Nutrition models of energy and protein utilisation have been developed for lactating pigs but there is little or no evidence in the scientific literature of the validation of such models. If there are systematic discrepancies in such models, then inappropriate supply of nutrients will either be an inefficient use of resources with oversupply or have an impact on piglet growth and subsequent reproductive performance of the sow with undersupply. In the current study, lactating gilts were fed isoenergetic diets differing in ileal digestible lysine: energy. Protein and energy utilisation were predicted using existing nutrition equations. The validity of the energy and protein utilisation equations was tested by determining if the predicted energy balance differed significantly from zero or if the predicted protein balance was not significantly less than zero.