot of insoluble fiber - found in whole grains, fruits and vegetables - had somewhat less risk of breast cancer. Fiber can modulate the absorption of carbohydrates.
"It leads me to believe that healthier carb sources, or at least diets containing fiber, would be less strongly associated with breast cancer," said Marji McCullough, a senior epidemiologist and nutrition expert at the American Cancer Society.
Experts say more research is needed through a study that, instead of relying on women's memories about what they ate, asks them to keep food diaries and then follows them for years afterward to see which ones develop cancer.
Finding dietary links to breast cancer is important because diet is one of the few risk factors a woman can easily modify.
"This study alone isn't enough for people to make changes in their diet, but it's a cautionary sign," Willett said.
He also said the study's findings don't mean that it is safe or healthful to eat lots of meat, cheese or fats, as many people who go on low-carb diets do.
"There are many concerns with eating diets high in animal fat," Willett said. "If people do want to cut back on carbohydrates, it's really important to do it in a way that emphasizes healthy fats, like salads with salad dressings."
The Institute of Medicine recommends that carbohydrates constitute 45 percent to 65 percent of calories, and that no more than 20 percent should come from added sugars, said Schlicker, who served on the panel that drafted the advice. New dietary guidelines are due to be released next year.