The WORLD study is a single-center, year-long randomized controlled trial in free-living women. Participants were randomly assigned to follow either a lower-fat (LF) diet or a moderate-fat (MF) diet for weight management in a parallel-arm design. The two phases of the study were a weight-loss phase (phase 1) and a weight-maintenance phase (phase 2) (Figure 1). During phase 1, months 1 through 4, participants consumed a hypo-caloric diet consistent with the 2005 Dietary Guidelines in the free-living environment. During phase 2, months 5 through 12, participants shifted into weight maintenance. It was hypothesized that a weight-loss intervention at the extremes of dietary fat recommendations of the 2005 Dietary Guidelines would be equally effective for weight loss while achieving comparable nutrient adequacy. Also, Overall, the lower-fat and moderate-fat diets would both be nutritionally adequate, based on the Healthy Eating Index.