Introduction: Obesity is a known risk factor forpostmenopausal breast cancer and is associated withpoorer prognosis for premenopausal andpostmenopausal patients; however, the aetiologicalmechanisms are unknown. Preclinical studies supportweight loss via caloric restriction and increasedphysical activity as a possible cancer control strategy,though few clinical studies have been conducted. Weundertook a feasibility trial among women recentlydiagnosed with stage 0-II breast cancer hypothesisingthat presurgical weight loss would be feasible, safe andresult in favourable changes in tumour markers andcirculating biomarkers. Methods and analysis: A two-arm randomisedcontrolled trial among 40 overweight or obese women,newly diagnosed with stage 0-II breast cancer andscheduled for surgery was planned. The attention controlarm received upper body progressive resistance trainingand diet counselling to correct deficiencies in nutrientintake; the experimental arm received the same pluscounselling on caloric restriction and aerobic exercise toachieve a weight loss of 0.68-0.919 kg/week. In additionto achieving feasibility benchmarks (accruing andretaining at least 80% of participants, and observing noserious adverse effects attributable to the intervention),we will explore the potential impact of an acute state ofnegative energy balance on tumour proliferation rates(Ki-67), as well as other tumour markers, serumbiomarkers, gene expression, microbiome profiles andother clinical outcomes (eg, quality of life). Outcomesfor the 2 study arms are compared using mixed modelsrepeated-measures analyses. Ethics and dissemination: Ethics approval wasreceived from the University of Alabama at BirminghamInstitutional Review Board (Protocol numberF130325009). Study findings will be disseminatedthrough peer-reviewed publications. Given that this isone of the first studies to investigate the impact ofnegative energy balance directly on tumour biology inhumans, larger trials will be pursued if results arefavourable.