There are so many
misconceptions surrounding the glycolytic product known as lactate. Lactate is
both produced and used by the muscles, its rate of production increases as the
exercise intensity increases and as more carbohydrate is used to fuel exercise.
In the body,
lactate usually exists in combination with sodium (as sodium lactate) not in the
acidic form of lactic acid.
·
The original thinking was that muscles only
began to produce lactate when their oxygen supply was inadequate to meet their
oxygen demand. The muscles therefore became anaerobic and as a result, the end
products of glycogen breakdown could not be metabolised in the mitochondria, as
oxygen is required in order for them to function.
·
The prevailing belief was that blood lactate
concentrations suddenly increased at a threshold exercise intensity, variously
called the anaerobic threshold, the lactate turnpoint, or the ventilation
threshold. The term ‘lactate threshold’ is limiting in that it identifies a
point that is determined visually, not mathematically, and that it suggests some
sort of abrupt threshold when no such threshold exists. With the lack of a
suitable alternative, we have continued to use the term ‘lactate turn point’.
The lactate turn point might be best defined as the exercise intensity or
running speed that produces a blood lactate concentration of 3mmol per litre per
minute during a progressive maximal exercise test to exhaustion. This is because
when exercise is continued at running speeds below that critical speed, blood
lactate concentrations do not rise further but tend to fall. This suggests that
exercise intensity can be sustained for some reasonable time.
·
When high-intensity exercise (greater than 85
–95% VO2max) is achieved, virtually all energy comes from carbohydrate
oxidation. This means that the rate of energy flow through the glycolytic
pathway increases steeply with increasing exercise intensity, the result is that
the rate of lactate production increases inside the muscle. The high rate of
glycolytic turnover also produces acidic hydrogen ions or protons, which acidify
the inside of the muscle cell. To counteract this, the protons are exported from
inside the cell into the blood stream but this process requires that blood
lactate is co.transported with the protons. In this way lactate appears in the
blood stream whenever the use of carbohydrate is high. The presence of lactate
in the blood stream is a by-product of a process which aims to prevent the
muscle from becoming acidic too rapidly.
Within an hour of an intensive interval training session,
during which blood lactates reach the highest achievable value (15mmol.l-1)
muscle lactate values will return to normal.
·
Lactate is a totally innocuous substance that,
if infused into the blood stream, has no noticeable effects. Rather it is the
excess acidic hydrogen ions released during rapid carbohydrate turnover that may
be related to fatigue during high intensity exercise.
·
As the exercise intensity increases, the rate
of carbohydrate use by the body increases.
As a result, glycolysis is activated; lactate production in
both Type 1 and Type 2 muscle fibres rises, so that at the lactate turnpoint
lactate production exceeds the rate of lactate consumption in the exercising
skeletal muscles. Lactate then appears in increasing amount in the arterial and
venous blood. This way, lactate is shuttled to other tissues, in particular the
liver, the heart and the inactive skeletal muscles. The liver may use lactate
for producing new glucose and glycogen, in the heart, lactate becomes the
preferred fuel for oxidative (mitocondrial) metabolism. The inactive muscles
store the lactate, thereby lowering the lactate concentrations in the blood and
active muscles. Lactate is metabolized to glycogen in the inactive skeletal
muscles and enters the blood stream; to be used by glycogen depleted active
muscles.
·
Blood lactate testing could be used to monitor
an athlete’s fitness level if repeated frequently during training. However, in
reality, it is relatively time- consuming to accurately identify the lactate
turnpoint in the laboratory. Also, it is probably a less accurate predictor of
marathon performance than either the peak treadmill running velocity measured
during the VO2max test; the prediction based on racing performance over shorter
distances and/or, even the athletes own prediction, provided that he or she is
an experienced marathon runner. Indeed, in equally well-trained cyclists,
measurements of blood lactate parameters during exercise were found to be of
negligible value in predicting their performances.
·
Despite the hype that surrounds the value of
lactate testing in predicting the performance in runners, the expectations have
not been met. Changes in blood lactate concentrations during exercise can give
some indication of whether the athletes fitness has improved or regressed, but
there is little added value in using blood lactate measurements for the
prediction of performance
The shortened extracts on lactate are a very small part of the chapters in the over 1000 pages of the book Lore of Running (4th edition).
Without the
detailed explanations of all aspects of the subjects discussed my collection of
bits and pieces hardly reflects the value of the book.
I shall try
to answer any questions from the information in the book.