LACTIC ACID

CAS No.: 50-21-5
EC No.: 200-018-0

Synonyms:
LACTIC ACID; lactic acid; laktik asit; LAKTİK ASİT; acidum lacticum; acidum sarcolacticum; aethylidenmilchsaeure; biolac; espiritin; ethylidene lactic acid; 2- hydroxy-2-methylacetic acid; 1-hydroxyethane carboxylic acid; (±)-2-hydroxypropanoic acid; 2-hydroxypropanoic acid; alpha-hydroxypropanoic acid; DL-2-hydroxypropanoic acid; dextro,laevo-2-hydroxypropanoic acid; 2-hydroxypropionic acid; a-hydroxypropionic acid; alpha-hydroxypropionic acid; (RS)-2-hydroxypropionsaeure; lactacyd; lactasol; laktik asit; LAKTİK ASİT; (±)-lactic acid; DL-lactic acid; lactic acid 50% FCC; lactic acid 80%, (naturals); lactic acid 88% USP heat stable (fermented); DL-lactic acid FCC; lactic acid natural; lactic acid powder; lactic acid synthetic; lactic acid, 88% USP; lacticacid; lactovagan; dextro,laevo-milchsaeure; DL-milchsaeure; milk acid; paramilchsaeure; propanoic acid, 2-hydroxy-; propel; propionic acid, 2-hydroxy-; tisulac; tonsillosan; lactic acid; 2-hydroxypropanoic acid; DL-Lactic acid; 50-21-5; 2-hydroxypropionic acid; Milk acid; Polylactic acid; lactate; Ethylidenelactic acid; Lactovagan; Tonsillosan; Racemic lactic acid; Propanoic acid, 2-hydroxy-; Ordinary lactic acid; Milchsaeure; Acidum lacticum; Kyselina mlecna; DL-Milchsaeure; Lactic acid USP; laktik asit; LAKTİK ASİT; laktik asit; LAKTİK ASİT; 1-Hydroxyethanecarboxylic acid; Aethylidenmilchsaeure; alpha-Hydroxypropionic acid; Lacticacid; Lactic acid (natural); FEMA No. 2611; 26100-51-6; Kyselina 2-hydroxypropanova; Milchsaure [German]; Propionic acid, 2-hydroxy-; 598-82-3; (RS)-2-Hydroxypropionsaeure; CCRIS 2951; HSDB 800; (+-)-2-Hydroxypropanoic acid; FEMA Number 2611; Kyselina mlecna [Czech]; Propanoic acid, hydroxy-; SY-83; DL- lactic acid; Propel; NSC 367919; laktik asit; LAKTİK ASİT; laktik asit; LAKTİK ASİT; AI3-03130; Purac FCC 80; Purac FCC 88; Kyselina 2-hydroxypropanova [Czech]; EINECS 200-018-0; EINECS 209-954-4; MFCD00004520; EPA Pesticide Chemical Code 128929; BRN 5238667; (R)-2-Hydroxy-propionic acid;H-D-Lac-OH; CHEBI:78320; Poly(lactic acid); C3H6O3; NSC-367919; NCGC00090972-01; 2-hydroxy-propionic acid; DL-Lactic acid, 90%; E 270; DSSTox_CID_3192; (+/-)-Lactic acid; alpha-Hydroxypropanoic acid; C01432; DSSTox_RID_76915; DSSTox_GSID_23192; Milchsaure; Polactide; Lacticum acidum; D(-)-lactic acid; Cheongin samrakhan; UNII-3B8D35Y7S4; CAS-50-21-5; Cheongin Haewoohwan; Cheongin Haejanghwan; Lactic acid [JAN]; Lactic acid [USP:JAN]; laktik asit; LAKTİK ASİT; lactasol; Propanoic acid, 2-hydroxy-, homopolymer; 1-Hydroxyethane 1-carboxylic acid; Biolac; Whey; 2-Hydroxy-2-methylacetic acid; Cheese whey; Lactide Polymer; Milk serum; MFCD00064266; Chem-Cast; Whey, cheese; L- Lactic acid; DL-Polylactic acid; laktik asit; LAKTİK ASİT; Lactate (TN); 3B8D35Y7S4; 2-Hydroxypropionicacid; 4b5w; Lactic acid, tech grade; Propanoic acid, (+-); DL-Lactic Acid, Racemic; HIPURE 88; (.