Lactic acid is a type of acid that our bodies produce when we exercise really hard or when there isn't enough oxygen available. It can make our muscles feel tired and sore. Full definition
Chen, H., et al., «Microbiological study of lactic acid bacteria in kefir grains by culture - dependent and culture - independent methods,» Food Microbiology 2008; 25 (3): 492 - 501. (doctorshealthpress.com)
Almudena García - Ruiz and her group studied 11 strands of lactic acid bacteria in wine and their ability to pass through environments similar to the mouth (saliva), stomach (gastric acid), and upper GI tract (bile) and end up in the intestinal mucosa to help fight infection and disease. (cavemandoctor.com)
Instead, their bodies seem to be doomed to feeling the soreness and burn of lactic acid build up no matter what they do. (goodhealthplanning.com)