How is it possible that the Red Bull Zero contains 0 gramms of fat, carbs and protein, but it still has 1.8 kcal of "energy". I always thought that the human body can gain energy only from 3 kinds of nutrients: fat, carbs and protein. Is there a 4th kind? Or they just display an energy value that's not accessible to the body?
The list of ingredients on the can mentions "Zuckerkulör," which is caramel colour, which can have 2 kcal/g, according to one producer.
Next, there is "Citronensäure," which is citric acid, which can, as other organic acids, have 2-3 kcal/g, according to this source.
There is also taurine, which is an amino acid-like compound, so it could, like proteins, have 4 kcal/g, but is, according to Taurine Metabolism in Man (Journal of Nutrition), poorly metabolized and probably has less than 0.2 kcal/g.
It is sometimes allowed, at least by U.S. Food and Drug Administration, to round the amounts of macronutrients (carbs, proteins, fats) smaller than 0.5 g per serving to zero, which is what they obviously did in this case, but they decided to keep the summary of calorie values of all ingredients exact.
It is usually said that only 3 types of nutrients contain energy: carbohydrates, proteins and fats.
Digestible carbohydrates (sugars and starch) provide 4 kcal/g. Undigestible, but fermentable, carbohydrates, such as soluble dietary fiber, sugar alcohols or polyols (maltitol, mannitol, sorbitol, xylitol, isomalt) and organic acids (citric, acetic acid, etc.), can provide 2 kcal/g of energy in average. On the other hand, some carbohydrates (such as "Sucralose" from the ingredient list) are neither digestible nor fermentable, so they do not provide any calories.
The "fourth" nutrient that can provide energy (7 kcal/g) is alcohol (ethanol), but is not considered a nutrient by some authors.
According to Food Label Accuracy of Common Snack Foods article (Obesity, 2014), the calories on the food labels should represent usable calories (metabolizable energy):
Of note, it is important to distinguish that food label calories actually represent metabolizable energy, which is total caloric content minus calories that are presumably not absorbed by the body and excreted as waste.