Lactic Acid: Superfuel for Muscles and Brain
Lactic acid myths: Lactic acid buildup in muscles after exercise causes cramps and sore muscles.
Lactic acid truth: Lactic acid is a metabolic superfuel. It generates quick energy for muscles and heart during exercise, aids in carbohydrate metabolism and can be used to replenish liver glycogen (Bergman et al. 2000, Brooks 2002, Gladden 2008, Cruz et al. 2012). In addition, the brain prefer lactic acid over glucose as a fuel.
Picture: "Say what about lactic acid?"
Lactic acid does not cause muscle aches, cramps or delayed muscle soreness. It actually can prevent muscle soreness.
So what does cause muscle soreness? Researchers are still arguing about that but it is likely a combination of microdamage to muscle fibers, the break down of damaged muscle tissues and a general inflammation caused by the muscle damage (Nosaka 2008). The good news is that research shows that repeated bouts of exercise protect against muscle soreness. Also, using the sore muscle does not cause more muscle damage (so there goes that excuse for not exercising) and usually helps alleviate pain.
Lactic Acid Overview
Acid whey is a good source for lactic acid. It contains 6-10% lactic acid by dry weight. In contrast, sweet whey only contains about 2% or less lactic acid by dry weight.
Lactic acid is formed during yogurt or cheese fermentation by culture organisms known as lactic acid bacteria. Lactic acid bacteria break down lactose, milk sugar, for energy and form lactic acid as a by product. As the lactic acid builds up it coagulates the casein protein of the milk to form the yogurt or cheese.
In the body, lactic acid is formed as a product of metabolism or energy production. Most lactic acid is produced within muscle cells. Muscle cells break down glucose to produce lactic acid. The muscle cells can use lactic acid for fuel or send it to the heart, liver or brain.
Lactic acid provides quick energy for the body
Lactic acid is used as a quick fuel by the muscles, heart and brain. During exercise your body prefers to use lactic acid as fuel. Muscles use up to 80% of the lactic acid they produce for energy (Gladden 2008). As exercise intensity increases the heart also uses more lactic acid.
The liver uses lactic acid in the blood to make glycogen. During periods of rest or marathon internet surfing, the liver transforms up to 30% of lactic acid into glycogen (Conner et al. 1982, Gladden 2008). Your body commonly stores excess energy as glycogen. It can use the excess glycogen later to help fuel activities.
Lactic acid is a small molecule that is easily passed between different body tissues. Muscle cells can release and take up lactic acid when needed. They can share unneeded lactic acid with other body tissues like the heart. This makes lactic acid a more versatile fuel than glucose. Muscle cells cannot release stored glucose to share with the rest of the body since the glucose is trapped within the muscle.
Lactic acid fuels the heart
The heart loves lactic acid. Lactic acid composes up to 60% of the fuel used by the heart during exercise (Stanley 1991, Gladden 2008). In mice, reduced lactic acid availability to the heart during endotoxic shock resulted in impaired cardiovascular performance and early death (Matejovic et al. 2007). This has led some researchers to suggest that lactic acid be explored as a possible therapeutic treatment for critically ill people with heart problems (Ventura-Clapie et al. 2004, Matejovic et al. 2007).
Your brain is supercharged by lactic acid
During exercise the brain is fueled by lactic acid (Quistorff et al. 2008, van Hall et al. 2009). Brain lactic acid use increases by 3-fold over 30 minutes of cycling (van Hall et al. 2009). At rest, 8% of the energy used by the brain was lactate; this increased to 27% after thirty minutes of exercise. The brain can also use lactic acid instead of glucose when blood glucose levels drop.
Fun Lactic Acid Fact:
Lactic acid was discovered in 1780 in sour milk by Carl Wilhelm Scheele, a Swedish chemist. Scheele is also created with discovering many other substances such as oxygen, chlorine, hydrogen cyanide and hydrofluoric acid (an acute poison). He was also the first person to note that light blackened the chloride in silver salts and that blackened silver chloride was insoluble in ammonia (ammonia can dissolve regular silver chloride). These discoveries would later prove significant in developing modern photography. Sadly, he did not receive credit for many of his most important discoveries leading Isaac Asimov to call him "hard-luck Scheele" (therefore proving that even imminent scientists are not immune to a little name calling).
Unfortunately, Scheele liked to sniff and taste the new substances he created in his poorly ventilated apothecary. This led to his untimely demise from what may have been heavy metal toxicity and/or poisoning at the age of 43. When someone is breathing toxic gases and licking poisonous metals it is hard to pin down an exact cause for their poor health.
What's the take home message from Scheele's early death? For better health feast on delicious dairy products instead of chemicals of questionable origin. Who knows what substances he may have discovered if he stuck with drinking sour milk instead of hydrogen cyanide. In spite of this bit of interesting history, the catch line 'Dairy, its not hydrofluoric acid!' failed to resonate with the general public who vastly preferred 'Got milk?'.
References:
Bergman BC, Horning MA, Casazza GA, Wolfel EE, Butterfield GE, Brooks GA. Endurance training increases gluconeogenesis during rest and exercise in men. American Journal of Physiology. 2000;278:E244–E251. Pubmed. Full paper. Brooks GA. Lactate shuttles in nature. Biochemical Society Transactions. 2002;30:258–264. Pubmed. Connor H, Woods HF. Quantitative aspects of L(+)-lactate metabolism in human beings. In: Porter R, Lawrenson G, editors. Metabolic Acidosis (Ciba Foundation Symposium 87). London (UK): Pitman Books Ltd. 1982;214–34. Wiley Online Library. Cruz RS, de Aguiar RA, Turnes T, Penteado Dos Santos R, de Oliveira MF, Caputo F. Intracellular shuttle: the lactate aerobic metabolism. Scientific World Journal. 2012;2012:420984. Pubmed. doi: 10.1100/2012/420984 (full paper) Gladden LB. A lactatic perspective on metabolism. Med Sci Sports Exerc. 2008;40:477-85. Pubmed. doi: 10.1249/MSS.0b013e31815fa580 Matejovic M, Radermacher P, Fontaine E. Lactate in shock: a high-octane fuel for the heart? Intensive Care Med. 2007;33:406-8. Pubmed. Nosaka K. Muscle Soreness and Damage and the Repeated-Bout Effect. In Tiidus PM. Skeletal muscle damage and repair. Human Kinetics. 2008;59–76. Full paper. Quistorff B, Secher NH, Van Lieshout JJ. Lactate fuels the human brain during exercise. FASEB J. 2008;22:3443–3449. Pubmed. doi: 10.1096/fj.08-106104 Stanley WC. Myocardial lactate metabolism during exercise. Med Sci Sports Exerc. 1991;23:920-4. Pubmed. van Hall G, Strømstad M, Rasmussen P, Jans O, Zaar M, Gam C, Quistorff B, Secher NH, Nielsen HB. Blood lactate is an important energy source for the human brain. J Cereb Blood Flow Metab. 2009 Jun;29(6):1121-9. Pubmed. doi: 10.1038/jcbfm.2009.35 Ventura-Clapier R, Garnier A, Veksler V. Energy metabolism in heart failure. J Physiol. 2004;555:1–13. Pubmed. Full paper.