The cost of dietary energy has greatly increased in the last few years and this trend of historically higher prices is expected to remain in the future. This has led to shifts in diet formulation and a need to reevaluate pig responses to changing dietary energy density. As part of the evaluation, nutritionists should review whether energy loadings for their ingredients are accurate. Is the relative change in dietary energy reflected accurately by changes in feed efficiency? If there is not a 1:1 relationship between dietary energy and feed efficiency, dietary energy is not being accurately represented. Because of the linear relationship between diet energy and feed efficiency, that portion of the economic equation is easy to measure. However, the impact of dietary energy on growth performance is more difficult to predict. Does daily gain increase linearly over the entire range of possible energy levels or, if not, when does it plateau? This relationship must be known or estimated to determine the value of changes in energy density. Furthermore, if alternative ingredients with high fiber content are used, at what point does fiber content prevent pigs from increasing feed intake to maintain energy intake? Another key practical concept with energy is how fast the pig adjusts feed intake to changes in diet energy density. Over the long term, pigs adjust their level of consumption to the energy density of the diet; however, it takes time for pigs to make this adjustment. Thus, in the short term, pigs consume the same amount of total feed resulting in increased energy intake and gain if dietary energy is increased or reduced energy intake if diet energy is reduced. When changing dietary energy, the lysine:calorie ratio and phosphorus:calorie ratio should be adjusted to maintain proper amino acid and phosphorus intake to support optimum growth. The high cost of energy has increased the search for other methods to increase pig performance. The ratios of several key amino acids to Lys should be reviewed to take advantage of their impact on feed intake (ex. Trp, Val, etc). From a practical standpoint, the rise in energy costs has led to the removal of added fat from most pig diets unless essential to maintain pellet quality. Alternative ingredients are being used wherever economical.