REstoring Metabolic FLEXibility Through Combined Exercise in Breast Cancer Survivors: A Randomized Controlled Trial. (REFLEX)

The REFLEX study is a randomized, controlled clinical trial designed to evaluate the impact of a 16-week supervised, combined exercise program on metabolic flexibility and cardiorespiratory fitness in breast cancer survivors. While cancer treatments have significantly improved survival rates, the disease and its therapies often cause systemic mitochondrial dysfunction and muscle loss. This leads to persistent "metabolic inflexibility"-a reduced ability to efficiently switch between burning carbohydrates and fats for energy. This inflexibility, exacerbated by sedentary behavior and cancer-related fatigue, can severely diminish a patient's functional autonomy and overall quality of life. The trial focuses on assessing whether exercise can restore this metabolic adaptability in women aged 18 to 70 with stage I-III breast cancer who are currently on hormonal treatment and completed neo/adjuvant chemotherapy within the past 6 months.

The study operates on the primary hypothesis that an individualized exercise intervention will significantly improve the participants' metabolic flexibility-measured via the metabolic crossover point-and overall aerobic capacity (VO2max) when compared to a control group. Additionally, the trial will analyze secondary outcomes, including changes in cardiovascular and metabolic risk factors, muscle strength, body composition (such as bone mineral density), and patient-reported outcomes assessing quality of life, fatigue, joint pain, anxiety, and depression.

A total of 44 participants will be randomly assigned to either the intervention or control group. Participants in the intervention group will complete a 16-week program consisting of two weekly supervised sessions. Each session lasts between 55 and 85 minutes and includes a tailored combination of aerobic and strength training based on the individual's baseline fitness and metabolic thresholds. Participants in the control group. will receive standard educational materials via email, featuring healthy lifestyle and exercise guidelines from the World Health Organization (WHO) and the American College of Sports Medicine (ACSM).

To evaluate the program's effectiveness, comprehensive medical and functional assessments will be conducted at baseline, immediately after the 16-week intervention, and at a 3-month follow-up. These evaluations include submaximal and maximal exercise tests on a cycle ergometer to analyze respiratory gases, detailed blood tests, body composition imaging (DXA scans), and maximum dynamic strength testing. Ultimately, the research aims to generate high-quality evidence to optimize standard exercise recommendations for breast cancer survivors and improve their long-term health and resilience.

Study Overview

• Status: Not yet recruiting
• Conditions: Cancer; Breast Cancer; Neoplasm; Metabolic Flexibility
• Intervention / Treatment: Behavioral: Exercise; Behavioral: Control

• Study Type: Interventional
• Enrollment (Estimated): 44
• Phase: Not Applicable

Contacts and Locations

• Study Contact: Name: Susana Aznar Laín; Phone Number: +34 669356600; Email: susana.aznar@uclm.es
• Study Contact Backup: Name: Alejandro Jiménez Marín; Phone Number: +34 685414255; Email: alejandro.jimenez@uclm.es

Study Locations

• Spain
  • Toledo
    • Toledo, Toledo, Spain, 45071
      • University of Castilla-La Mancha (PAFS Research Group)
      • Contact: Susana Aznar Laín; Phone Number: +34 669356600; Email: susana.aznar@uclm.es

Participation Criteria

Eligibility Criteria

• Ages Eligible for Study: Adult; Older Adult
• Accepts Healthy Volunteers: No

Description

Inclusion Criteria:

• Women aged between 18 and 70 years.
• Confirmed diagnosis of breast cancer in stages I to III.
• Received neo-adjuvant or adjuvant chemotherapy and completed the chemotherapy treatment within the 6 months prior to study enrolment.
• Residency within a distance of less than 40 km from the Hospital Universitario de Toledo.

Exclusion Criteria:

• Diagnosis of stage IV breast cancer.
• Undergone major surgery within the 4 months prior to enrolment or scheduled for breast reconstruction surgery during the study period.
• Diagnosis of diabetes mellitus with poor glycaemic control, defined as an HbA1c value greater than or equal to 8.5%.
• Presence of an acute infectious disease.
• Absolute or relative medical contraindications for physical exercise or maximal exercise stress testing, according to the American College of Sports Medicine (ACSM) guidelines.
• Serious cardiac or cardiovascular disease, including a history of myocardial infarction, heart failure, or known cardiomyopathy.
• Regular consumption of beta-blockers or any medication that alters the functioning of the nervous system.
• Currently active, defined as engaging in more than 150 minutes per week of structured physical exercise during the 6 months prior to enrolment.

Study Plan

How is the study designed?

Design Details

• Primary Purpose: Prevention
• Allocation: Randomized
• Interventional Model: Parallel Assignment
• Masking: Single

Number of Arms

2

Arms and Interventions

Participant Group / Arm	Intervention / Treatment
Other: CONTROL GROUP	Behavioral: Control; Participants assigned to the control group receive standard care typically provided to patients in this phase of recovery. Instead of a supervised exercise program, these individuals are sent educational materials via email. This material consists of a video and official documents containing basic guidelines and recommendations for a healthy lifestyle, based on standards from the World Health Organization (WHO) and the American College of Sports Medicine (ACSM) for cancer survivors. The documents provide general instructions on physical activity, reducing sedentary behavior, and maintaining overall health habits. Like the intervention group, participants in the control group undergo all clinical and functional evaluations at the start of the study, after 16 weeks, and during a 3-month follow-up.
Experimental: EXERCISE GROUP	Behavioral: Exercise; The intervention consists of a supervised, in-person, combined exercise program lasting 16 consecutive weeks, conducted at a specialized exercise unit. Participants attend two sessions per week, with at least 48 hours of recovery between sessions, and each session lasts between 55 and 85 minutes. The program follows a "combined" methodology, meaning every session integrates both resistance (strength) and aerobic training. To ensure safety and precision, sessions are led by a graduate in Physical Activity and Sport Sciences with specialized training in oncology, and participants work in small groups of 6 to 8 people. Each session is structured into four distinct parts: a 10-minute warm-up involving low-to-moderate intensity activity and joint mobility; a resistance training block; an aerobic training block; and a cool-down featuring static stretching for 30-40 seconds per muscle group. The resistance training portion utilizes a "full body" approach.