+/-.)-Lactic acid; EC 200-018-0; Lactic acid (7CI,8CI); ACMC-1B0N9; Lactic acid (JP17/USP); Lactic acid, 85%, FCC; Lactic Acid, Racemic, USP; NCIOpen2_000884; L-( pound<<)-Lactic acid; .alpha.-Hydroxypropanoic acid; .alpha.-Hydroxypropionic acid; laktik asit; LAKTİK ASİT; KSC269O0T; (RS)-2-hydroxypropanoic acid; Lactic Acid (Fragrance Grade); INS NO.270; L-(+)-Lactic acid, 98%; CC(O)C([O])=O; CHEMBL1200559; DTXSID7023192; Lactic acid, natural, >=85%; (+/-)-2-hydroxypropanoic acid; BDBM23233; CTK1G9709; laktik asit; LAKTİK ASİT; HSDB 8244; Lactic Acid, 85 Percent, FCC; DL-Lactic acid, ~90% (T); INS-270; DL-Lactic acid, AR, >=88%; DL-Lactic acid, LR, >=88%; L-(+)-Lactic Acid, High Purity; Lactic Acid, 10 Percent Solution; KS-00000WI6; EINECS 295-890-2; Propanoic acid, 2-hydroxy- (9CI); Tox21_111049; Tox21_202455; Tox21_303616; ANW-43668; BBL027466; NSC367919; SBB065762; STL282744; AKOS000118855; AKOS017278364; Tox21_111049_1; AM87208; DB04398; LS-2145; MCULE-5387110670; VC30148; DL-Lactic acid, 85 % (w/w), syrup; Propanoic acid,2-hydroxy-,(.+/-.)-; laktik asit; LAKTİK ASİT; NCGC00090972-02; NCGC00090972-03; NCGC00257515-01; NCGC00260004-01; 163894-00-6; AK164446; I487; Lactic Acid, 85 Percent, Reagent, ACS; SC-18578; SC-86055; DB-071134; LS-180647; laktik asit; LAKTİK ASİT; 235-EP2269610A2; 235-EP2269986A1; 235-EP2269988A2; 235-EP2270000A1; 235-EP2270006A1; 235-EP2270008A1; 235-EP2270014A1; 235-EP2272516A2; 235-EP2272537A2; 235-EP2272817A1; 235-EP2272822A1; laktik asit; LAKTİK ASİT; 235-EP2272835A1; 235-EP2272844A1; laktik asit; LAKTİK ASİT; 235-EP2275401A1; 235-EP2275413A1; 235-EP2275421A1; laktik asit; LAKTİK ASİT; E-270; FT-0624390; FT-0625477; FT-0627927; FT-0696525; FT-0774042; L0226; Lactic acid solution, ACS reagent, >=85%; Lactic acid solution, USP, 88.0-92.0%; Lactic acid solution, p.a., 84.5-85.5%; Lactic acid, meets USP testing specifications; D00111; DL-Lactic acid, SAJ first grade, 85.0-92.0%; Propanoic acid, 2-hydroxy-, (+-)-, homopolymer; Q161249; DL-Lactic acid, JIS special grade, 85.0-92.0%; Lactic acid solution, Vetec(TM) reagent grade, 85%; laktik asit; LAKTİK ASİT; F2191-0200; BC10F553-5D5D-4388-BB74-378ED4E24908; Lactic acid, United States Pharmacopeia (USP) Reference Standard; ALPHA/BETA HYDROXY ACIDS (LACTIC ACID) (ALPHA/BETA HYDROXY ACIDS); laktik asit; LAKTİK ASİT; Lactic acid, Pharmaceutical Secondary Standard; Certified Reference Material; laktik asit; LAKTİK ASİT

