The cost of feed has been rising and while research has focused on amino acid requirements and cost, research on the cost of energy itself has been behind. Energy can come from starch, fat, protein or fibre, but the efficiency varies depending on which source is used to obtain the energy. Energy is used by the pig for either maintenance or growth. Maintenance energy requirement calculations look at Fasting Heat Production, but it is important to remember additional costs like temperature regulation, immune challenges, and social stressors. Growth can be achieved through protein or lipid deposition and, while their efficiencies are similar, lean accretion is much more efficient than fat accretion due to the water requirement. Still, lean pigs are not necessarily the most cost effective if they are gaining weight slowly, as they will require extra barn time and maintenance energy. There are variations between herds in ability to maintain growth rate with lower energy density foods. Generally, herds that are able to increase feed intake to maintain energy intake will maintain growth, and will be desirable continuing into the future.