ISSA Nutritionist Practice Exam

What is the primary source of energy for the brain?

1. Fatty acids

2. Protein

3. Glucose

4. Lactate

The correct answer is: Glucose

The primary source of energy for the brain is glucose. This carbohydrate is crucial for brain function as it supports the high metabolic demands of neuronal activity. The brain, despite making up only about 2% of body weight, consumes roughly 20% of the body's energy, predominantly in the form of glucose, because neurons rely on this sugar as their main fuel. While fatty acids and proteins can be used for energy in other parts of the body, they are not the primary energy source for the brain under normal circumstances. Fatty acids can lead to the formation of ketone bodies, which may serve as an alternative energy source during prolonged fasting or low-carbohydrate diets but are not generally the main supply. Protein is primarily used for tissue building and repair rather than as an energy source, especially in the context of brain metabolism. Lactate, often produced during intense exercise, can be used by the brain as an alternative energy source, but it is not the primary one under typical conditions. Therefore, glucose remains the foremost energy substrate critical for optimal brain function.