Carb-conscious dieters may be lowering their risk of breast cancer while they're shedding pounds, based on the findings of research published in this month's edition of the journal Cancer Epidemiology, Biomarkers & Prevention.

In a case-control study of 1,866 women in Mexico, those who derived 57 or more percent of their total energy intake from carbohydrates incurred a risk of breast cancer 2.2 times higher than women with more balanced diets. Dietary patterns in Mexico are characterized by higher consumption of carbohydrates and lower intake of fat and animal protein than those in more affluent western countries.

The team of researchers from the Instituto de Salud Pública in Cuernavaca, Mexico, and the Harvard School of Public Health in Boston, suggests that the association between carbohydrates and breast cancer may be related to elevated levels of insulin and insulin-like growth factor binding proteins in the blood.

"Scientists have long suspected that diet was among the factors contributing to breast cancer," said study co-authors Walter Willett, Dr. P H, the Fredrick John Stare Professor of Epidemiology and Nutrition at the Harvard School of Public Health adding, "Now, with studies like ours, we are beginning gradually to understand what elements of diet specifically are associated with the disease, and to grasp the chemical and biological processes that contribute to it at the cellular level."

Of all the carbohydrate compounds, sucrose and fructose demonstrated the strongest association with breast cancer risk in the study. Sucrose is derived from sugar cane, sorghum and the sugar beet; it is most commonly found in table sugar and sweetened prepared foods and beverages. Fructose is a component of sucrose and is also found in fruit.

Eating sweets and starches causes a rapid rise in the body's blood sugar levels, which in turn cues the production of insulin and triggers a biological process that ultimately can influence carcinogenesis by causing cells to proliferate.

Insulin and an insulin-like growth factor also may contribute to higher circulating levels of biologically active estrogens, a risk factor for breast cancer in pre-menopausal women. Ninety per cent of breast tumours are insulin-receptor positive and over-express the insulin-like growth factor.

The prevalence of type 2 diabetes, often associated with obesity, reflects an underlying insulin resistance in the Mexican population generally. Among urban Mexicans, nearly one-third of women between the ages of 12 and 49 are overweight. Yet when the research team took into account body mass index and other such potentially confounding factors as socioeconomic status, age at first birth, number of children, and family history of breast cancer, the relationship between carbohydrate intake and breast cancer remained the same.

Dietary fat - certainly a contributor to obesity - fared well in the research, showing no significant association with breast cancer risk overall. Willett noted, however, that the intake of polyunsaturated fat by the women in the study group was only about half that of the United States population.

Insoluble fiber intake was associated with lower risk of breast cancer, possibly because fiber may modulate the absorption of carbohydrates and thus affect the glycemic response.

"This study raises important questions about high carbohydrate diets, particularly among populations or individuals prone to insulin resistance. However, one study is not enough to make major changes in diet, and more work on this topic is urgently needed," Willett said.