The Lactate Shuttle and Blood Lactate Measures
As you are sitting and reading this you are producing lactate, and at the same time you are using it and moving it around the body but you are not building up high levels and you are not aware of the process. Lactate production within your muscles occurs in healthy, well-oxygenated individuals at all times. Coaches and athletes, however, are not so much concerned with rest as to what happens during exercise and in the recovery from exercise.
During exercise, an individual's lactate level may be assessed by taking a small sample of blood and measuring the concentration of lactate. In the past, these lactate values have sometimes been incorrectly used to make projections on lactate production. Since lactate has the capacity to be both introduced and removed from the blood, the lactate level which we measure at any one time is actually a measure of accumulation, not production. The concentration of lactate in the blood represents the combined result of all those processes which add lactate to and remove it from the circulation.
Blood Lactate Accumulation = Lactate added to Blood - Lactate removed from Blood
At rest, blood lactate values are about 1 millimole per litre, mmol/L, of blood and can rise to in excess of 25 mmol/L during and after intense exercise. Blood lactate levels are low and stable in resting individuals because lactate production is balanced by utilisation and removal. The rates of lactate production in skeletal muscle increases as submaximal exercise intensity increases. At the same time, there is a rise in the rate of lactate conversion to pyruvate within the muscle. This production and removal is in such dynamic equilibrium that lactate does not 'spill' over to the systemic circulation and, consequently, blood lactate levels are relatively unaffected.
As exercise intensity increases there is a slow rise in the level of blood lactate. The small rises observed are despite dramatic increases in lactate production within the muscle meaning that other processes are combining together. It has been recorded that lactate production within exercising muscles may rise to five times the resting rate, or more, during exercise and yet give the same value for blood lactate concentration.
"In fact, lactate may be the most dynamic metabolite produced during exercise; lactate turnover exceeds that of any other metabolite yet studied" Billat LV, 1996
In summary, we can say that during and after the most intense exercise, muscle and blood lactate levels can rise to their highest values. We now understand that this accumulation above resting levels represents the balance of production and removal. It tells us nothing about whether the accumulation is due to an increased rate of production or decreased rate of removal, or both. In the same way, if lactate concentrations in the blood do not rise above resting levels during or immediately following exercise it also infers nothing about lactate production during that activity. It may be that lactate production is many times higher than at rest but that it is matched by its removal resulting in no visible increase.
OBLA and the Lactate Threshold
There is a point, in increasing intensity of exercise, where individuals may show a rapid increase in the levels of blood lactate. This turning point is referred to as the Onset of Blood Lactate Accumulation, or OBLA. The OBLA implies that the dynamic equilibrium which can exist between lactate addition to, and removal from, the blood has been upset. It is not clear at this time what processes have been disturbed or may have reached 'critical' levels to cause the OBLA.
In the past, this point in the progression of increasing intensity of exercise where blood lactate production increases dramatically was called the anaerobic threshold. The idea that the cell suddenly switches over from aerobic to anaerobic metabolism is simplistic and misleading, since this does not reflect accurately what is taking place in the muscle and the term anaerobic threshold is not now commonly used. Instead, the onset of blood lactate accumulation is referred to as the Lactate Threshold.
"...intense exercise generates big lactate loads, and the body adapts by building up mitochondria to clear lactic acid quickly. If you use it up [as an energy source], it doesn't accumulate." - George A. Brooks, UCB press release, 2006
Developing the Lactate Energy System
To develop the lactate shuttle and the lactate energy system optimally, training at fluctuating intensities is necessary. The recovery during this type of training is best when lactate utilisation and clearance is encouraged and this removal from the body is accelerated when lactate is shuttled to areas of high oxidative activity while maintaining either an active cycling pace as in Fartlek Interval Training. In this active environment, the lactate may optimally revert to pyruvate and in the mitochondria be oxidised in the Krebs cycle and the electron transport chain to produce water and carbon dioxide.
To put it more simply, the faster cycling sections will raise lactate availability and the recovery intervals train the body to efficiently utilise and clear the lactate. The cycle then becomes, 'Create more lactate, then use it as fuel, create more lactate, then use it as fuel and so on'. In the recovery, the use of lactate as a fuel has cumulative training effects. These cumulative effects of the active roll-on recoveries of the Shuttle Interval Training include:
more efficient mitochondria
more efficient MCTs
raised OBLA or Lactate Threshold
improved running economy