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Lactic acid or lactate is a by-product of the lactic acid anaerobic metabolism. It is a compound toxic to cells, whose accumulation in the blood stream correlates to the appearance of the so-called muscle fatigue.
Lactate is already produced starting from low intensity of exercise; the red blood cells, for example, form it continuously, even in conditions of complete rest.
Lactic Acid and Lactate are Synonyms?
Lactic Acid and Lactate are not exactly synonymous.
Lactic acid, in effect, is a weak acid, with a dissociation constant "pK" of about 3.7; therefore, at the conditions of muscle and blood pH (pH 6.4 - 7.4), more than 99% of the lactic acid is dissociated in the form of lactate ion and H + hydrogenion, as shown in the figure:
![](http://energymedresearch.com/img/fisiologia-dellallenamento/194/acido-lattico.gif)
Lactate is therefore the ion that derives from the depronation of lactic acid itself. Therefore, at the physiological pH values, lactic acid is completely deprotonated in solution, lowering its pH.
Production and Metabolism
A normally active adult man produces about 120 grams of lactic acid a day; of these, 40 g are produced from tissues having an exclusively anaerobic metabolism (retina and red blood cells), while the rest are produced by other tissues (especially muscle) based on the actual availability of oxygen.
The human body has defense systems to protect itself from lactic acid and can convert it back to glucose thanks to the activity of the liver. The heart is instead able to metabolize lactic acid for energy purposes.
From these statements it can be deduced that lactic acid, although toxic, is not a real waste product. Thanks to a whole series of enzymatic processes this substance can in fact be used for intracellular glucose resynthesis.
The latest studies point out that lactic acid is actually only indirectly involved in the increase in blood acidity. The main culprit of this phenomenon is the hydrogen ion H +, which during high-intensity physical exercise is released in large quantities due to the increase in ATP hydrolysis. Consequently the "reserves" of bicarbonates in the blood decrease.
H + + HCO 3 - ↔ H 2 CO 3 ↔ CO 2 + H 2 O
![](http://energymedresearch.com/img/fisiologia-dellallenamento/194/acido-lattico.jpg)
The Cori cycle is the mechanism responsible for the conversion of lactic acid into glucose, it takes place in the liver and follows the steps shown in the figure.
In the under-strength muscle, the production of lactic acid is massive especially in the fast or pale fibers that have an anaerobic glycolytic power higher than the red or resistant ones. It is no coincidence that particularly brilliant athletes in the lactic acid anaerobic tests such as track-tracking in cycling and 400-1500 meters in athletics produce over 20% more lactic acid than a normal person.
Role in Sport
At the same intensity of exercise, the quantity of lactic acid produced is inversely proportional to the degree of training of the subject. This means that if an athlete and a sedentary run at the same speed, the latter produces much more lactic acid than the first and disposes of it with greater difficulty.
During strenuous muscular work when the aerobic metabolism is no longer able to satisfy the increasing energy demands, an accessory pathway is activated for the production of ATP called lactate anaerobic mechanism. This phenomenon, while partially overcoming the lack of oxygen, increases the amount of lactic acid produced which in turn exceeds the capacity of neutralization by the body. The result of this process is a sharp increase in the amount of lactate present in the blood which roughly corresponds to the subject's anaerobic threshold frequency.
The blood lactate concentration in the blood is normally 1-2 mmol / L at rest but during an intense physical effort it can reach and exceed 20mmol / L. The anaerobic threshold, measured by the haematic concentration of lactic acid, is made to coincide with the heart rate value so that in the course of an incremental exercise the concentration of 4mmoles / L is reached.
Lactic acid begins to accumulate in the muscles and blood when the synthesis rate exceeds the disposal speed. Roughly, this condition triggers when during an intense physical exercise the heart rate exceeds 80% (for the untrained) and 90% (for the most trained) of the maximum heart rate.
Increase tolerance to lactic acid
Athletes engaged in anaerobic lactate disciplines (effort duration between 30 and 200 seconds) are forced to compete in conditions of maximum production and accumulation of lactate. Their performance is therefore correlated to the efficiency of the anaerobic lactate metabolism and of the disposal systems in blood, muscle and liver.
The purpose of training aimed at increasing these characteristics is to saturate the muscles with lactic acid in such a way that they get used to working in conditions of strong acidity. At the same time, this approach improves the effectiveness of blood buffer systems (bicarbonate) in neutralizing blood acidosis.
The athlete has two training techniques available to improve the lactic acid anaerobic performance:
- one based on continuous effort (20-25 minutes) with heart rate values close to the anaerobic threshold (± 2%)
- one based on the working method at intervals: in athletics 2-6 repeated for 1-4 series from 150-400 meters at race pace or higher interspersed with partial recoveries between repetitions (45-90 seconds) and complete between the series ( 5-10 minutes).
Lactic acid is disposed of within 2 or 3 hours, and its quantity is halved every 15-30 minutes depending on training and the quantity of lactic acid produced.
- Contrary to what is often stated, lactic acid is not responsible for the muscular pain felt the day after a very intense workout. This pain is caused by muscle micro-lacerations that give rise to inflammatory processes; moreover, there is an increase in blood and lymphatic activities that increase sensitivity in the most stressed muscle areas.
Lactic acid is a strong stimulus for the secretion of anabolic hormones such as GH and testosterone. For this reason exercises with high intensity weights, interspersed with not too long pauses, maximize muscle mass gain.
In addition to the Cori cycle, there is an additional system to dispose of lactic acid, preventing it from accumulating in the muscle. It is the haematic filling mediated by bicarbonate (see: Bicarbonate).
65% of the lactic acid produced is converted to carbon dioxide in water, 20% is converted to glycogen, 10% in proteins and 5% in glucose.
Curiosity
Did you know that ... Lactic acid is used in the food industry as an acidity regulator.
In the mouth, among the various bacteria present, the lactobacillus acidophilus ( Lactobacillus acidophilus ) has the highest cariogenic power. This bacterium feeds on the glucose present in the food residues, forming lactic acid as a waste product. Thanks to its acidity, this substance is able to gradually dissolve the dental enamel by denting the dentin.