LACTIC ACID

Lactic acid
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Lactic acid
7 Milchsäure.svg
L-Lactic acid molecule spacefill.png
Names
Preferred IUPAC name
2-Hydroxypropanoic acid[1]
Other names
Lactic acid[1]
Milk acid
Identifiers
CAS Number    
50-21-5 check
79-33-4 (l) check
10326-41-7 (d) check
3D model (JSmol)    
Interactive image
3DMet    
B01180
Beilstein Reference    1720251
ChEBI    
CHEBI:422 check
ChEMBL    
ChEMBL330546 check
ChemSpider    
96860 check
ECHA InfoCard    100.000.017 Edit this at Wikidata
EC Number    
200-018-0
E number    E270 (preservatives)
Gmelin Reference    362717
IUPHAR/BPS    
2932
KEGG    
C00186
PubChem CID    
612
RTECS number    
OD2800000
UNII    
33X04XA5AT ☒
UN number    3265
CompTox Dashboard (EPA)    
DTXSID7023192 Edit this at Wikidata
InChI[show]
SMILES[show]
Properties
Chemical formula    C3H6O3
Molar mass    90.078 g·mol−1
Melting point    18 °C (64 °F; 291 K)
Boiling point    122 °C (252 °F; 395 K) at 15 mmHg
Solubility in water    Miscible[2]
Acidity (pKa)    3.86,[3] 15.1[4]
Thermochemistry
Std enthalpy of
combustion (ΔcH⦵298)    1361.9 kJ/mol, 325.5 kcal/mol, 15.1 kJ/g, 3.61 kcal/g
Pharmacology
ATC code    G01AD01 (WHO) QP53AG02 (WHO)
Hazards
GHS pictograms    GHS05: Corrosive[5]
GHS hazard statements    H315, H318[5]
GHS precautionary statements    P280, P305+351+338[5]
Related compounds
Other anions    Lactate
Related carboxylic acids    Acetic acid
Glycolic acid
Propionic acid
3-Hydroxypropanoic acid
Malonic acid
Butyric acid
Hydroxybutyric acid
Related compounds    1-Propanol
2-Propanol
Propionaldehyde
Acrolein
Sodium lactate
Ethyl lactate
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
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Infobox references
Lactic acid is an organic acid. It has a molecular formula CH3CH(OH)COOH. It is white in the solid state and it is miscible with water.[2] When in the dissolved state, it forms a colorless solution. Production includes both artificial synthesis as well as natural sources. Lactic acid is an alpha-hydroxy acid (AHA) due to the presence of a hydroxyl group adjacent to the carboxyl group. It is used as a synthetic intermediate in many organic synthesis industries and in various biochemical industries. The conjugate base of lactic acid is called lactate.

In solution, it can ionize, producing the lactate ion CH
3CH(OH)CO−
2. Compared to acetic acid, its pKa is 1 unit less, meaning lactic acid is ten times more acidic than acetic acid. This higher acidity is the consequence of the intramolecular hydrogen bonding between the α-hydroxyl and the carboxylate group.

Lactic acid is chiral, consisting of two enantiomers. One is known as l-(+)-lactic acid or (S)-lactic acid and the other, its mirror image, is d-(−)-lactic acid or (R)-lactic acid. A mixture of the two in equal amounts is called dl-lactic acid, or racemic lactic acid. Lactic acid is hygroscopic. dl-Lactic acid is miscible with water and with ethanol above its melting point, which is around 16, 17 or 18 °C. d-Lactic acid and l-lactic acid have a higher melting point. Lactic acid produced by fermentation of milk is often racemic, although certain species of bacteria produce solely (R)-lactic acid. On the other hand, lactic acid produced by anaerobic respiration in animal muscles has the (S) configuration and is sometimes called "sarcolactic" acid, from the Greek "sarx" for flesh.

In animals, l-lactate is constantly produced from pyruvate via the enzyme lactate dehydrogenase (LDH) in a process of fermentation during normal metabolism and exercise.[6] It does not increase in concentration until the rate of lactate production exceeds the rate of lactate removal, which is governed by a number of factors, including monocarboxylate transporters, concentration and isoform of LDH, and oxidative capacity of tissues.[7] The concentration of blood lactate is usually 1–2 mM at rest, but can rise to over 20 mM during intense exertion and as high as 25 mM afterward.[8][9] In addition to other biological roles, l-lactic acid is the primary endogenous agonist of hydroxycarboxylic acid receptor 1 (HCA1), which is a Gi/o-coupled G protein-coupled receptor (GPCR).[10][11]

In industry, lactic acid fermentation is performed by lactic acid bacteria, which convert simple carbohydrates such as glucose, sucrose, or galactose to lactic acid. These bacteria can also grow in the mouth; the acid they produce is responsible for the tooth decay known as caries.[12][13][14][15] In medicine, lactate is one of the main components of lactated Ringer's solution and Hartmann's solution. These intravenous fluids consist of sodium and potassium cations along with lactate and chloride anions in solution with distilled water, generally in concentrations isotonic with human blood. It is most commonly used for fluid resuscitation after blood loss due to trauma, surgery, or burns.

