The results of this study revealed an association between muscle strength and physical performance with the HRQoL domains of women with breast cancer. Muscle strength was associated with cognitive function and loss of appetite, while physical performance was associated with the domains of global health status and functional scale, which were assessed by the EORTC QLQ-C30. Likewise, muscle strength and physical performance were associated with the scale of specific symptoms of breast cancer, when the EORTC QLQ-BR23 was applied.
In this study, increased muscle strength was associated with better cognitive function. A possible mechanism behind this association involves the serum level of brain-derived neurotrophic factor (BDNF). When the level of BDNF was reduced, cognitive impairment was identified in cancer patients receiving chemotherapy [37]. BDNF is a neurotrophin widely expressed in the brain, particularly in the prefrontal cortex and hippocampus, and is secreted in response to muscle contraction [37, 38]. Therefore, muscle strength, which measures the effort of muscle contraction, is positively related to serum BDNF levels and, consequently, to cognitive impairment [39].
Yang et al. [39], when investigating the relationship between HGS and cognitive function in elderly women who survived breast cancer, observed that higher HGS was associated with better performance in cognitive tests, which corroborates our own study. HGS was also associated with loss of appetite. However, few studies in the literature have directly investigated the relationship between muscle strength and loss of appetite. A systematic review analyzing the correlations between muscle strength and HRQoL domains showed that HGS was correlated with the EORTC-QLQC-30 domains, including loss of appetite [13]. Another study carried out in women with breast cancer on neoadjuvant chemotherapy, which evaluated the effect of a nutritional intervention on HRQoL, concluded that the intervention not only preserved HGS but also promoted a reduction in loss of appetite [2]. Loss of appetite is an important symptom of nutritional impact and should be monitored, since its occurrence is closely related to malnutrition in cancer patients [40].
As for physical performance, measured by TUGT, our results showed a relationship with global health status and the functional scale of HRQoL, assessed by the EORTC QLQ-C30. Defined as “a whole-body function objectively related to mobility” [41], physical performance goes beyond measures of muscle function; it is a a multidimensional concept [7, 41] that involves many other organs and body systems (bones, balance, neurological information, cardiovascular aspects, motivation). Decreased physical skills and functional decline can lead to higher levels of dependence and disability. These adverse conditions, which can be caused by musculoskeletal conditions such as sarcopenia, are unfavorable and undesirable, having a direct impact on HRQoL [12].
When considering the EORTC-QLQ-BR23, we observed that the scale of specific symptoms of breast cancer was inversely associated with HGS and linearly so with TUGT, indicating that in the worsening of symptoms there was less muscle strength and worse physical performance. The EORTC-QLQ-BR23 symptom scale considers side effects of systemic therapy, breast and arm symptoms, and concerns about hair loss. These symptoms are well correlated with the stages of antineoplastic treatments. Therapies have adverse effects, including a negative effect on muscle function, which justifies the association identified in the present study of specific symptoms resulting from the disease and its treatments with muscle strength and physical performance [13].
In a prospective cohort study, Binotto et al. [42] investigated the main changes in the HRQoL of women with breast cancer, who were assessed one week before the start of chemotherapy and during the third month of treatment. A worsening was observed in different domains of HRQoL including specific symptoms of breast cancer, namely, body image, sexual function, and sexual pleasure. The authors also identified an increase in side effects related to systemic therapy, despite a decrease in breast and arm symptoms, which highlights the various changes that occur during chemotherapy in this population.
Another important component of sarcopenia is muscle mass. Although low skeletal muscle mass is known to be associated with bad outcomes related to cancer morbidity and mortality [43], evidence of its impact on HRQoL is less established. In this study, the amount of muscle mass, measured by the ASMMI, was the only component of sarcopenia that was not associated with any HRQoL domain. In fact, this relationship is even less described in women with breast cancer. Our hypothesis is that there is a relationship between the amount of muscle mass and HRQoL, so that low muscle mass negatively impacts the HRQoL of women with breast cancer, mainly by leading to reduced or interrupted response to treatment, hospitalizations, and low functionality [5, 12]. However, the different cutoff points used to classify low muscle mass in the literature limits the interpretation of this relationship, meaning that the prevalence of low muscle mass in cancer patients varies depending on the type of muscle measurement considered [44, 45]. Therefore, there is a need for prospective and longitudinal studies analyzing the interaction between skeletal muscle mass and HRQoL, in particular regarding the causal pathways and the interrelationship between body composition, specifically muscle mass, and HRQoL in the context of treatment for breast cancer.
As a contribution, this study demonstrated a relationship between sarcopenia components and HRQoL domains, which should aid clinical practice in the development of care strategies for patients with probable sarcopenia or in other degrees, a scarcely investigated subject. On the other hand, this study has limitations. The first one refers to its cross-sectional design, which restricts the ability to infer causality and only allows the identification of associations. The second limitation refers to HRQoL, a subjective and complex variable to be estimated. Therefore, we decided to apply the EORTC QLQ-C30, a validated instrument widely used in the literature to assess HRQoL and the EORTC BR23, which is specific for breast cancer. Finally, the third limitation is related to the cutoff points used to classify HGS and TUGT, which are not specific for cancer patients.