What it's actually good for
Electrolyte replacement during exercise is one of the least controversial topics in sports nutrition. When you sweat, you lose water and electrolytes — primarily sodium, with smaller amounts of potassium, magnesium, and chloride. Replacing these during prolonged or intense exercise maintains blood volume, supports muscle function, and prevents the performance decline that accompanies dehydration. This is basic physiology backed by decades of research and the consensus position of every major sports medicine organization.
The key electrolyte is sodium. Sweat sodium losses vary enormously between individuals (from 200 to over 2,000 mg per liter of sweat), which is why there is no single "correct" dose — heavy sweaters in hot environments need substantially more than light sweaters in cool conditions. Plain water is fine for short, low-intensity sessions, but for exercise lasting more than an hour, or any session involving heavy sweating, an electrolyte solution outperforms water for both fluid retention and performance maintenance. The addition of small amounts of glucose further enhances intestinal absorption via sodium-glucose cotransport — the same mechanism behind oral rehydration therapy. This is an A-grade category because the underlying science is robust, the practical applications are clear, and the consensus is strong.