Contents
1    History
2    Production
2.1    Fermentative production
2.2    Chemical production
3    Biology
3.1    Molecular biology
3.2    Exercise and lactate
3.3    Metabolism
4    Blood testing
5    Polymer precursor
6    Pharmaceutical and cosmetic applications
7    Foods
8    Forgery
9    Cleaning products
10    See also
11    References
12    External links
History
Swedish chemist Carl Wilhelm Scheele was the first person to isolate lactic acid in 1780 from sour milk.[16] The name reflects the lact- combining form derived from the Latin word lac, which means milk. In 1808, Jöns Jacob Berzelius discovered that lactic acid (actually l-lactate) also is produced in muscles during exertion.[17] Its structure was established by Johannes Wislicenus in 1873.

In 1856, the role of Lactobacillus in the synthesis of lactic acid was discovered by Louis Pasteur. This pathway was used commercially by the German pharmacy Boehringer Ingelheim in 1895.

In 2006, global production of lactic acid reached 275,000 tonnes with an average annual growth of 10%.[18]

Production
Lactic acid is produced industrially by bacterial fermentation of carbohydrates, or by chemical synthesis from acetaldehyde.[19] In 2009, lactic acid was produced predominantly (70–90%)[20] by fermentation. Production of racemic lactic acid consisting of a 1:1 mixture of d and l stereoisomers, or of mixtures with up to 99.9% l-lactic acid, is possible by microbial fermentation. Industrial scale production of d-lactic acid by fermentation is possible, but much more challenging.

Fermentative production
Fermented milk products are obtained industrially by fermentation of milk or whey by Lactobacillus bacteria: Lactobacillus acidophilus, Lactobacillus casei, Lactobacillus delbrueckii subsp. bulgaricus (Lactobacillus bulgaricus), Lactobacillus helveticus, Lactococcus lactis, and Streptococcus salivarius subsp. thermophilus (Streptococcus thermophilus).

As a starting material for industrial production of lactic acid, almost any carbohydrate source containing C5 and C6 sugars can be used. Pure sucrose, glucose from starch, raw sugar, and beet juice are frequently used.[21] Lactic acid producing bacteria can be divided in two classes: homofermentative bacteria like Lactobacillus casei and Lactococcus lactis, producing two moles of lactate from one mole of glucose, and heterofermentative species producing one mole of lactate from one mole of glucose as well as carbon dioxide and acetic acid/ethanol.[22]

Chemical production
Racemic lactic acid is synthesized industrially by reacting acetaldehyde with hydrogen cyanide and hydrolysing the resultant lactonitrile. When hydrolysis is performed by hydrochloric acid, ammonium chloride forms as a by-product; the Japanese company Musashino is one of the last big manufacturers of lactic acid by this route.[23] Synthesis of both racemic and enantiopure lactic acids is also possible from other starting materials (vinyl acetate, glycerol, etc.) by application of catalytic procedures.[24]

Biology
Molecular biology
l-Lactic acid is the primary endogenous agonist of hydroxycarboxylic acid receptor 1 (HCA1), a Gi/o-coupled G protein-coupled receptor (GPCR).[10][11]

Exercise and lactate
During power exercises such as sprinting, when the rate of demand for energy is high, glucose is broken down and oxidized to pyruvate, and lactate is then produced from the pyruvate faster than the body can process it, causing lactate concentrations to rise. The production of lactate is beneficial for NAD+ regeneration (pyruvate is reduced to lactate while NADH is oxidized to NAD+), which is used up in oxidation of glyceraldehyde 3-phosphate during production of pyruvate from glucose, and this ensures that energy production is maintained and exercise can continue. During intense exercise, the respiratory chain cannot keep up with the amount of hydrogen ions that join to form NADH, and cannot regenerate NAD+ quickly enough.

The resulting lactate can be used in two ways:

Oxidation back to pyruvate by well-oxygenated muscle cells, heart cells, and brain cells
Pyruvate is then directly used to fuel the Krebs cycle
Conversion to glucose via gluconeogenesis in the liver and release back into circulation; see Cori cycle[25]
If blood glucose concentrations are high, the glucose can be used to build up the liver's glycogen stores.
However, lactate is continually formed even at rest and during moderate exercise. Some causes of this are metabolism in red blood cells that lack mitochondria, and limitations resulting from the enzyme activity that occurs in muscle fibers having high glycolytic capacity.[25]

