Energy density values, which vary from 0 to 9 kcal/g, are influenced by the water content and macronutrient composition of foods. This is illustrated using 1-g scale weights in which each dot on the scales represents 1 kcal. 

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... prevent gradual weight gain over time, the 2005 Dietary Guidelines for Americans recommend small decreases in energy from foods and beverages and increases in physical activity (1) . For individuals who need to lose weight, the guidelines encourage a slow, steady weight loss by decreasing energy intake while maintaining an adequate nutrient intake and increasing physical activity. Strategies individuals have typically used to reduce energy intake include limiting portion sizes, food groups, or certain macronutrients. Clinical trials have found that restrictive approaches such as low-fat or low-carbo- hydrate regimens, because of decreased caloric intake, have led to weight loss in the short term (6 mo or less) (2,3) . Restrictive approaches may, however, result in feelings of hunger or dissatisfaction, which can limit their acceptability, sustainability, and long- term effectiveness (4–6) . An alternative positive strategy to manage energy intake is for individuals to eat more foods that are low in calories for a given measure of food—that is, they are low in energy density (kcal/g). Encouraging individuals to eat these types of foods is one of the dietary strategies recommended in the Dietary Guidelines for Americans to manage energy intake. In the following sections, energy density as a dietary strategy for management of weight is discussed. Energy density is the amount of energy in a particular weight of food. It is generally presented as the number of calories in a gram (kcal/g). Foods with a low energy density provide less energy relative to their weight than foods with a high energy density. Therefore, for the same amount of energy, a larger, more satisfying portion size of food can be consumed of a food low in energy density, compared with a food high in energy density. Energy density values, which are influenced by the moisture content and macronutrient composition of foods, range from 0 kcal/g to 9 kcal/g (Fig. 1). The component of food with the greatest impact on energy density is water (7) . Water has an energy density of 0 kcal/g, as it contributes weight but not energy to foods. Fiber also has a relatively low energy density, providing 1.5 to 2.5 kcal/g, and can lower the energy density of foods. On the opposite end of the energy density spectrum, fat is the most energy-dense component of food. Fat provides 9 kcal/g, more than twice as much energy as carbohydrates or protein, which provide 4 kcal/g. Although most high-fat foods have a high energy density, increasing the water content lowers the energy density of all foods, even those high in fat. The energy density of a food can be calculated easily by using information that is readily available on the Nutrition Facts Panel of food labels. In order to better understand which foods are low or high in energy density, Table 1 classifies foods into four catego- ries. Water-rich foods, such as nonstarchy fruits and vegetables and broth-based soups, are very low in energy density (<0.6 kcal/g) (8,9) and should constitute a large proportion of each meal, be eaten as snacks, and be chosen as appetizers. In addition to foods with a very low energy density, low-energy-dense foods (0.6 to 1.5 kcal/g) such as starchy fruits and vegetables, cooked grains, legumes, lean meat and fish, and low-fat mixed dishes should accompany and/or be incorporated with very-low-energy-dense foods as the primary focus of meals. Foods with a medium (1.5 to 4.0 kcal/g) and high (4.0 to 9.0 kcal/g) energy density should be consumed less frequently and attention should be given to limiting their portion size. By consuming a diet low in energy density, caloric intake can be reduced without strictly limiting food portions. Figure 2 depicts the total amount of food that can be consumed on a 1600-kcal diet depending on the overall energy density of the diet. The energy density values in this figure correspond to a low- (1.4 kcal/g), medium- (1.9 kcal/ g), or high-energy-dense (2.2 kcal/g) diet, as defined by the average dietary energy density of a representative group of US adults (10,11) . When consuming a diet with an energy density of 1.4 kcal/g, which would be rich in low-energy-dense foods, more than 1100 g of foods can be consumed for 1600 kcal. However, only 725 g can be consumed on the high-energy-dense, 2.2 kcal/g diet. At any energy level, the lower the energy density of the diet, the greater the amount of food that can be consumed. This review will focus on several types of scientific evidence, including population- based studies, laboratory-based studies, and clinical trials, showing the influence ...