Energy value of nutrients

Today we will talk about the energy value, Atwater factors, metabolizable energy and kilocalories.

Our energy value come 30-70% from carbs, 15-50% from fats and 10-25% from proteins. The Adwater factors are:

-gross energy - all the energy present in the food products (not all is available)
-digestible energy - amount of energy absorbed, dependent of the digestibility of the food (carbs 98%, fats 95%, proteins 92%)
-metabolizable energy - what is left after accounting the energy in feces and urine.

So, the gross energy is made by the digestible energy and the one not available. The digestible energy is made by the metabolizable energy and the energy eliminated through feces and urine. The energy eliminated is mostly made from proteins (30% of it as urea). The calories we see on the food package are in fact equal with the metabolizable energy.

In terms of kilo-calories (kcal), we have the carbohydrates and proteins with 4 kcal, fats with 9 kcal and alcohol with 7 kcal. In terms of dietary fiber, the insoluble one is subtracted from the carbs content, and the soluble ones have 4 kcal per gram. According to the EU rules, all the fiber is considered to have 2 kcal per gram.

The Atwater factor is an average, but in real life the digestibility can be different. For example, from the thermic effect of food point of view (dietary induced thermogenesis), the transient increase in energy expediture following meal consumption, caused by digestion, absorption and processing of the nutrients, it is higher for proteins than for carbohydrates and fats. Also, proteins, fats and carbs have different effects on hunger and satiety.

In conclusion, the traditional view that all energy is equal regardless of the source (a calorie is a calorie) is overly simplistic and often wrong in real life.

Next post: energy expediture.