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Medicine LibreTexts

1.11: Calories (Food Energy)

Food energy is measured in kilocalories (kcals), commonly referred to as calories. This terminology is technically incorrect, but is used so commonly that we will refer to them as calories throughout the course. A kilocalorie is the amount of energy needed to raise 1 kilogram of water 1 degree Celsius. A food’s kilocalories are determined by putting the food into a bomb calorimeter and determining the energy output (energy = heat produced). The first link below is to an image of a bomb calorimeter and a link to a video showing how one is used.

 

Web Links

Bomb Calorimeter

Video: Bomb Calorimetry (2:19)

 

Among the nutrients, the amount of kilocalories per gram that each provide are shown below.

 

Energy (kcal/g) No Energy 
Carbohydrates (4) Vitamins
Proteins (4) Minerals
Lipids (9) Water


As can be seen, only carbohydrates, proteins, and lipids provide energy. However, there is another energy source in the diet that is not a nutrient… alcohol. Just to re-emphasize, alcohol is NOT a nutrient! But it does provide energy.

Figure 1.112 Wine is a type of alcohol.

The following table lists the energy sources in the diet from lowest calories per gram to the highest calories per gram. Knowing these numbers allows a person to calculate/estimate the amount of calories the food contains if you know the grams of the different energy sources.

 

Energy Sources (kcal/g)

Carbohydrates → 4

Protein → 4

Alcohol → 7

Lipids → 9