Which nutrient is the slowest to provide energy but can provide energy for a long time when supplies are ample?

• A. Protein
• B. Fat
• C. Carbohydrates
• D. Water

Answer: (B)

Fat provides energy more slowly but can sustain you for a long time when energy supplies are ample. Fat stores are large and dense in energy, so they can keep a steady supply during long-duration, lower-intensity activity. However, tapping into fat requires multiple steps—mobilizing fatty acids, transporting them to muscles, and processing them through beta-oxidation and the citric acid cycle with sufficient oxygen—which makes the rate of ATP production slower than carbohydrates. Carbohydrates, by contrast, are quicker to mobilize and burn for rapid energy, but their stores are limited and get depleted with sustained effort. Water provides no energy, and protein contributes energy only in prolonged energy deficits, not as a primary sustained source.