Sugar affects energy levels during exercise by providing a quick yet brief source of energy. Simple carbohydrates break down rapidly, leading to a sharp but temporary increase in blood glucose levels. While this can enhance performance during short, intensive activities, overconsumption can lead to a rapid decline in energy, known as a "sugar crash." This often results in fatigue and decreased performance. For sustained energy, it's better to combine carbohydrates with protein or opt for complex carbohydrates that provide steadier, long-lasting energy throughout workouts.