(S)-Malic acid ((S)-2-Hydroxysuccinic acid) is a tart-tasting organic dicarboxylic acid found in many sour foods, such as apples, and contributes to the sourness of green apples and tartness of wine, although its concentration decreases with fruit ripeness (wikipedia). In its ionized form, it is called malate, an intermediate in the TCA cycle alongside fumarate, and can be formed from pyruvate through anaplerotic reactions. In humans, malic acid is derived from food sources and synthesized in the body via the citric acid cycle in mitochondria, playing a crucial role in energy production under both aerobic and anaerobic conditions. Under aerobic conditions, malate is oxidized to oxaloacetate, providing reducing equivalents via the malate-aspartate redox shuttle, while during anaerobic conditions, its simultaneous reduction to succinate and oxidation to oxaloacetate removes excess reducing equivalents, reversing hypoxia's inhibition of glycolysis and energy production. Studies on rats have shown that tissue malate depletes following exhaustive physical activity, suggesting that malic acid deficiency may cause physical exhaustion. Administering malic acid to rats has been shown to elevate mitochondrial malate, increasing mitochondrial respiration and energy production.