Catabolism and inflammation play a role in the physiopathology of heart failure with reduced ejection fraction and are more pronounced in the advanced stages of the disease. Our aim was to demonstrate that in patients with stable heart failure with reduced ejection fraction adequately treated, a direct relation exists between functional impairment, as evaluated by left ventricular ejection fraction (LVEF) and the 6-minute walking distance (6MWD), and catabolic and inflammatory markers. In 151 outpatients with heart failure and a LVEF of ≤40% (median age 64 years, LVEF 29%, and 6MWD 290 m) we measured the laboratory and body composition parameters that indicate directly or indirectly inflammatory activation, anabolic-catabolic balance, and nutritional status. We performed an analysis stratified by quartiles of LVEF and 6MWD and linear regression analysis to explore our hypothesis. In the linear regression analysis, after adjusting for age, gender, and etiology, LVEF was not related to the metabolic, inflammatory, or nutritional parameters. The 6MWD was directly related to albumin (p = 0.002) and log transformation of dehydroepiandrosterone (p = 0.013) and inversely to adiponectin (p = 0.001) and the log-transformation of high-sensitivity C-reactive protein (p = 0.037). In conclusion, in a population with stable heart failure with reduced ejection fraction, the 6MWD was related to the degree of inflammatory activity and catabolism, but LVEF was not. Even a slightly diminished functional capacity implies underlying inflammation and catabolic activation.