We evaluated 5 female non-professional Caucasian ultra-swimmers during a 12-hour swim to see whether a change in body mass, percent body fat, skeletal muscle mass, percent total body water and specific gravity of urine occurred. Urinary proton magnetic resonance spectroscopy (1H-NMR) of pre and post race urine samples was performed in order to detect alanine, lactate and 3-hydroxybutyrate metabolic leftovers. The 5 swimmers achieved an average distance of 23.8 (SD = 5.0) km. From the parameters of body composition, body mass decreased by 1.26 kg (− 1.8 %), fat mass decreased by 0.66 kg (− 2.8 %), skeletal muscle mass decreased by 0.57 kg (− 2 %) and total body water decreased by 2.7 l (− 6.8 %). Urinary specific gravity remained stable at 1.007. In the post race 1H-NMR-urinalysis we found no increase in ketone bodies. We presume that these female ultra-swimmers covered their energy demand with a consumption of energy rich substrates stored in their skeletal muscles and in their body fat. However, we could not detect an increase in ketone bodies in their post race urinary samples as a sign of increased lipolysis of adipose subcutaneous tissue.