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Endurance training

My most favorite sport is cycling and being an enthusiast on understanding energy metabolism, there’s an interesting triangle to form with the field of ketosis and sport performance.

I originally started with thinking there’s a performance benefit when combining eating low carb with endurance due to the higher level of fat burning achieved and possible mitochondrial biogenesis but now I think they are pretty much on par for the majority of endurance sports. And looking back at it, it makes sense. Biologically both glucose and fatty acids can be used as fuel. Our bodies are flexible in using both to create energy. The redox state kind of dictates a limitation to keep everything within safe limits.

We also can’t expect major differences between both fuel types or nature would have preferentially selected one over the other. That is how evolution tends to work, it seems to always go for the most energy efficient pathways and has kept both glucose and fatty acids in its design.

Mitochondrial biogenesis (or mitogenesis) is important as we will see below but when it comes to a ketogenic diet (KD), another aspect of nature is that it only invests in what is needed and no more than that. Research shows a KD can increase mitochondrial mass but I suspect this is only in situations where there is a shortage. People who are already engaged for months or years in their endurance activities, will already have excess capacity when at rest.

The only incentive to further adapt and increase that mitochondrial mass is by chronically pushing the point that signals insufficiency. This means engaging in your endurance activity as much as possible.

So how do we go about it? A lot has been written but I never found base training explained completely so that me, or anyone else, would understand why it works the way that it is prescribed. And by understanding why, I hope to share with you how to properly conduct base training so that the quality of your base training increases and you’ll be able to see improvement (again).

  1. Why base training?
  2. How to improve the quality?
  3. Mitochondria
  4. Consistency
  5. Damage
  6. High Intensity adaptation
  7. Nutrition
  8. Finally

Why base training?

There are a lot of other adaptations but I want to focus on the mitochondria as this is where the energy is produced. Broadly speaking, there are 2 types of adaptation. One that improves quantity and one that improves quality of the energy production. Both will result in improved aerobic ATP production but they are not equal.

Both can be delivered by high intensity exercise but if you tried, you know it will easily lead to injury. The force on your body is too much too sudden. Stephen Seiler noticed this from his research work into how elite athletes train. It will lead to increased time where you cannot train and that will go at the cost of adaptation.

I will get back on this at the end but it is not the intention of this article to go through the difference and compare if high intensity is better than base training but suffice to say I’ll be focusing on the latter.

What we can say is that high intensity is like a jumping board on top of your base aerobic level. It is generally lost after around 2 weeks of abstaining from it. That indicates it is energy intensive to maintain. Instead, we can reserve this investment for when we really need it and that is to prepare for competition.

Let me jump straight into defining where your heart rate should be. Stephen Seiler talks about a 3 zone model instead of the 5 zones that you may know.

They are defined by the level of lactate reached. 2 mmol/L also referred to as lactate threshold 1 or LT1 and the next point is your maximum steady state lactate threshold or LT2. This is usually around 4mmol/L but I believe it can vary a bit more per individual and trained status.

For base training you want to look at the point where 2mmol/L is reached. Substract around 5bpm for your upper limit and then 5bpm for your lower limit. This will be your high aerobic training zone. Substract another 5 for your upper limit and again 5 for your lower limit and now you have your low aerobic training zone.

Why 2 mmol?  Below is an example shown on the GCN show ( from one of their presenters.  Any fluctuation below the 2 mmol/L threshold is considered within the capacity of the aerobic system to avoid a raised level of lactate in the blood.  Crossing this point means that the exercising muscles no longer can fully process the lactate so you get a lactate spill-over into the rest of the body.  Increasing aerobic fitness will delay this point because that adaptation increases the lactate processing capacity (both increasing clearance and reducing production).  Because this spill-over can be measured by crossing that 2 mmol/L, it is a valid measurement for everybody.  And because that blood lactate level is closely linked with your blood pH, your heart rate at this point is fixed.  So by testing once, you can be fairly certain that it is always correct.

The 4 mmol/L (roughly estimated) delineating zone 2 from zone 3 is where we see the lactate clearance capacity exceeding within the whole body.  Other type I muscles in the body can take up lactate, the brain takes up lactate, the liver does as well but at some point it reaches the maximum clearance capacity and from that moment onwards, any higher sustained effort will lead to continuously increasing lactate levels.

! Note: Because the 2 mmol/L represents the moment where your muscles cannot process the lactate alone, using muscles differently (for example climbing vs flat cycling or big gear vs small gear) will actually result in different power output. Each muscle has a different lactate clearance level (or aerobic fitness level) so as you change position and the way you engage your muscles, you will note that difference in wattage if you maintain heart rate.

Below you can see my own data.

As you can see, the 2 mmol/L crossover point is at the same heart rate for both 2012 and 2016.  But in the second picture you see that the power output in wattage is higher.

2019 is different and because of 3 possible effects: 

  • A first is that I switched to a ketogenic diet by the end of 2016.  The acidic ketones and higher circulating fatty acids may contribute to a bit more downward pressure on pH leading to a higher HR for the same blood lactate level. 
  • The second variable is that the incremental protocol was different.  For the first 2 the wattage was increased with 40W every 3 minutes while the test in 2019 increased 40W every 8 minutes.  It is possible that there is some delay in adaptation which may only fully take effect between that 3rd and 8th minute.  Measurements were taken after every 4 minutes and indicated a further increase of HR at the 8th minute compared to 4th minute.  I would recommend this slower 8 minute increase in wattage as it allows to reflect the steady state lactate level.
  • And third, the first 2 tests were done on the same machine in a university lab while the 2019 test was done in a basement on a zwift machine with a home device for measuring lactate so we can expect some deviation between both circumstances.

