Physical activity does not alter prolactin levels in post-menopausal women: results from a dose-response randomized controlled trial

BACKGROUND: Increased circulating levels of prolactin have been associated with increased risk of both in situ and invasive breast cancer. We investigated whether or not physical activity had a dose–response effect in lowering plasma levels of prolactin in postmenopausal women. METHODS: Four hundred previously inactive but healthy postmenopausal women aged 50–74 years of age were randomized to 150 or 300 min per week of aerobic physical activity in a year-long intervention. Prolactin was measured from fasting samples with a custom-plex multiplex assay. RESULTS: A high compared to moderate volume of physical activity did not reduce plasma prolactin levels in intention-to-treat (Treatment Effect Ratio (TER) 1.00, 95% Confidence Interval (CI) 0.95 – 1.06) or per-protocol analyses (TER 1.02, 95% CI 0.93 – 1.13). CONCLUSIONS: It is unlikely that changes in prolactin levels mediate the reduced risk of breast cancer development in post-menopausal women associated with increased levels of physical activity. TRIAL REGISTRATION: clinicaltrials.gov identifier: NCT01435005.