3.9: Energy and Nutrition

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The energy you need to survive is derived from the carbohydrates, the main source of fuel for the body, proteins, and fats that you eat throughout the day. In this section, we'll discuss the units of energy and how it relates to your nutrition.

Units of Energy

The SI unit of energy and work is the joule (J). The Joule is a very small amount of energy so scientists predominantly use the kilojoule, or 1000 joules, when discussing energy. However, you may be more familiar with the terminology of a calorie (cal). A calorie is the amount of energy needed to raise the temperature of 1 gram of water by 1 degree celsius. The two units can be related with the following equality.

1 cal = 4.184 J (exact)

As you can imagine this is also a fairly small number. Therefore, when discussing nutrition a new unit was introduced as the nutritional calorie or Calorie (Cal) which is 1000 calorie units, 1 kcal. This is the unit that you know as a calorie on the back of nutritional labels.

1 Cal = 1 kcal = 1000 cal

These equalities can also be written in terms of joules.

1 Cal = 4.184 kJ = 4184 J

The majority of energy required to do work in the body is obtained from the three main macronutrients carbohydrates, proteins, and fats. However, these are not equal in the amount of energy that they provide. The following table lists the typical energy values in terms of either kilocalories (kcal) or kilojoules (kJ) per gram of macronutrient for the three major food types.

Food Type	kcal/g	kJ/g
Carbohydrate	4	17
Protein	4	17
Fat	9	38

The energy from a food source can be calculated by multiplying the grams of each of the three major food groups by their typical energy value and then added together. An example of this can be seen below.

Summary

Identify the units of Energy
Define the the Energy Values for Foods