Healthy dietary habits are of paramount importance for physical and mental health. The identification of properties of foods or dietary patterns related to positive health outcomes helps policymakers to develop strategies for the promotion of healthy diets.

The energy density of a diet, defined as the amount of available dietary energy per unit of weight consumed, seems to play a role in regulating food intake¹. Lower energy density is associated with lower energy consumption: individuals consuming a greater amount of energy are more likely to meet their nutrient needs. However, although restricting energy intake could, theoretically, lead to nutrient imbalance, several recent reports have suggested that low energy density diets are associated with higher diet quality^{2, 3} and favourable health outcomes such as prevention of obesity and diabetes in the general population^{4, 5}.

Association of energy density with diet quality and other lifestyle variables

We analyzed dietary data, obtained through a food frequency questionnaire, and other lifestyle variables from a random sample of the 25- to 74-year-old population of Gerona, Spain⁶. Low energy density diets were associated with a healthier lifestyle: participants who adhered to a low energy density diet spent more time in leisure physical activity, drank less alcohol, and were more likely to be nonsmokers than those following a high energy density diet.

In recent years, a holistic approach based on food consumption has been developed to quantify overall diet quality, including the creation of composite indices of dietary behaviours⁷. These indices combine a large amount of information about dietary behaviours into a single indicator of diet quality. Although based on different concepts, the Mediterranean Diet Score and the Healthy Eating Index are measurement tools that rank adherence to a healthy diet. Most importantly, high adherence to these diet quality indices has been associated with lower risks of cardiovascular heart disease, obesity, diabetes type 2, hypertension, and mortality^{8, 9}. In our study, adherence to low energy density diets showed a high correlation to both diet quality indices in men and women. In contrast, those reporting high-energy density diets exhibited poor diet quality. Furthermore, compliance with dietary recommendations proposed by the Spanish Society for Community Nutrition was higher among participants following a low energy density diet as compared with their peers following a high energy density diet. Hence, a high consumption of low energy density foods can be considered an important factor for diet quality. An interesting model for a healthy diet would be rich in low energy density foods, contain modest amounts of foods of moderate energy density such as fish and pulses, and include only a low to moderate amount of vegetable-based high energy density foods.

However, food choice is influenced by a large number of factors, including taste, cost, convenience and, to a lesser extent, health and variety10. It has been shown that economic constraints lead to the consumption of less healthy diets characterized by high energy density and palatability11. The consumption of vegetables and fruit accounted for more than one third of the total costs of the average diet in our study population12. Furthermore, low energy density diets were directly associated with monetary costs. In an effort to identify and promote healthy diets, monetary costs should be considered. Otherwise, promotion of this new model will run the risk of being in vain, particularly in low income groups.

Diet quality and energy density in the elderly

Due to the amorphous nature of energy density, freely selected low energy density diets might differ considerably in food and nutrient composition across populations. Because ingestion of most nutrients increases with increasing energy consumption, and energy density increases with energy intake, low energy density diets are, theoretically, more prone to nutrient inadequacy than are high energy density diets. Aging has been associated with altered sensations of thirst, hunger, and satiety, and a loss of appetite is generally observed in the elderly. Data from a study performed in free living elderly men and women indicate that following a low energy density diet had a generally favourable micronutrient intake pattern13. Furthermore, diet adequacy, measured as compliance with RDA for 19 nutrients, was remarkably higher among elderly men and women on low energy density diets as compared with their peers following a high energy density diet.