In 2004, Robergs et al. maintained that lactic acidosis during exercise is a "construct" or myth, pointing out that part of the H+ comes from ATP hydrolysis (ATP4− + H2O → ADP3− + HPO2−
4 + H+), and that reducing pyruvate to lactate (pyruvate− + NADH + H+ → lactate− + NAD+) actually consumes H+.[26] Lindinger et al.[27] countered that they had ignored the causative factors of the increase in [H+]. After all, the production of lactate− from a neutral molecule must increase [H+] to maintain electroneutrality. The point of Robergs's paper, however, was that lactate− is produced from pyruvate−, which has the same charge. It is pyruvate− production from neutral glucose that generates H+:

Polymer precursor
Main article: polylactic acid
Two molecules of lactic acid can be dehydrated to the lactone lactide. In the presence of catalysts lactide polymerize to either atactic or syndiotactic polylactide (PLA), which are biodegradable polyesters. PLA is an example of a plastic that is not derived from petrochemicals.

Pharmaceutical and cosmetic applications
Lactic acid is also employed in pharmaceutical technology to produce water-soluble lactates from otherwise-insoluble active ingredients. It finds further use in topical preparations and cosmetics to adjust acidity and for its disinfectant and keratolytic properties.

Foods
Lactic acid is found primarily in sour milk products, such as koumiss, laban, yogurt, kefir, and some cottage cheeses. The casein in fermented milk is coagulated (curdled) by lactic acid. Lactic acid is also responsible for the sour flavor of sourdough bread.

In lists of nutritional information lactic acid might be included under the term "carbohydrate" (or "carbohydrate by difference") because this often includes everything other than water, protein, fat, ash, and ethanol.[40] If this is the case then the calculated food energy may use the standard 4 kilocalories (17 kJ) per gram that is often used for all carbohydrates. But in some cases lactic acid is ignored in the calculation.[41] The energy density of lactic acid is 362 kilocalories (1,510 kJ) per 100 g.[42]

Some beers (sour beer) purposely contain lactic acid, one such type being Belgian lambics. Most commonly, this is produced naturally by various strains of bacteria. These bacteria ferment sugars into acids, unlike the yeast that ferment sugar into ethanol. After cooling the wort, yeast and bacteria are allowed to “fall” into the open fermenters. Brewers of more common beer styles would ensure that no such bacteria are allowed to enter the fermenter. Other sour styles of beer include Berliner weisse, Flanders red and American wild ale.[43][44]

In winemaking, a bacterial process, natural or controlled, is often used to convert the naturally present malic acid to lactic acid, to reduce the sharpness and for other flavor-related reasons. This malolactic fermentation is undertaken by lactic acid bacteria.

While not normally found in significant quantities in fruit, lactic acid is the primary organic acid in akebia fruit, making up 2.12% of the juice.[45]

As a food additive it is approved for use in the EU,[46] USA[47] and Australia and New Zealand;[48] it is listed by its INS number 270 or as E number E270. Lactic acid is used as a food preservative, curing agent, and flavoring agent.[49] It is an ingredient in processed foods and is used as a decontaminant during meat processing.[50] Lactic acid is produced commercially by fermentation of carbohydrates such as glucose, sucrose, or lactose, or by chemical synthesis.[49] Carbohydrate sources include corn, beets, and cane sugar.[51]

Forgery
Lactic acid has historically been used to assist with the erasure of inks from official papers to be modified during forgery.[52]

Cleaning products
Lactic acid is used in some liquid cleaners as a descaling agent for removing hard water deposits such as calcium carbonate, forming the lactate, Calcium lactate. Owing to its high acidity, such deposits are eliminated very quickly, especially where boiling water is used, as in kettles. It also is gaining popularity in antibacterial dish detergents and hand soaps replacing Triclosan.

See also
Hydroxybutyric acid
Acids in wine
Alanine cycle
Biodegradable plastic
Dental caries
MCT1, a lactate transporter
Thiolactic acid

Lactic acid, or lactate, is a chemical byproduct of anaerobic respiration — the process by which cells produce energy without oxygen around. Bacteria produce it in yogurt and our guts.  Lactic acid is also in our blood, where it's deposited by muscle and red blood cells.

It was long thought that lactic acid was the cause of muscle soreness during and after an intense period of exercise, but recent research suggests that's not true, said Michael Gleeson, an exercise biochemist at Loughborough University in the U.K., and author of "Eat, Move, Sleep, Repeat" (Meyer & Meyer Sport, 2020). 

