Energy density

Energy density is the number of calories per gram of food. A low-density food (cucumber ~0.15 kcal/g) provides few calories for a lot of volume; a very dense food (oil ~9 kcal/g, chocolate ~5.5) packs a lot of calories into little volume.

The original angle (volume fills you up, not calories): a key finding from the work of Barbara Rolls (volumetrics): the stomach fills and signals fullness mainly according to the volume and weight of food, not its calorie content.

In other words, for the same calories, a large volume of low-density food fills you up far more than a small volume of dense food. Eating 500 g of vegetables + lean protein (~400 kcal) keeps you full for longer; 80 g of crisps (~400 kcal) leaves you hungry.

Water and fibre, volume allies: foods rich in water (fruit, vegetables, soups) and fibre have a low energy density: they “trick” the stomach with volume at a low calorie cost. A soup as a starter reduces the total calorie intake of the meal.

Application to weight management: favouring foods with a low energy density lets you eat until you are full (filling volume) while controlling calories, without the frustration of small portions. It is the principle behind many volumetric diets.

A caveat: dense foods are not “bad”. Nuts, avocado and olive oil are dense AND healthy, but need to be portioned. Energy density is a tool for fullness, not a judgement of nutritional value (to be weighed against nutrient density).