Introduction
Energy is a key factor in nutrition as it powers all the activities of the human body, from breathing to walking to thinking. Food provides energy, but not all of it is usable by the body. Terms like combustible energy and metabolizable energy help us understand how much energy is available from food and how much of that energy our body can actually use. In this answer, we’ll explore both concepts and provide a simple example.
What is Combustible Energy?
Combustible energy, also known as gross energy, is the total amount of energy present in a food item. It is measured by completely burning the food in a laboratory device called a bomb calorimeter.
Features of Combustible Energy:
- Measured by burning 1 gram of food and capturing the heat released.
- Represents the total potential energy in food, including both usable and non-usable parts.
- Includes the energy lost in feces, urine, and gases.
Example: When 1 gram of carbohydrate is burned in a calorimeter, it releases approximately 4.1 kcal (gross energy).
What is Metabolizable Energy?
Metabolizable energy is the actual energy available to the body after subtracting the losses that occur during digestion, absorption, and excretion. It is the energy that the body can utilize for metabolism, movement, and growth.
Features of Metabolizable Energy:
- Calculated by subtracting energy lost in feces, urine, and gases from gross energy.
- This is the energy listed on food labels and used in dietary guidelines.
- Represents usable energy that fuels body functions.
Example: The metabolizable energy from 1 gram of carbohydrate is approximately 4 kcal (slightly less than the gross energy).
Illustrative Example: Carbohydrates
| Component | Energy (kcal/g) |
|---|---|
| Gross (Combustible) Energy | 4.1 |
| Losses (Feces, urine, gas) | 0.1 |
| Metabolizable Energy | 4.0 |
So, although the bomb calorimeter shows 4.1 kcal for 1 gram of carbohydrate, the body only gets 4.0 kcal of usable energy due to small losses.
Energy Values for Major Nutrients (Approximate Metabolizable Energy)
- Carbohydrates: 4 kcal/g
- Proteins: 4 kcal/g (more energy is lost in urine)
- Fats: 9 kcal/g
- Alcohol: 7 kcal/g
Conclusion
In summary, combustible energy is the total energy a food contains when completely burned, while metabolizable energy is the energy your body actually uses after accounting for digestive losses. Understanding this difference is important for accurate dietary planning and assessing the true energy contribution of food to our daily needs.