Background: Creatine supplementation is an effective ergogenic nutrient for athletes, as well as people for people starting a health or fitness program. Resistance training previously been identified as an important method of increasing muscle mass and strength among people, specially in people with cancer to avoid sarcopenia. The potential of creatine supplementation for adaptations produced by strength training in cancer patients are still unknown.

Objective: the primary aim of this study is to evaluate the effectiveness of a 16-week supervised resistance training program intervention with and without creatine supplementation in patients with breast cancer.

Methods: A multicentre, randomized, double-blind, placebo-controlled study designed to evaluate the effects of creatine supplementation in addition to resistance training in breast cancer patients. Patients will be randomly assigned to 3 groups: a control group (CG) and two experimental groups. The first resistance training group (RG) will perform resistance training, while the second resistance-creatine experimental group (RCG) will perform the same resistance training as the RG and will also receive a 5 g/day supplementation of creatine for the duration of the exercise the 16-week intervention. RG participants will follow the same daily dosing protocol, but in their case, with dextrose/maltodextrin. Resistance training will be a 16-week supervised workout that will consist of a series of resistance exercises (leg press, knee extension, knee bends, chest press, sit-ups, back extensions, pull-ups, and shoulder press) that involved the largest muscle groups of the body and will be performed three times a week on non-consecutive days. Both the RG and the RCG will receive a supplement of soluble protein powder (20-30 g) daily.

Discussion: The results of this intervention will help to better understand the potential of non-pharmacological treatment for improving strength and wellbeing values in breast cancer patients with and without creatine supplementation.

Inclusion Criteria:

• Stage pTis-T1-T2-T3 pN0-1 M0 (early stage).
• Patients who have completed local treatment [surgery + radiotherapy (RT)] undergoing clinical follow-up in outpatient clinics with or without adjuvant hormone therapy treatment.
• Age: ≥18-60 years.
• ECOG Performance status from 0 to 1 (IK ≥80%).
• Body mass index above than 18.5.
• Without excessive alcohol consumption (men > 21 and women > 14 units/week).
• No current or previous illness or injury that may prevent participation and training.
• No recent systematic strength training.
• Not taking medications that are known to alter body composition (corticosteroids, metformin...).
• People who carry out some aerobic training will not be excluded (the activity carried out will be recorded weekly).

Exclusion Criteria:

• Having received adjuvant or neoadjuvant chemotherapy.
• Metastatic carcinoma of the breast.
• Recurrence or second primary breast.
• History of previous cancer except previously treated basal cell carcinoma of the skin.
• Decompensated heart disease, uncontrolled hypertension (TAS>200 or TAD>110), heart failure (NYHA II or greater), heart failure or constrictive pericarditis, neutropenia, severe anemia (Hb<8.0 g/dl), blood count platelets <50,000 microL.
• Other health problems in which exercise is contraindicated
• Perform regular physical activity (150 min/week of moderate activity or 75 of vigorous activity), measured with the PVS questionnaire.
• Pregnancy.
• Criterion in the opinion of the investigator of inability to adequately understand the involvement and participation in this clinical study.