Lactate

Lactate is a substance produced by your muscles during intense exercise when oxygen levels are low. It results from the process of breaking down sugar (glucose) for energy without enough oxygen, known as anaerobic metabolism. Rather than being just a waste, lactate can be used as fuel by the heart and other muscles. Its presence often causes feelings of muscle fatigue or soreness, but it is a natural part of certain physical activities and helps your body adapt to increased activity levels.