Cancer is considered a major global public health problem. It was estimated that in 2022 approximately 19.9 million new cancer cases were diagnosed worldwide, and this number is expected to increase over the next two decades to 28.0 million (1). Specifically, breast cancer (BC) represents the highest incidence worldwide, with approximately 2.3 million new cases diagnosed in 2022 (1).

A higher incidence of BC is observed in developed countries, which may be due to high rates of obesity, alcohol and tobacco consumption, early onset of puberty, the use of contraceptives and hormonal therapies, low levels of physical activity, and giving birth at later ages (2,3). In addition to the factors mentioned above, hereditary factors and age also represent risk factors for cancer development (2,3). Finally, the presence of family members with breast and/or ovarian cancer carrying mutations in the BRCA1 or BRCA2 genes, among others, which increase the likelihood of tumor proliferation, as well as age over 40 years, also increase the probability of developing BC (2,3).

Specifically, there is a molecular subtype that does not respond to hormonal receptors or HER2 and may be more aggressive and have fewer specific treatment options, known as triple-negative breast cancer (TNBC).

Metabolic flexibility (MF) is described as the ability of the body to adapt to energy demands in different contexts. During chemotherapy and after surgery, significant changes may occur, such as increased body fat, loss of muscle mass, cancer-related fatigue, metabolic alterations, and decreased quality of life. These changes may persist even years after treatment and may affect both well-being and recovery. It could therefore be suggested that metabolic flexibility in muscle fibers in patients with TNBC may be reduced, particularly in patients undergoing systemic treatment, with potential difficulties adapting to different intensities and energy demands in daily life. A decrease in muscle metabolic flexibility would also imply a reduction in muscle strength and physical function, significantly impairing quality of life.

Therefore, the main objective of this study is to analyze muscle metabolic flexibility at different stages of early disease and to evaluate whether different types of exercise training can improve these outcomes.

To achieve this, assessments will be conducted at four time points during the early stages of the disease: at diagnosis, after neoadjuvant treatment, after surgery, and following an exercise intervention. The assessments will include blood analyses, body composition measurements, cycling exercise tests to evaluate oxygen consumption and the utilization of fat and glucose, measurements of muscle strength, and questionnaires assessing fatigue and quality of life.

After surgery, participants will be randomly assigned to one of four groups for 12 weeks: a control group receiving general physical activity recommendations; a moderate-intensity cardiovascular exercise group focused on maximal fat oxidation; a high-intensity interval cardiovascular exercise group; and a progressive resistance training group. The final objective is to determine which type of exercise most effectively improves metabolic flexibility, muscle strength, body composition, and overall well-being.

Participation in the study is voluntary and does not affect standard medical care. All assessments and training sessions will be supervised by qualified exercise professionals to ensure participant safety.

Cancer is recognized as a major global socio-health issue. In 2022, it was estimated that approximately 19.9 million new cancer cases were diagnosed worldwide, and this number is projected to rise to 28.0 million over the next two decades (1). Breast cancer (BC) specifically exhibits the highest incidence globally, with around 2.3 million new cases reported in 2022 (1). According to the Spanish Society of Medical Oncology (SEOM), 37,682 new BC cases are expected to be diagnosed in Spain in 2025 (1).

Currently, BC subtypes are classified based on their molecular characteristics (2,3). The triple-negative (TN) molecular subtype is defined by the absence of estrogen receptors (ER), progesterone receptors (PR), and human epidermal growth factor receptor 2 (HER2). TNBC accounts for 10-20% of invasive BC cases and is considered more biologically and clinically aggressive than other subtypes (2,3). It also presents a poorer prognosis and more limited treatment options (2,3).

Metabolic flexibility (MF) refers to the body's capacity to adapt to energy demands under varying conditions (4,5). Mitochondria, as the primary cellular organelles responsible for energy production, facilitate substrate oxidation to generate ATP according to required intensity levels (5). In contrast, metabolic inflexibility in muscle fibers is characterized by impaired lactate clearance, reduced lipid oxidation capacity, and rapid switching from fat to carbohydrate (CHO) oxidation (6).

Some studies suggest that cancer induces systemic mitochondrial dysfunction across multiple tissues, influenced both by disease pathophysiology and the toxicity of oncological treatments (7). Additionally, decreased PGC-1α levels have been observed in patients receiving neoadjuvant chemotherapy (NAC). PGC-1α is a key transcriptional coactivator regulating mitochondrial biogenesis, and its reduction may contribute to inefficient energy production, resulting in muscle dysfunction and loss of both mass and function in cancer patients (7,8).

Moreover, women with BC who undergo chemotherapy are more likely to gain fat mass compared to age-matched women without BC (7,9,10,11). Excess adipose tissue is linked to metabolic disease and elevated pro-inflammatory cytokines, further contributing to mitochondrial and metabolic dysfunction. Increased fat mass has also been associated with higher risks of recurrence, disease progression, and mortality in BC studies (7,9,10,11).

