Understanding Energy Units in Nutrition
What are Kilojoules and Calories?
Both kilojoules (kJ) and calories (cal) are units used to measure energy, particularly in the context of food and nutrition. They tell us how much energy our bodies can get from consuming particular foods and drinks.
Kilojoule (kJ)
- The standard SI unit for measuring energy
- Used in Australia, New Zealand, and many European countries
- 1 kilojoule = 0.239 kilocalories (food calories)
Calorie (cal)
- Traditionally used in the United States and some other countries
- The "calories" on food labels are actually kilocalories (kcal)
- 1 kilocalorie = 4.184 kilojoules
The Historical Context
The calorie was first defined in 1824 as the amount of heat needed to raise the temperature of 1 gram of water by 1°C. The kilojoule, being part of the metric system, was later adopted as the standard scientific unit of energy.
Most countries have transitioned to using kilojoules on food labels as part of adopting the metric system. However, the United States and a few other countries continue to use calories as the primary unit of measurement for food energy.
Important Distinction: Small vs. Large Calories
In scientific contexts, there are actually two types of calories:
- Small calorie (cal) - The amount of energy needed to raise 1 gram of water by 1°C
- Large Calorie (Cal or kcal) - The amount of energy needed to raise 1 kilogram of water by 1°C
The "calories" referred to on food labels and in dietary contexts are actually kilocalories (kcal) or large Calories. This means that when a food label says "100 calories," it's actually referring to 100 kcal, which equals 100,000 small calories.
Energy Content of Foods
Different macronutrients provide different amounts of energy:
- Carbohydrates: 4 kcal/g (17 kJ/g)
- Proteins: 4 kcal/g (17 kJ/g)
- Fats: 9 kcal/g (37 kJ/g)
- Alcohol: 7 kcal/g (29 kJ/g)
Why Both Units Are Still Used
The continued use of both systems can be attributed to:
- Historical precedent and cultural familiarity
- Different governmental regulations around the world
- The gradual nature of metric conversion in some countries