Where does lactate come from?
The term lactate is usually associated with the easily measured lactate concentration in the blood. However, the amount of lactate and its significance within the cells far exceeds that in the blood!
In our organism, in the cells, the splitting of 1 molecule of glucose into 2 molecules of lactic acid constantly produces 2 molecules of ATP – the chemical fuel of our organism. The reaction can be described as follows:
Glucose + 2 ADP + 2Pi → 2 lactic acid + 2 ATP (the intermediate step via pyruvate is omitted). As you can see, no oxygen is consumed. The formation of the chemical fuel, ATP, takes place anaerobically.
The energy content of the reaction: ATP → ADP + Pi is 7.3 Kcal / mol
At a normal blood pH value of 7.4, lactic acid itself is practically non-existent in our organism; it splits almost completely into lactate and hydrogen ions.
Generations of doctors and sports scientists have learned that lactate is only formed to a significant extent when there is a lack of oxygen, as it is formed anaerobically from glucose. The energy yield in the formation of lactate with 2 molecules of ATP when splitting 1 molecule of glucose is extremely low compared to the oxidation of glucose, which produces 32 molecules of ATP per molecule of glucose. Anaerobic glycolysis was therefore regarded as an emergency reserve of the organism in the event of a lack of oxygen, and lactate as a waste product of anaerobic metabolism, which is reused in downstream processes, primarily in the liver, for gluconeogenesis.
The first increase in lactate in the blood during exercise is regarded as the beginning of an acid deficiency in the stressed muscles and was described by many authors as the aerobic threshold. As the load increases further, more lactate is measured in the blood. After the aerobic-anaerobic transition area, the anaerobic metabolism is reached with the 2nd lactate threshold, the anaerobic threshold.
We have already mentioned that anaerobic metabolism plays an important role in the chapter “Energy metabolism during exercise” section. After all, oxygen consumption / watt remains practically constant up to VO2max. So how can anaerobic energy production play a significant role in the formation of lactate?
The fact that the formation of lactate also takes place when the muscle cells are supplied with a completely sufficient amount of oxygen is discussed in the chapter : “What is the significance of lactate” in detail.
At the same time, however, the lactate is also absorbed by the cells – or more correctly, reintroduced into the energy metabolism. As a result of formation and breakdown, the lactate concentration in the blood is between 0.6 and 2.4 mmol/l under resting conditions.
Lactate is by no means a waste product of anoxidative metabolism, as is often claimed. Rather, it is used by many organs as an important source of energy – it is metabolized oxidatively, i.e. using oxygen. As you can read in the chapter: “What is the significance of lactate” up to 60% of the energy used by the heart during exercise comes from the combustion of lactate! The brain also uses considerably more lactate to generate energy during exercise than under resting conditions. Today we must even assume that the lactate is not only used by the unstressed muscles, no – even by the stressed muscles (probably in the slow fibers) for energy production!
An increase in lactate is always seen when the release of lactate from the stressed muscles increases more than the removal and utilization of lactate in the tissues.
As described in the chapter Energy metabolism during exercise our organism has an “emergency reserve” of energy-rich phosphates for a few seconds, which allows it to adapt immediately to high physical exertion. As it takes some time before the organism can offer the muscles considerably more oxygen than is required at rest, energy is also produced in the meantime without the use of oxygen, anaerobically during glycolysis, the splitting of glucose ultimately into 2 molecules of lactic acid plus 2 molecules of ATP. The lactate that remains largely in the stressed muscle cells is ultimately an intermediate product of the oxidative breakdown of glucose.
With increasing physical exertion, the stressed cells react with increased lactate production. The breakdown of glucose without the use of oxygen ultimately to lactic acid, glucolysis, is accompanied by the production of 2 molecules of ATP per molecule of glucose. The oxidation of glucose, i.e. the combustion of glucose using oxygen, is extremely much more effective, leading to the formation of 32 molecules of ATP per molecule of glucose. However, most of the lactate remains intracellular, is transported into the mitochondria ( for details see here) and is then used for oxidative phosphorylation. This “detour of glucose degradation” has no influence on the oxygen balance! Only the lactate that enters the bloodstream represents anaerobic energy production.
As described in the chapter: Energy metabolism during exercise the proportion of anaerobic energy production at the beginning of the increased lactate production is less than 5 per mill, and at maximum load still less than 5 percent of the energy required for the load! Energy preparation is probably not the reason for the increased lactate production!