What is the study measuring?

Primary Outcome Measures

Outcome Measure	Measure Description	Time Frame
Changes in Cardiorespiratory Fitness	Assessment of cardiorespiratory capacity determined through a maximal cardiopulmonary exercise test (stress test) on a cycle ergometer	7 months
Change in Crossover point	Assessment of the metabolic crossover point determined through an individualized, submaximal incremental exercise test on a cycle ergometer with continuous measurement of expired gases (indirect calorimetry) to calculate substrate oxidation rates.	7 months

Secondary Outcome Measures

Outcome Measure	Measure Description	Time Frame
Change in Lower Body Muscular Strength	Estimated one-repetition maximum (1RM) for the leg press. Measurements will be obtained through force-velocity profiling using a linear position transducer.	7 months
Change in Upper-Body Muscular Strength	Estimated one-repetition maximum (1RM) for the vertical press. Measurements will be obtained through force-velocity profiling using a linear position transducer.	7 months
Change in 6-Minute Walk Test	Evaluation of physical performance using the 6-minute walk test.	7 months
Change in Lower-Body Muscular Endurance (30-sec CST)	Evaluation of physical performance using the 30-second chair stand test.	7 months
Change in Grip Strength	Evaluation of upper-body isometric strength using a handheld dynamometer.	7 months
Change in Body Mass Index (BMI)	BMI will be calculated as weight in kilograms divided by height in meters squared (kg/m2).	7 months
Change in EQ-5D-5L Visual Analogue Scale (VAS)	Evaluation of self-rated health using the EuroQol Visual Analogue Scale (EQ VAS). The scale is a vertical line with minimum value 0 (worst imaginable health) and maximum value 100 (best imaginable health). Higher scores indicate a better perceived health outcome.	7 months
Change in EQ-5D-5L Index Score	Evaluation of health-related quality of life based on the EuroQol 5-dimension 5-level (EQ-5D-5L) descriptive system. This system assesses five dimensions: mobility, self-care, usual activities, pain/discomfort, and anxiety/depression. Responses are converted into a single index score using a valuation algorithm. The index scale typically ranges from less than 0 to 1, where higher scores indicate a better outcome.	7 months
Change from Baseline in Cancer-Related Fatigue (FACIT-F)	Evaluation using the Functional Assessment of Chronic Illness Therapy-Fatigue (FACIT-F) scale. Total scores range from 0 to 52, where higher scores indicate lower levels of fatigue.	7 months
Change in Hospital Anxiety and Depression Scale (HADS)	Evaluation using the Hospital Anxiety and Depression Scale (HADS). The total score ranges from 0 to 42, where higher scores indicate greater psychological distress.	7 months
Change in maximal fat oxidation	Assessment of the maximal fat oxidation rate determined through an individualized, submaximal incremental exercise test on a cycle ergometer with continuous measurement of expired gases (indirect calorimetry).	7 months
Change in maximal carbohydrate oxidation	Assessment of the maximal carbohydrate oxidation rate determined through an individualized, submaximal incremental exercise test on a cycle ergometer with continuous measurement of expired gases (indirect calorimetry).	7 months
Change in Body Fat Percentage	Assessment of total body fat percentage using Dual-Energy X-ray Absorptiometry (DXA) scan.	7 months
Change in Lean Mass Percentage	Assessment of total lean muscle mass percentage using Dual-Energy X-ray Absorptiometry (DXA) scan.	7 months
Change in Bone Mineral Density	Assessment of bone mineral density (g/cm²) determined through a Dual-Energy X-ray Absorptiometry (DXA) scan.	7 months

Collaborators and Investigators

• Sponsor: University of Castilla-La Mancha

Study record dates

Study Major Dates

• Study Start (Estimated): September 1, 2026
• Primary Completion (Estimated): August 1, 2027
• Study Completion (Estimated): December 1, 2027

Study Registration Dates

• First Submitted: January 27, 2026
• First Submitted That Met QC Criteria: February 4, 2026
• First Posted (Actual): February 12, 2026

Study Record Updates

• Last Update Posted (Actual): May 28, 2026
• Last Update Submitted That Met QC Criteria: May 25, 2026
• Last Verified: January 1, 2026

More Information

Terms related to this study

• Additional Relevant MeSH Terms: Neoplasms by Site; Skin Diseases; Breast Diseases; Skin and Connective Tissue Diseases; Neoplasms; Breast Neoplasms; Motor Activity; Movement; Musculoskeletal Physiological Phenomena; Musculoskeletal and Neural Physiological Phenomena; Exercise
• Other Study ID Numbers: 2024-UNIVERS-12844

Drug and device information, study documents

• Studies a U.S. FDA-regulated drug product: No
• Studies a U.S. FDA-regulated device product: No