Final answer:
The assertion that brain cells can only use carbohydrates for energy is false. While glucose is the primary fuel, brain cells can also use ketone bodies as an alternative during periods of low carbohydrate availability. This allows the brain to continue functioning even when glucose is scarce.
Step-by-step explanation:
The statement that under almost all circumstances, brain cells can only use carbohydrates for energy is false. While it's true that the major fuel for the brain is the carbohydrate glucose, and indeed, carbohydrates are broken down into glucose which provides energy as ATP through metabolic pathways, it is not the only source of energy that can be used. When glucose is scarce, the brain can utilize ketone bodies as an alternate energy source. This metabolic adaptation occurs during periods when carbohydrate intake is very low, such as fasting or a ketogenic diet.
In the presence of oxygen, glucose is metabolised to produce ATP, which is the usable form of energy for your cells. However, most cells, including those in the heart and different organs, can use fatty acids as an alternate energy nutrient when glucose is limited. During prolonged fasting or starvation, glucose stores become depleted, and the body begins to generate ketone bodies from fats, which can be used as a preferential fuel source to spare glucose for the brain.
Therefore, the brain does primarily rely on glucose, but in times of scarcity, it can adapt to use ketone bodies for energy.