The assessment of changes in body protein content over time allows us to estimate amino acid requirements much more precisely. Assessment of energy requirements similarly will be based on carcass lipid and muscle content. The body is not composed solely of protein and lipid, but also includes the gain of water and ash, as well as gut fill. The objective of this study was to determine the changing proportions of body weight composition as the pigs grew from 24 to 120 kg. A series of mathematical models were developed which allowed us to characterize the deposition of protein, lipid, water, ash and gut fill. The portion of body lipid (fat) almost tripled, increasing from 10% at 24 kg to 28% at 120 kg. Over the same weight range, protein was much more constant, 17 % at 24 kg and 15% at 12 kg. Increased information on body composition will form the basis for diet formulation in the future. Basic knowledge of how modern commercial genotypes grow will be essential, as diet formulation becomes more precies, allowing for maximum carcass quality at the lowest possible cost.