Energy density

Energy density tells you how many calories there are in one gram of a food – and it explains why some things fill you up and others don't. Cucumber has about 0.1 kcal per gram, apple 0.5, cooked pasta 1.5, bread 2.5, cheese 3.5, chocolate 5.5, nuts 6 and oil 9. The difference comes mainly from the water and fat content: lots of water and fibre push the energy density down, lots of fat and sugar drive it up.

Nutrition researcher Barbara Rolls roughly divides foods into four groups: very low (under 0.6 kcal/g: vegetables, fruit, soups), low (0.6 to 1.5: potatoes, pulses, lean meat, yoghurt), medium (1.5 to 4: bread, cheese, pizza) and high (4 to 9: crisps, biscuits, chocolate, nuts, oil).

The trick: your stomach responds to weight and volume, not to calories. People eat a similar number of grams from day to day – if the energy density is lower, they automatically take in fewer calories without being any hungrier. Studies show that this approach alone leads to significant weight loss over several months.

In practice, this means “bulking out” meals with vegetables, salad, soup or fruit, choosing tomato-based rather than cream-based sauces, roasted vegetables instead of chips, and a soup or raw veg as a starter before the main course. Energy-dense foods aren't forbidden – nuts and olive oil are actually very healthy – but they belong on your plate in small, mindful amounts.

With children, use common sense: for growing, they also need energy-dense foods such as nut butter, whole milk or avocado. The approach suits adults who want to lose weight – for the family, it simply means more vegetables for everyone.