Lactate

Lactate is a substance produced during the anaerobic breakdown of glucose or glycogen. This process is also known as the anaerobic glycolysis. It is continuously produced and can be used to generate more ATP in aerobic metabolism or transported to the liver to be converted to glucose. Contrary to popular belief, lactate is not responsible for metabolic acidosis, or acid buildup in the muscle.

The decrease in pH is caused by the increase in concentration of H+ ions which are produced during the conversion of ATP to ADP. However, during intense exercise lactate concentrations do rise because lactate clearance capacity can not keep up with the increased lactate production. A process known as lactate threshold.

Reducing the increased lactate concentration during or after exercise is thought to be one of the reasons for an elevated metabolism after training, and one of the reasons high intensity exercise greatly aids during weight loss.

References:

-Baechle, T.R., Earle, W.R. (2008). Bioenergetics of Exercise and Training. Essentials of Strength Training and Conditioning. Third Edition. USA. Human Kinetics.