Guanidinoacetic acid (GAA) is a fundamental intermediate in cellular bioenergetics, with circulating levels of GAA often reflects disturbances in its conversion due to many intrinsic and extrinsic factors, including gender or age. Here, we evaluated serum GAA in 172 healthy women aged 18 to 65 years, with age found to significantly predict serum GAA concentrations (r=0.29; P=0.03). This perhaps nominates serum GAA as a novel gender-specific proxy of impaired bioenergetics with aging.