In this study, we showed that a high percentage of predicted body fat mass was associated with increased risk of CVDs among 5-year breast cancer survivors. Furthermore, a high percentage of predicted lean body mass and predicted appendicular skeletal mass was associated with reduced risk of CVDs. In all three components of body composition, blood pressure showed to be the most important mediator for the association. To our knowledge, this is the first study, in this scale, to show comprehensively that low percentage of body fat mass and high percentage of lean body mass and appendicular skeletal mass are associated with decreased risk of CVDs among breast cancer survivors.
The results from previous studies are in accordance with increased risk of CVDs associated with a high percentage of predicted body fat mass observed in our study. For example, in a study investigating the association between adipose tissue distribution with CVDs risk, each standard deviation increase in visceral or intramuscular adiposity was associated with an increase in CVDs risk (aHR, 1.15 [95% CI, 1.03 to 1.29] and aHR, 1.21 [95% CI, 1.06 to 1.37]), respectively) [8]. Similarly, in our study, each 1% increase in pBFMP was associated with an increased risk of CVDs (aHR: 1.05, 95% CI 1.03–1.07, p < 0.01). Additionally, a relatively recent report observed the association of body mass index, central obesity, and body composition with mortality in black breast cancer survivors [25]. High body mass index, central obesity (waist-to-hip ratio and waist circumference), and body composition (percent body fat and fat mass index) were associated with higher overall mortality (1.57 [95% CI, 1.11–2.22], 1.74 [95% CI, 1.26–2.41], and 1.53 [95% CI, 1.09–2.15], respectively) and breast cancer-related mortality. However, in this study, association with CVDs was not investigated and there were potential confounding variables, because the number of subjects was limited to 1,891, and data on underlying heart disease were not collected.
Breast cancer patients experience cardiotoxic agents, and it has been suggested that CVD risk may also be related to anti-estrogen therapy such as aromatase inhibitors [26]. Breast cancer survivors tend to also have other risk factors for CVDs. Increased body fat mass may lead to metabolic disturbances related to increased adiposity including insulin resistance, dyslipidemia, and chronic inflammation [27]. Also, visceral fat has been suggested to alter cardiac autonomic activity in breast cancer survivors [28]. Specifically, diminished parasympathetic activity and heart rate variability have been associated with loss of cholinergic anti-inflammatory pathway, enabling enhanced cytokine responses to the otherwise normal stimuli [29]. Finally, increased intramuscular fat was associated with reduced peak exercise capacity in cancer survivors [30] [31]. This, in addition to breast cancer-related skeletal muscle damage, may have reduced exercise-based opportunities for CVD risk reduction.
Reduced muscle mass, which is prevalent in breast cancer survivors, is a risk factor for mortality in early breast cancer patients [32, 33]. A UK study examined appendicular skeletal muscle mass to have a curvilinear association with CVD events in women [34]. In our study of breast cancer survivors, both lean body mass and appendicular skeletal mass were associated with reduced risk of CVDs. Though not significant, this was also evident in the lower risk of CVDs observed in those with higher lean body mass (High BFM-High LBM) compared to those with lower lean body mass (High BFM-Low LBM) among those with high body fat mass. In breast cancer survivors who had received radiation therapy, there was a significantly higher CVD risk was associated with higher pBFMP and lower risk in higher pLBMP and pASMP. The risk of CVD may have been more prominent in this subgroup due to the significant changes seen in body composition post-radiation [35].
Blood pressure, among the three confounders that were adjusted for in combinations, was observed to be the most important mediator for the association of pLBMP, pASMP, and pBFMP with CVD risk. Though further research is warranted to elucidate the mechanism in which body composition and metabolic changes lead to CVD, studies have already examined a close link between body composition and blood pressure. Both lean body mass, the majority constituted by muscle mass, and fat mass, at a lesser degree, were significant determinants of blood pressure level; relatively high muscle mass was associated with high blood pressure levels in this report [36]. In a study on middle-aged adults, higher visceral adiposity was associated with higher blood pressure level with lower variability, independent of BMI, and this persistently elevated blood pressure may impose cardiac burden [37].
Our study is not without limitations. First, body composition based on the prediction equation is an imperfect measurement. However, a previous large validation study based on Korean National Health and Nutrition Examination Survey showed high predictive values, including low bias, high intraclass correlation coefficient, high adjusted R2, and low SEE, allowing it to apply to large-scale research and epidemiological studies. Second, body composition was observed at one time point. Third, the results of our studies were limited to Asians and may not represent other ethnicities. Body composition, metabolic syndrome, and the nature of cancer varies among ethnicities. A major strength of our study was the large sample size and inclusion of other indices (lean body mass and appendicular skeletal mass) in addition to body fat mass. Body composition was investigated without the use of imaging modalities and presented in terms of percentile. This allows for a cost-effective means, without radiation exposure, to help identify and modify body composition in breast cancer survivors, including those who are normal weight or have few comorbidities.
In conclusion, in this cohort study, a high percentage of predicted lean body mass and predicted appendicular skeletal mass and a low percentage of predicted body fat mass were associated with reduced risk of CVDs. Blood pressure showed to be the most important mediator for the association.