High blood sugar can significantly affect energy levels during exercise by reducing the body's ability to utilize glucose effectively. When blood sugar is elevated, the body struggles to convert glucose into energy, leading to feelings of fatigue and sluggishness. This can hinder performance, making it difficult to maintain intensity or sustain prolonged activity. Furthermore, the body may start to rely on fat as an alternative energy source, which can also affect stamina negatively. Monitoring and managing blood sugar levels is essential for optimal energy during exercise and overall athletic performance.