It is well established that type 2 diabetes, a condition characterized by abnormally high levels of blood sugar, is a risk factor for breast cancer. Less well known perhaps is that high blood sugar can increase breast cancer risk even in lean women without diabetes. In other words, diets with high glycemic load appear to increase the risk of breast cancer even in women who are not overweight. Previous studies have reported that relatively high consumption of sweets and sugary sodas increase the risk of breast cancer.

These associations have been explained as the result of an insulin-related mechanism since consuming sweet foods causes a temporary rise in circulating insulin. High insulin exposure is thought to increase the risk of breast cancer. However, there is some evidence that high glucose in and of itself can interact with cancer cells in ways that promote their growth. Now a new study has reported that high fasting blood sugar at diagnosis predicts worse breast cancer prognosis than normal blood sugar.

Latest research finds any degree of high blood sugar worsens prognosis

The retrospective study referenced at the beginning of this news story was designed to investigate the influence of blood glucose level and body mass index (BMI) on breast cancer prognosis. The study included 1,261 women diagnosed with stage I to stage III breast cancer and treated at Italy's National Cancer Institute in 1996, 1999 and 2000. The women were followed until year-end 2009. Data concerning blood tests and follow-up were obtained electronically.

The women were divided in five groups (quintiles) according to their fasting glucose levels around the time of diagnosis. The risks of distant metastasis were found to be significantly higher for every quintile compared to the lowest glucose quintile (<87 mg/dL). For the highest blood sugar levels, the risk of breast cancer was almost double (1.99 times) that of the lowest quintile. Comparable risks for the other quintiles were 1.85, 1.73 and 1.91. The risk of local recurrence was also higher for all other glucose quintiles compared to the first. Finally, the risk of death was also higher in the second, fourth and fifth quintiles compared to the first quintile. In other words, any degree of high blood glucose had a negative impact on prognosis.

The authors then examined the question of how body weight interacted with blood glucose in influencing prognosis. Overweight and obese women (with BMI ≥ 25 kg/m2) were found to have significantly higher risks of local recurrence and distant metastasis than those with BMI < 25 kg/m2, regardless of blood glucose level. The increased risk persisted without much variation during follow up. Further analysis determined that glucose and BMI had independent effects on prognosis. This means that even lean women might be better off avoiding eating habits that result in blood sugar spikes. The authors conclude that high fasting glucose and obesity are associated with less favorable outcomes in women with breast cancer. Maintaining healthy blood glucose levels and normal weight might improve prognosis, according to the authors.

Please see our articles on glycemic load and type 2 diabetes for more information.