• Boris Clark

Glycolytic flux - An overview of lactate

When we talk about lactate levels, glycolytic flux, and glycolytic capacity what do we mean? And why does it matter? How can it help you?

Put simply, glycolysis is the process of splitting a molecule of glucose into two molecules of pyruvate, which can turn into lactate (These two are almost the same thing so from now on for simplicity we will refer to lactate). Glycolytic flux is the rate at which this process is happening, while glycolytic capacity is a term we use to describe the ability of your body to use this process.

But why does this matter? To understand this we need to give a basic overview of energy in the body and a little bit of chemistry.

The body has essentially 2 main fuel sources. Fat and carbohydrate. We can also use some protein for energy in the form of amino acids, through a process called gluconeogenesis we turn amino acids into glucose (a carbohydrate) to be used. This process makes up a minimal amount of total fuel for the body and is one we really want to limit as athletes as there is a high chance the protein we use will come from our muscle tissue. So for the purposes of this discussion, fat and carbohydrate are our fuel sources.

The body does not simply run on fat or carbohydrate however. The body uses adenosine triphosphate (ATP) in order to use energy. Using ATP lets us contract our muscles, allowing us to exercise. The higher the intensity of exercise, the more energy is needed, and hence, the more ATP our body will require. The body will break down both fat and carbohydrate order to generate ATP.

ATP. This is what lets you exercise (and perform any movement at all!)

At rest and low intensities a large portion of our energy needs can be met by the breakdown of fat to form ATP. Even at rest we will still use some carbohydrate to create ATP (in fact fat requires carbohydrate in order to be burnt i.e. fat burns in a carbohydrate fueled flame). There are several steps which limit the rate at which we can utilise fat to generate ATP, along with the fact that it requires oxygen to use. These factors can be influenced and improved through training and diet, however this process is still far slower than using carbohydrate.

The rate of glycolytic flux determines how much lactate is produced through anaerobic glycolysis which can be used aerobically as fuel, providing oxygen is available. When the lactate for fuel is insufficient for the bodies energy demand, the body will require another energy source, which will be fat, when too much lactate is produced, only lactate will be used, leading to only carbohydrate being used for fuel, and if exercise is continued at this intensity then acidosis will occur (failure to keep turning pyruvate in lactate in very simple terms), we fatigue, and we slow down or stop altogether. This means that the point at which combustion of lactate for fuel and lactate production are equal, is our maximal lactate steady state, or more commonly known as ‘threshold’.

One of the problems we face here is that while even in a lean athlete, fat is almost unlimited, carbohydrate is extremely limited, with only somewhere in the region of 500 grams available. This can be burned through in well under 2 hours in a fit person with a high energy turnover (i.e. high power output or running/swimming pace).

To understand why this is a problem we need a quick chemistry lesson. Below is the formula for one glucose molecule.


Here we have 6 carbons, 12 hydrogens, and 6 oxygens. All perfectly stable and ready to be used.

In order to yield ATP from this glucose molecule we must break it down. This process uses 2 ATP of energy to do, but yields us 4, so a net gain of 2 ATP. This process is extremely quick, and does not require any oxygen to assist in the reaction. Sounds great right? Well, sort of. What we end up with is this:

C3H4O3 and C3H4O3

The above is two molecules of pyruvate. By combining this pyruvate with oxygen we can create acetyl-coA, which then goes into the Krebs cycle, giving us a couple more ATP and some by-products, which can then go into the electron transport chain generating 34 more ATP, as long as we can combine it with oxygen.

Steps of anaerobic glycolysis to end up with Pyruvate

But if you look at the chemical formulas above you will notice something is missing. 4 molecules of hydrogen seem to have disappeared. These hydrogen ions essentially split off, and are acidic. This is what is responsible for your burning muscles as you exercise intensely.

So where does lactate come into all of this? Well, in order to buffer these acidic hydrogen ions unused pyruvate can get the hydrogen ions back to become lactate and become the below:


Great! Less acidic hydrogen ions floating around our muscles, for now . . . As we continue to demand ATP production, we will continue to have a build-up of hydrogen ions which lactate can only buffer some of, and eventually we will reach a level where the body cannot form and utilise lactate any faster from the muscles, meaning we will have a build-up of hydrogen ions, a decrease in pH (increase in acidity), our muscles will burn, and cells will stop functioning properly forcing us to slow down.

How strong we want this anaerobic capacity to be depends on what sport we do, and what our goals are in the sport.

If our glycolytic capacity is high, we have a high rate of glycolytic flux, even at low intensities. This means an early build-up of lactate, leading to a lower threshold, higher carbohydrate utilisation, and reduced ability to recover from hard efforts. Unfortunately, the driver of respiration is the removal of CO2 rather than the intake of O2 so we cannot simply breathe more, this is why our lactate threshold is not at 100% of VO2Max. A high glycolytic capacity will therefore lower the anaerobic threshold due to the shortfall between oxygen demand in the muscles and oxygen uptake from respiration. So do we want to reduce the glycolytic capacity?

Maybe, but not necessarily.

A glycolytic capacity that is too low will result in a low rate of glycolytic flux, this means less acidity, higher threshold, and higher ability to use fat for fuel, but also less ability to utilise carbohydrate through anaerobic glycolysis to generate ATP quickly, so a higher threshold, quicker recover between efforts, but less ability to do high intensity work above threshold.

Developing the VO2Max will always be useful to improve performance, but the ideal glycolytic capacity will vary depending on what we hope to achieve. For a given VO2Max, an athlete with a low glycolytic capacity will have a higher threshold, be able to recover quicker (even while at a higher intensity), and prolong time to fatigue through lower carbohydrate usage, while the high glycolytic capacity athlete will have a lower threshold, less ability to recover from hard efforts, and higher carbohydrate usage at all intensities, but has the capacity to do a large amount of work above threshold for a short-moderate time.

Ever wonder why cycling sprinters don’t usually time trial well despite their obvious power, or why a cyclist with pretty obvious leg strength like Tony Martin does not sprint well? Why middle distance runners can’t complete a marathon as fast as a pure marathoner despite their obvious speed advantage, or why marathon runners can’t run 5000m as fast despite the fact the distance is basically a sprint for them? The answer is the level of each of these athletes glycolytic capacity.

We know Elia Viviani is a great sprinter with enormous power, and clearly a very good aerobic capcity, so why he is not a great time trialest? The answer is the very reason he is a good sprinter, a high anaerobic capcity/glycolytic flux rate

Through lactate testing and analysis we can track where your current performance fits into all of this, track it over time, and individualise your training for your specific goals based on this information. Knowing this information means less time wasted, more efficient training, and ultimately, better results!