‘乳酸’ (rǔ suān) literally means ‘milk acid’, combining 乳 (rǔ), meaning ‘milk’, and 酸 (suān), meaning ‘acid’. It is the chemical compound lactic acid, produced naturally in muscles during intense physical activity when oxygen levels are low, and also found in fermented dairy products like yogurt and sour milk. In biochemistry, it plays a key role in energy metabolism and muscle fatigue.

In everyday Chinese, 乳酸 appears frequently in health, sports science, and food contexts — for example, discussing post-exercise soreness (‘乳酸堆积’), probiotic benefits of fermented foods, or food labeling (e.g., ‘含乳酸’). While the character 乳 suggests a dairy origin, lactic acid is not exclusive to milk; it’s also generated by bacterial fermentation in vegetables (e.g., pickles) and human tissues. Its usage is neutral and technical, common in both scientific writing and general health discussions.