What is the basis of weight loss or gain?
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For most people, the basis for weight loss and weight gain is energy or calorie balance (See Energy Balance Figure). To an economist, it would be a simple model of supply and demand; for us, it allows us to use those algebra skills we developed in high school. If the calories contained in the food we eat (supply or positive) exceeds the calories expended (“burned”) by our body (demand or negative), then we will store the surplus. (Printable Calorie Balance) |
Quantifying the energy content of foods is easy. We can simply read the food label or look at a calorie chart. A food’s energy content is the sum total of the energy contributions of its protein, carbohydrate, fat, and alcohol. However, quantifying the energy that we expend over the course of a single day and assessing how our energy expenditure may fluctuate over time with respect to different situations is a bit more complicated.
How do we know how much energy is in food?
When scientists want to know the energy content of a food, they can place the food in an insulated chamber, called a bomb calorimeter, and “combust” or it. Combustion requires oxygen and the products of combusting foods in a bomb calorimeter include carbon dioxide, water and heat. In addition, if the food contains protein or amino acids, some nitrogen-containing gases will also be produced.
Since heat energy is typically measured in calories it is applied to food energy and the energy used in our body. In separate experiments, scientists can also determine the individual amounts of carbohydrate, protein, fat, and alcohol in a given food. The approximate energy equivalent of 1 gram of these substances is as follows:
- 1 gram of carbohydrate = 4 calories
- 1 gram of protein = 4 calories
- 1 gram of alcohol = 7 calories
- 1 gram of fat = 9 calories
If we were to add up the energy contribution of the individual energy nutrients in a food, it should approximate the total calories of heat measured by the bomb calorimeter.
Do we generate the same amount of energy when using energy nutrients in our body as in a bomb calorimeter?
We combust or "burn" energy nutrients in our cells and in the process generate the same amount of energy as in the bomb calorimeter. In fact, the reason we bring oxygen into our body is so that it can be used in the burning of energy nutrients within our cells. Furthermore, carbon dioxide is produced during the burning of these energy nutrients in our cells and we must breathe it out.
Despite several similarities between the burning of energy nutrients in a bomb calorimeter and in our cells there are a couple of fundamental differences as follows:
- When amino acids and proteins are burned in a bomb calorimeter, nitrogen-containing gases are produced. Contrarily, when amino acids are used for energy in our cells, most of the nitrogen is ultimately used to make urea.
- The burning of energy nutrients in a bomb calorimeter is for the most part an instantaneous process, while the burning of energy nutrients occurring within our cells happens over a series of many chemical reactions (energy pathways).
- Unlike a bomb calorimeter, when we burn energy nutrients in our cells, we capture roughly 40 percent of the energy released in the formation of ATP and to a lesser degree GTP. Meanwhile, the remainder of the energy released in the breakdown of energy nutrients is converted to heat.
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