Our body is constantly burning energy. Even when we’re sat on the sofa or sleeping, we carry on burning energy. Therefore, we need to consume energy, and we do this thanks to the food we eat. We are able to extract energy from some of the nutrients that make up our food (carbohydrates, fats and proteins). The amount of energy we get from these nutrients is about 4 kcal per gram for protein and carbohydrates, and 9 kcal per gram for lipids. To keep our body weight constant it is necessary to consume food with a quantity of energy equivalent to the energy we spend. This is known as a “normocaloric” diet, meaning the energy introduced is equal to the energy expended. The amount of energy recommended to be consumed in a normocaloric diet is not the same for everyone; it depends on how much energy each person burns. Energy expenditure is largely linked to resting (or basal) metabolism, which constitutes about 65% of total expenditure and depends on various factors, including mainly gender (it is greater in men than women), age (it is proportionally greater in children than in adults and the elderly) and body surface area.

To determine total energy expenditure, add your resting metabolic rate to the expenditure from physical activity, which does not only mean sports activities but any other activity (walking, doing housework, etc.). The average energy requirement for a 1.80 m tall adult weighing 73 kg is around 2500 kcal if the level of physical activity is low, but can exceed 3500 kcal for high levels of physical activity (Reference Intake Levels for Nutrients and Energy*). Equally, for a 1.70 m tall woman, weighing 65 kg, the average energy requirement can vary from around 2000 kcal to nearly 3000 kcal depending on how much physical activity she performs (Reference Intake Levels for Nutrients and Energy*).

If our diet is normocaloric, our weight will not change. But if we consume more energy via our food than we burn, it will be stored as fat. And so our weight will increase. This happens if our diet contains too many calories, regardless of whether they are introduced as fats, carbohydrates or proteins. It’s the total amount of energy that counts!
On the contrary, if I consume less energy than what I need, my body will draw on fat reserves, and I will lose weight. But watch out. If I am on a “zero calorie” diet, ie I am fasting, or if I am consuming very little energy, before drawing from adipose tissue the body will use proteins as an energy source. So I will lose weight, but not so much fat loss as muscle loss! Never fast or skip meals!

In nutrition, energy is measured in kilocalories (kcal) or in kilojoules (kj). These two units do not represent the same amount of energy, but 1 kcal is equivalent to a little over 4 kj. Be careful, therefore, when reading the nutritional labels: it is not enough to look at the number, we must also consider the unit of measurement.

Alessandra Bordoni
Department of Agricultural and Food Sciences
University of Bologna

*Italian Society of Human Nutrition (SINU): LARN – Reference Intake Levels for Nutrients and Energy for the Italian population – 4th edition. SICS ed.