Atrial natriuretic peptide (ANP) is known for its natriuretic, diuretic, and vasodilatory properties. However, ANP also has metabolic effects stimulating lipolysis and lipid oxidation. Overweight individuals have decreased circulating ANP concentrations. It has been proposed that this potential ANP deficiency could have biological consequences in overweight-related disorders, including decreased lipolysis and lipid oxidation. The purpose of this study was to investigate the relationships between ANP, exercise-induced lipid oxidation, and cardiorespiratory fitness in 562 20-28-year-old healthy community-based women and men. We measured fasting plasma concentrations of mid-regional proANP (MR-proANP), a stable marker of ANP secretion, the respiratory exchange ratio (RER) during sub-maximal exercise, which provides an estimate of lipid oxidation, and maximal oxygen consumption (VO2-max) at the end of a maximal exercise test, which is a measure of cardiorespiratory fitness. An increase of 10 pmol/L in fasting plasma MR-proANP concentrations was related to an increase in relative VO2-max of 0.78 (95% CI 0.36-1.09) ml O-2/min/kg and a decrease in RER of -0.0094 (-0.014 to -0.0045) in age- and sex-adjusted analysis (P<0.001). Further adjusted for body mass index, a rise of 10 pmol/L in fasting plasma MR-proANP concentrations was associated with a rise in relative VO2-max of 0.60 (0.28-0.92) ml O-2/min/kg and a fall in RER of -0.0096 (-0.015 to -0.0048) (P<0.001). Fasting plasma MR-proANP concentrations associate with lipid oxidation during exercise and cardiorespiratory fitness in healthy young adults. The data support the existence of important connections between the endocrine heart, hemodynamics, and metabolism