A diet high in carbohydrates may be linked to an increased risk of breast cancer, researchers report. In a study of Mexican women, those who consumed the highest percentage of carbohydrates - particularly sugars - were most likely to develop breast cancer.

But it is probably premature to use the findings in the debate over low-carbohydrate diets.

“The most important point is that this should be regarded as preliminary data,” study co-author Dr. Walter Willett of Harvard Medical School in Boston told Reuters Health. “A prospective study of this relation should be conducted in Latin America because the results could be extremely important.”

Willett noted that most of the elevated cancer risk was related to consumption of sugar and refined starches. “There is already good reason to keep intakes of these low for the prevention of heart disease and type 2 diabetes,” he noted.

Willett’s group compared 475 Mexico City women who had breast cancer with 1,391 female residents who had not been diagnosed with cancer. On average, participants got 57 percent of their total calories from carbohydrates, 28 percent from fat and 15 percent from protein.

Carbohydrate intake was directly related to the risk of breast cancer, the researchers report in the medical journal Cancer Epidemiology, Biomarkers and Prevention. Women whose diet contained the highest percentage of carbohydrates were more than twice as likely to have breast cancer as women who consumed the least carbohydrates.

The risk was increased even though the team accounted for other factors that can influence the risk of breast cancer, including age, total calorie intake, socioeconomic status, family history of breast cancer and the number of times a woman has giving birth.

Sucrose - table sugar - was the carbohydrate most strongly associated with an increased risk of breast cancer. The association between sugar and breast cancer was stronger among women who had gone through menopause.

Carbohydrates may influence the risk of breast cancer by their effect on the body’s insulin pathway, according to the researchers, who were led by Dr. Isabelle Romieu at the Instituto Nacional de Salud Publica in Cuernavaca, Mexico.

After a person eats starchy or sugary carbohydrates, blood sugar rises. This spike in glucose triggers the secretion of the sugar-processing hormone insulin. Elevated levels of insulin may in turn lead to increased levels of a protein called IGF-I, and recent research has shown that IGF-I may be related to an increased risk of breast cancer, particularly in premenopausal women, according to the report.

Although some studies have suggested a link between fat and breast cancer risk, the study found that consumption of polyunsaturated fat - found in fish and vegetable oils - was associated with a reduced risk of breast cancer, particularly among postmenopausal women.

In addition, although body weight has been associated with the risk of breast cancer in some studies, body mass index, or BMI - a measure of weight in relation to height, was not related to breast cancer risk in this study.

The researchers point out that carbohydrates make up a high proportion of the Mexican diet. A recent survey found that urban Mexican women obtained an average of 64 percent of total calories from carbohydrates.

SOURCE: Cancer Epidemiology, Biomarkers and Prevention, August 2004.