The word calorie comes from the French word chaleur (heat), and it is a unit of energy. One calorie of heat energy is required to raise the temperature of 1 gram of water by 1 degree Celsius. This makes it a simple and logical unit of energy. It was introduced in 1824 by the French scientist Nicolas Clément and was used worldwide as the official unit of energy until 1929, when it was replaced by the joule. Today, the energy content of most foods is expressed in joules, but in nutrition, the calorie is still stubbornly in use. The conversion between the two is 4.2. So, if we eat food containing 1000 calories, that equals 4200 joules, or 4.2 kilojoules.

What is the optimal amount?

People’s energy needs are not the same. They depend on body weight, lifestyle, and also age. Older people require less energy because their bodies are no longer engaged in as many building processes, whereas children need much more energy in relation to their body weight. For example, the daily energy requirement of a petite, slim woman with a sedentary job is around 1600 kcal, while that of a tall, 80-kg man with an active lifestyle is exactly double. The average need usually falls between 2000 and 2500 kcal.

Our bodies derive energy from burning, or oxidizing, nutrients. After several transformations, this energy ultimately leaves the body as heat. The three main sources of energy-providing nutrients are fats, carbohydrates, and proteins. Carbohydrates and fats are fully burned during use, while proteins are not only used for energy but also for building and repairing the body. Fats carry the most energy, but carbohydrates burn more quickly.

For adults, the optimal situation is, of course, to consume exactly as much energy as they expend each day. If this balance is not achieved, the body will store the excess nutrients, and weight gain begins. If one is overweight, then for a sustained period, energy intake should be lower than energy expenditure.

Measuring both energy intake and expenditure precisely can indeed help solve or prevent weight problems, but counting calories can never replace healthy intuition and body awareness. In some cases, however, knowing exact energy intake is essential – for example, for athletes or people with diabetes.