ISSA Nutritionist Practice Exam

Which fuel source is the most effective at producing ATP?

1. Amino acids

2. Carbohydrates

3. Fatty acids

4. Proteins

The correct answer is: Fatty acids

Fatty acids are the most effective fuel source for producing ATP because they undergo a process called beta-oxidation, which allows them to be broken down efficiently in the mitochondria of cells. Each molecule of fatty acid can yield a significant amount of ATP, often more than carbohydrates or proteins. The high energy density of fats means that they contain more ATP-generating potential per molecule compared to carbohydrates or proteins. When fatty acids are oxidized, they produce acetyl-CoA, which enters the citric acid cycle, and subsequently leads to a greater generation of ATP through oxidative phosphorylation. While carbohydrates also provide a rapid source of energy and are metabolized quickly, their overall ATP yield per unit weight is lower than that of fatty acids. Proteins can also be used for energy, but they are primarily reserved for use when carbohydrate and fat stores are depleted, as utilizing protein for fuel can lead to muscle loss and is less efficient compared to fats and carbohydrates. In a well-fed state, the body favors the use of carbohydrates for quick energy, but when energy needs are sustained or during prolonged exercise, fatty acids become the preferred source due to their effective ATP production and energy efficiency.