"Lactate has always been thought of as the bad boy of exercise," Gleeson told Live Science. 

Contrary to that reputation, lactic acid is a constant, harmless presence in our bodies. While it does increase in concentration when we exercise hard, it returns to normal levels as soon as we're able to rest — and even gets recycled back into energy our body can use later on, Gleeson said. 

CLOSE
How muscles produce lactic acid
Throughout most of the day, our body burns energy aerobically — that is, in the presence of oxygen. Part of that energy comes from sugar, which our muscle cells break down in a series of chemical reactions called glycolysis. (We also get energy from fat, but that involves a whole other chemical process). The end product of glycolysis is pyruvate, a chemical that the body uses to produce even more energy. But energy can be harvested from pyruvate only in the presence of oxygen. That changes during hard exercise.

Related: Muscle spasms and cramps: Causes and treatments

When you break into an all-out sprint your muscles start working overtime. The harder you work, the more energy your muscles need to sustain your pace. Luckily, our muscles have built-in turbo-boosters, called fast-twitch muscle. Unlike slow-twitch muscle, which we use for most of the day, fast-twitch muscle is super-effective at producing lots of energy quickly and does so anaerobically, Gleeson said. Fast-twitch muscle also uses glycolysis to produce energy, but it skips harvesting energy from pyruvate, a process that takes oxygen. Instead, pyruvate gets converted into a waste product, lactic acid, and released into the bloodstream.

It's a common misconception that muscle cells produce lactic acid when they can't get enough oxygen, Gleeson said. "That's not the case. Your muscles are getting plenty of oxygen," he said. But in times of intense energy needs, muscles switch to anaerobic respiration simply because it's a much quicker way to produce energy.

Other sources of lactic acid
Muscle cells aren't the only sources of lactic acid. Red blood cells also produce lactic acid as they roam the body, according to the online text Anatomy and Physiology published by Oregon State University. Red blood cells don't have mitochondria — the part of the cell responsible for aerobic respiration — so they only respire anaerobically.

Many species of bacteria also respire anaerobically and produce lactic acid as a waste product. In fact, these species make up between 0.01-1.8% of the human gut, according to a review published in the Journal of Applied Microbiology. The more sugar these little guys eat, the more lactic acid they produce. 

Slightly more insidious are the lactic acid bacteria that live in our mouths. Because of the acidifying effect they have on saliva, these bacteria are bad news for tooth enamel, according to a study published in Microbiology.

Finally, lactic acid is commonly found in fermented dairy products, like buttermilk, yogurt and kefir. Bacteria in these foods use anaerobic respiration to break lactose — milk sugar — into lactic acid. That doesn't mean that lactic acid itself is a dairy product, however — it's 100% vegan. It happens to get its name from dairy simply because Carl Wilhelm, the first scientist to isolate lactic acid, did so from some spoiled milk, according to a study published in the American Journal of Physiology.

A young girl eating yogurt out of a cup.

Lactic acid is found in fermented dairy products, like yogurt, but lactic acid itself isn't dairy — it's 100% vegan. (Image credit: Shutterstock)
Your body on lactic acid
It's common to feel a burning in your legs after you squat with heavy weights, or complete a hard workout. But contrary to popular belief, it's not lactic acid that causes the soreness, Gleeson said. 

Lactic acid is processed by the liver and the heart. The liver converts it back into sugar; the heart converts it into pyruvate. During exercise, concentrations of lactic acid in the body do spike because the heart and liver can't deal with the waste product as quickly as it's produced. But as soon as we're done exercising, lactic acid concentrations go back to normal, Gleeson said.

Related: Feel the pain? Don't blame lactic acid. 

Muscle soreness after exercise most likely has more to do with tissue damage and inflammation, Gleeson said. Hard exercise physically breaks down your muscles, and it can take days for them to recover.

Lactic acid can build up to life-threatening levels in the body, according to a review published in the Mayo Clinic Proceedings. But this condition, called acute lactic acidosis, happens because of acute illness or injury, not exercise. When tissues are deprived of blood due to a heart attack or sepsis, for example, they tend to go into anaerobic respiration, producing lactic acid.

"They get starved of oxygen," Gleeson said.

But Gleeson said he's never heard of a case of life-threatening lactic acidosis because of exercise. "That would be most unusual."  

Additional resources: 

Read about anaerobic respiration on Khan Academy.
Find out why you feel so sore after a workout.
Learn about acute lactic acidosis on Medscape.