You’ve probably noticed this while doing your shopping. The energy value of a food is expressed in kilojoules (K J) or in kilocalories (calories). This indication is present on the food product packaging and allows the consumer to know how much energy a food will provide them. In fact, the energy of a food corresponds to the “body fuel” to function properly. In fact, on a daily basis, when we want losing weight, we look at much more the number of kcal than the number of kJ. These two units do not have the same value. For simplicity :
► The kilocalorie (calories) is a unit of measurement for energy that is equivalent to 1,000 calories. Defined by the chemist and physicist Nicolas Clément, the calorie was used in the past in the field of physics and corresponded to the quantity of energy necessary to raise the temperature of a gram of liquid water by +1°C. But today, the term “calorie” is rather used in the field of dietetics and no longer has the same meaning: it is an energy reference which is used to balance one’s diet. In common parlance, we do an abuse of language and we speak of “calories” for “kilocalorie”.
- 1g of lipid provides 9 kcal.
- 1g of protein provides 4 kcal.
- 1g of carbohydrate provides 4 kcal.
► Kilojoule (kJ) correspond to 1,000 joules, a unit of measurement of energy invented by physicist James Prescott Joule. Scientifically, one joule is the work required to move one kilogram over one meter in one second. Nutritionally, the joule corresponds to the energy value provided by lipids, carbohydrates and proteins.
- 1g of lipid brings 37.66 kJ,
- 1g of protein brings 6.74 kJ
- 1g of carbohydrate brings 16.74 kJ
As you will have understood, there is a scale relationship between the two units: 1 kj is equal to 4,184 kJ.
How many calories do we need per day?
The daily intake a person needs based on their age, gender and physical activity. According to the ANSES recommendations, it is necessary:
- Around 2,200 kcal for a woman aged 20 to 40
- Around 2,700 kcal for a man aged 20 to 40
- Around 2,000 kcal for a woman aged 40 to 60
- Around 2,500 kcal for a man aged 40 to 60
These figures simply give an indicative range, which must be varied according to the individual’s activity level. A sedentary person will need fewer calories than an athletic person.