Fat has 38.1 kJ per gram and has by far the highest energy content. The term "joule" comes from the English physicist JP Joule that indicates the energy required to increase the temperature of 1g of water by 1 ° C. 1,000 joule equals 1 kilojoules (kJ). In everyday use, however, the more common unit is the kilocalorie (kcal). Where: 4.184 kJ = 1 kcal .
Thus, fat has a calorific value of 38.1 kJ = 9.1 kcal. By comparison, proteins and carbohydrates provide just 17.1 kJ (4.1 kcal) per gram. Even alcohol is 29.3 kJ (7.1 kcal), under the energy content of fat.
To burn now 1 kg of body fat, you would first have to identify the physiological calorific value (energy content) of 1 kg human adipose tissue. If we extrapolate the physiological caloric value of fat to one kilogram, we get about 38,100 kJ, equivalent to 9,100 kcal. But, does one have to save 9,100 calories to lose one kilogram of body fat? No. Since the human adipose tissue is not 100 percent pure fat, but about 25 percent water, which is the physiological caloric value of body fat "only" is about 6,900 kcal.
To lose a kilogram body fat, you must thus save around 7,000 calories. With the aim to lose one kilogram of fat in a week, that would be 1,000 calories per day. This is not an easy task and even not recommended. But what is the best way to proceed? Eat less? Or just move more?