Final answer:
The athlete is likely experiencing decreased energy and poor performance due to inadequate carbohydrate intake, not receiving enough energy to sustain athletic performance.
Step-by-step explanation:
When an athlete's dietary history shows a significant increase in the intake of high-protein foods such as chicken, tuna, and beef, but they experience decreased energy and poor performance, the likely cause of these symptoms is b. inadequate carbohydrate intake. Although protein is crucial for muscle repair and growth, carbohydrates are the primary fuel source for both the brain and muscles during exercise. The athlete's increased protein intake, without sufficient carbohydrates, may lead to depleted glycogen stores, resulting in decreased energy levels during the day and at practice. Quite simply, the body is not receiving enough of its preferred source of energy to sustain high levels of athletic performance.