Purpose: Both weight gain and insulin resistance have been associated with poorer prognosis in women receiving adjuvant therapy for early stage breast cancer, however, interactions between weight gain and insulin resistance have not been explored longitudinally throughout the breast cancer treatment continuum.

Methods: One hundred non-diabetic women with early stage breast cancer receiving adjuvant chemotherapy and /or hormonal therapy were enrolled in this prospective, observational study. Metrics of weight, body composition (BMI, waist/hip circumference ratio (WHR)), and cardiometabolic health (fasting insulin, glucose and triglycerides) were obtained prior to adjuvant therapy (baseline) and repeated 6, 12, and 24 months post-diagnosis. Insulin resistance was calculated using the Homeostatic Model Assessment of Insulin Resistance (HOMA-IR).

Results: Complete data were available for 95 participants. Compared to baseline, body weight was significantly higher at the 12-month time-point (75.3 ± 15.7 vs. 76.2 ± 16.7, p = 0.03), however there was no difference in waist circumference (p = 0.96) or WHR (p = 0.52). HOMA-IR tended to increase 6 months after diagnosis (2.36 ± 2.17 vs. 2.70 ± 2.83, p = 0.06), largely driven by adverse responses in patients treated with chemotherapy (mean change + 0.53 (chemotherapy) vs - 0.64 (no chemotherapy), p = 0.005). Despite 12-month weight gain, the 6-month increase in HOMA-IR was fully abrogated 12 months after diagnosis.

Conclusion: Breast cancer patients experience small but significant weight gain in the year following diagnosis, and those who receive chemotherapy experience significant short-term metabolic impairments suggestive of insulin resistance. While the acute insulin resistance appears to attenuate over time, the long-term ramifications are unclear and may help explain weight gain in this population.

Keywords: Adjuvant therapy; Breast cancer; Insulin resistance; Weight gain.