Evidence highlights the crucial role physical exercise plays in cellular metabolism. Studies demonstrate that exercise significantly influences glucose levels, insulin resistance, growth factors, fat oxidation rates, and lactate clearance (12,13). Dysregulation of these factors may activate tumor signaling pathways, posing a risk for tumor proliferation (12,13,14). Consequently, modulating metabolism through physical activity could be fundamental in influencing cancer progression (12,13,14).

Exercise is key for activating the AMPK signaling pathway, an enzyme triggered under high-energy demands. Activation of AMPK promotes GLUT4 translocation, enhancing glycolysis, fatty acid (FA) oxidation, and mitochondrial biogenesis, including upregulation of PGC-1α (15,16). Continuous endurance training has been shown to sustain AMPK activation for hours post-exercise, with longer durations producing greater benefits (15,17). Similarly, high-intensity cardiovascular training appears to elevate AMPK levels hours after the activity (15,17).

Alongside increased AMPK and PGC-1α activity, training has been associated with greater mitochondrial content and function in muscle fibers (4,6,16,18). This enhances fatty acid oxidation capacity in mitochondria, thereby improving metabolic flexibility (4,6,18). Altogether, these adaptations increase the ability of muscle fibers-now with more mitochondria-to utilize multiple substrates efficiently, improving MF (4,6,18,19).

Recent research also indicates that increasing muscle strength and preventing fat mass gain are essential for maintaining metabolic health and optimizing treatment response, as they are associated with improvements in markers such as insulin sensitivity, glycemic control, and acute anti-inflammatory responses (20,21). Importantly, exercise-induced benefits on mitochondrial and metabolic health appear independent of weight loss from fat reduction (20,21).

In summary, cancer patients-particularly those with TNBC-may experience systemic metabolic dysfunction due to disease pathophysiology, treatment toxicity, and suboptimal lifestyle habits (4,5,7,8,10,14,18,22,23,24,25,26). However, to date, no studies have described the metabolic response to exercise in this population.

Thus, the primary aim of this study is to describe the metabolic flexibility of patients with early-stage TNBC across different phases of the disease. This approach may allow indirect determination of the preferred energy substrate in muscle fibers and identification of the optimal intensity for fatty acid oxidation to improve metabolic profiles in these patients.

Additionally, the study will evaluate the effects of two cardiovascular training interventions and one strength training intervention on the metabolic profile of patients with early-stage TNBC.

This pilot study will initially adopt a descriptive, longitudinal design, followed by an open, randomized experimental phase in early-stage TNBC patients. First, a descriptive observational analysis will be conducted, followed by an experimental study with four groups using a pre-post design, including a control group (CG).

A group consisting exclusively of newly diagnosed TNBC patients (D1) meeting inclusion criteria will be established. This group will receive only general physical activity recommendations provided by the World Health Organization (WHO) during neoadjuvant treatment and prior to breast surgery.

The sample will include 40 TNBC patients selected by convenience sampling from Hospital Universitario Severo Ochoa (Av. de Orellana, s/n, 28914 Leganés, Madrid), Hospital Universitario Infanta Leonor (Av. Gran Vía del Este, 80, Vallecas, 28031 Madrid), and Hospital de la Princesa (Calle de Diego de León, 62, Salamanca, 28006 Madrid), all located in the Community of Madrid.

To determine the most effective exercise intervention for enhancing metabolic flexibility and function, the study will consider: the group with the highest number of patients demonstrating decreased RER and lactate during the exercise test (ET) at the end of each incremental protocol stage post-surgery compared with pre-intervention; the group with the most patients showing increased FATox and CHOox, as well as associated kcal values, at the same stage-defined ET points post-surgery; and the experimental group exhibiting statistically significant changes in these variables.

Inclusion criteria:

• Female between 20-55 years of age.
• Confirmed histological new diagnosis of stage I to III triple-negative breast cancer and a candidate to receive systemic neoadjuvant treatment with chemotherapy +/- immunotherapy.
• Must not have started systemic treatment for the neoplastic disease.
• Participating in any external physical exercise program or sports activities during the experimental study phase.
• Inability to understand and provide written Informed Consent (IC)

Exclusion Criteria:

• Having neurological or orthopedic disease at the time of recruitment or during the study evaluation and intervention.
• Inability to understand Spanish language.
• Presenting absolute contraindications for performing a cardiopulmonary exercise test (CPET) such as heart failure, myocarditis, acute pericarditis, severe aortic stenosis, aortic dissection, vascular toxicity, uncontrolled severe arterial hypertension, uncontrolled severe cardiac arrhythmias, pulmonary thromboembolism, or severe anemia.
• Have relative contraindications for performing a CPET such as bradyarrhythmias or tachyarrhythmias, moderate valvular stenosis, inability to perform physical or mental exertion, chronic infectious diseases, musculoskeletal disabilities, ventricular aneurysm, second- or third-degree atrioventricular block, or severe arterial hypertension.