The nutrient essential for maintaining hydration balance is water. It plays a crucial role in various bodily functions, including temperature regulation, nutrient transport, and waste elimination. Maintaining adequate hydration levels is vital for optimal performance, especially during exercise, as even mild dehydration can lead to decreased energy, impaired cognitive function, and increased risk of injury. Electrolytes like sodium and potassium are also important, as they help maintain fluid balance and nerve function, further underscoring the necessity of proper hydration in health and fitness.