Insulin Resistance Linked to Increased Breast Cancer Risk

SAN FRANCISCO—Insulin resistance, as determined by C-peptide levels, appears to be linked to increased breast cancer risk, Celia Byrne, PhD, said at the 93rd Annual Meeting of the American Association for Cancer Research (abstract 1179). In a study of 1,226 women, those who developed breast cancer were more likely than controls to have elevated concentrations of C-peptide, considered an indication of insulin secretion.

"Women with the highest levels of C-peptide (in the highest quintile) had a risk of breast cancer that was 68% greater than that of women with the lowest levels of C-peptide (in the lowest quintile)," said Dr. Byrne, instructor in medicine, Harvard Medical School and the Channing Laboratory of Brigham and Women’s Hospital, Boston.

Nurses Health Study

Dr. Byrne and her colleagues at Harvard and McGill University identified 463 women diagnosed with breast cancer and 763 controls, matched for age, menopausal status, time of blood collection, and fasting status, from the 32,826 participants in the Nurses Health Study.

All of the women were between 43 and 69 years of age when they gave a blood sample in 1989 or 1990. The breast cancer cases were diagnosed within a 5-year period following the collection of blood samples.

C-peptide levels increased with age, body mass index, and postmenopausal status, and decreased with greater alcohol intake and physical activity. The increased risk of breast cancer associated with elevated C-peptide levels appeared somewhat stronger among women who were premenopausal when they gave the blood sample, Dr. Byrne said.

"In general, C-peptide levels increased slightly with age, and they increased quite dramatically with a higher body mass index," Dr Byrne said. "Women who were physically active tended to have lower levels of C-peptide, which may add to the evidence that physical activity lowers breast cancer risk."

At a media briefing on the study, Joyce O’Shaughnessy, MD, of Baylor Sammons Cancer Center and US Oncology, Dallas, said that the study "provides us with evidence that there is an increasingly tight link between obesity and breast cancer risk."

Based on these and other findings, she said, public health advocates should stress the dangers of overweight and obesity to the American populace. "We should be telling people even more forcefully that physical activity should be increased because it lowers estrogen levels and improves insulin resistance," Dr. O’Shaughnessy said.

Stressing the importance of physical activity and a moderate diet is especially important for schoolchildren, Dr. O’Shaughnessy said. "We need to start early in life when we can truly influence diet and activity habits," she said.