Same Intensity — Different Lactate Readings

Why can lactate readings differ at the same exercise intensity? Several key factors are at play.

Changes in Blood Volume

When a person exercises, various adaptations can cause blood volume to change — both increase and decrease. Heat training and altitude training have a particularly strong effect. In endurance sports, we should aim for increased blood volume, but this doesn't always happen.

We measure lactate and other blood markers per milliliter or liter of fluid — we assess concentration, not total amount. This is why, when doing any blood tests, we must understand that changes in blood volume can also change the values measured per ml/L of blood.

This applies not only to lactate but also to other markers. For example, at altitude you might get a blood test showing significantly higher hemoglobin levels, but this doesn't mean you've achieved positive progress — the total amount of hemoglobin remains the same, only the blood volume has changed. At altitude, due to the characteristics of the air, we lose more fluid through breathing, leading to dehydration. For data accuracy, we need to know the blood volume — this can be measured but is not available in regular laboratories.

The same applies to heart rate, which is also a concentration indicator rather than a volume indicator. This is precisely why we see a significant increase in heart rate when, for example, we are dehydrated.

Blood lactate units are mmol/L (millimoles per liter of blood), and blood volume itself can change with adaptation to heat stress and other circumstances. For example, during the off-season it can decrease quite quickly, which may lead to higher measured lactate concentrations.

Let's look at what happens when plasma volume changes. Example: an athlete records 1 mmol/L with a blood volume of 5 L — the athlete's blood contains a total of 5 mmol of lactate. If the volume increases to 5.5 L, the concentration drops to 5 mmol / 5.5 L = 0.91 mmol/L, which seems insignificant. If a test results in blood lactate of 15 mmol/L with a blood volume of 5 L, then the total lactate is 75 mmol. If the same test is performed with 5.5 L of blood, the resulting value is 75 mmol / 5.5 L = 13.6 mmol/L. And if the volume changes, for example, from 5 L to 7 L, the difference would be even more substantial.

Energy Availability

At the cellular level, trillions of chemical reactions occur every second that enable us to move. We cannot control these reactions, and most of them cannot even be measured. But maintaining these reactions requires a large amount of fuel. With each hard training session, we raise the stress level, and if the body cannot cope with this level, there will be no training progress.

To control this stress, we measure lactate levels. But as we discussed in a previous article, lactate is a consequence of carbohydrate oxidation. If we have a carbohydrate deficit, the training session may not be properly executed, and lactate may be lower than usual at the initial stage, while becoming higher in subsequent sessions due to mitochondrial dysfunction.

Impact of Heavy Training

Any heavy workload that leads to significant stress, followed by insufficient recovery, can cause mild to severe mitochondrial dysfunction, which will affect lactate utilization.

Mitochondria are the powerhouses of your cells. Their job is to convert the nutrients you consume into energy by producing adenosine triphosphate (ATP) — a coenzyme used by your cells for a range of functions, from breathing to physical exercise. When you increase the size and number of your mitochondria through mitochondrial biogenesis, the mitochondria can more efficiently convert energy into ATP, meaning more energy is available for working muscles.

Any negative change in mitochondrial structure will affect lactate readings, and at the same intensity you will see different numbers.

Sample Collection Errors

We discussed how to properly take a sample in previous articles. Let us remind you once again: when measuring lactate, the most important things are strip sterility and a clean blood sample without contamination from sweat or alcohol from a wipe. Even the slightest amount of alcohol or sweat will affect the result — our sweat also contains lactate, but it is very unstable, and alcohol content affects the reaction between the strip and blood.

It is also very important that the sample comes from a single drop and that the blood opening in the strip is filled in one attempt — the blood should enter only the opening and not spread across the entire strip.