So focus on 2012 and 2016, the point I wanted to show is that there is a clear improvement in how much watt is produced for a given heart rate while the heart rate at 2 mmol stays the same under both fitness levels.

I do train based on the 2019 data as I am still on a KD so what did I do?  To be on the safe side I took the crossing point at around 157~158bpm.  To keep the numbers easy I have set my high aerobic zone between 145 and 150bpm.  This way I have a bit of margin if I would go over 150 but I definitely want to stay below 150.  My low aerobic zone is set between 135 and 140bpm with some flexibility, trying not to cross 130 and 145 on either side. 

There’s no need to be freakishly precise but don’t go any further than the safety zones to ensure quality base training.

How to improve the quality?

Doing base training requires you to stay in the zone at all times. Brief moments of higher intensity will reduce the quality because it will lead to the release of catabolic hormones.

The adaptation towards mitogenesis depends greatly on muscle glycogen depletion. When those catabolic hormones are activated, they will increase fuel delivery. This is needed to engage in the high intensity but when you return back to your base training effort, those hormones still remain active for a while. Iñigo San Millán (coach of Tadej Pogačar) estimates this effect to last for around 30 minutes. During this time you reduce the glycogen depletion.

What we want to achieve is pressure to adapt by depleting the muscle glycogen levels. This will create the necessary signaling to adapt the aerobic fitness level. When you are staying within zone 1, the delivered fuel from the blood circulation will be insufficient to cover the total demand.

A good marker is when you feel hungry towards the end of your ride.

Another effect is that as you approach depletion, lactate production will go up. If you have a power meter, you’ll notice by sticking to your heart rate that your power output goes down. This is a good sign.

Below are a couple of examples to demonstrate the rise in catecholamines after 2 mmol/L.

Between the 175 and 200W point, the subjects cross the 2mmol lactate.  From 200W onwards we see both epinephrine and norepinephrine levels increase.


Not all serum glucose is processed aerobically.  Some of it is processed anaerobically and results in lactate.  The skeletal muscle will adapt over time by increasing mitochondrial mass so that more glucose can be processed aerobically and thus increases the point where the level of power results in spill-over.

The length of baseline training doesn’t need to be done for endless hours depending on how you start.  Since the goal is to reach muscle glycogen depletion, does it make sense to start exercise just after a heavy carbohydrate breakfast?  If you do then it will take much longer to reach that depletion and signaling so you will have to make the ride multiple hours long.  If you don’t and start fasted, then around 2 to 2,5 hours may be sufficient.  It could even be shorter depending on your muscle glycogen state.

During winter, you can combine resistance exercise and base training with low carb eating.  Keep in mind though, it is not because you eat low carb and drastically increase your fat burning that you therefor have an elevated aerobic ATP production.  You don’t.  You just have a different mixture of fuel and it has about an equal effect on saving muscle glycogen and replenishing. 

Yes you have easier access to a larger storage of fuel but that doesn’t mean the fuel quantity that is reaching the mitochondria is therefore doubled.

But the combination with resistance exercise will increase depletion and absorption of glucose in those other muscles so that less is available for your legs.  If you eat carbs, then you increase serum glucose levels so much that there is plenty for all muscles to absorb and restore their glycogen levels.


Between your base training you’ll be feeding and sleeping.  This allows the regeneration of the muscle glycogen stores.  We need to apply base training consistently and frequently in order to keep pressure on those glycogen levels.  If you ride after a meal for 4 or 5 hours and then wait a couple of days for your next ride, it won’t work as well as riding 1.5 to 2.5 hours every day in a fasted state. 

It takes longer to deplete glycogen levels which are fully regenerated than when they are only 50% or 75% regenerated.


Coming back to high intensity training. We want to avoid damage.  High intensity sessions also help to deplete muscle glycogen but the force applied causes more damage to the muscle tissue and tendons.  You’ll notice this when issues start to pop up like knee pain, inflammation etc..  This needs to be avoided as it will force you to pause your training, something you may not want to do at first and make the damage even worse.  By sticking to base training, the force is low enough that any kind of micro damage has a good chance of healing while you keep training.

To take the boredom out of base training, you can have a small sprint session at the very end.  Short enough to not cause much damage compared to a full blown HIIT or SIT session and intense enough to cause a bit more of that muscle glycogen depletion.

When you get closer to competition, I’m in favor of the polarized training.  About 2 to 3 weeks in advance you can start adding in more high intensity training sessions (mimicking what you’ll experience during competition) and reduce the intensity of your base training to the lower part.  But don’t overdo it, be mindful about the increased damage so maximum 3 sessions per week and count race days as one of those sessions.  Purely to make more room for healing while at the same time allow for that adaptation to the high intensity.  After the competition, you can pick up base training again like before or if you now have a whole series of competition coming then you continue with that polarized approach, long low aerobic intensity with 2 to 3 high intensity training sessions and count competitions as a high intensity training.

The 2 days before competition day are all about healing damage so that you can perform at your strongest during the competition.

High Intensity adaptation

High intensity also causes muscle glycogen depletion and even faster than base training.  So the adaptation towards increasing mitochondrial mass will be triggered which is great.  In addition, it will also cause adaptation to the efficiency of the mitochondrion so that some of the energy that is lost will be reduced creating a higher aerobic yield in ATP production.

So why not apply high intensity training all the time?  We’ve already covered the damage part and the need to heal so we cannot apply it all the time.  Also the short life of this efficiency gain was covered.  During winter, if you take some time off training for 1 or 2 weeks, all those efforts are lost so only invest in it when you approach competition.

Because the efficiency is additive to your base fitness, you’d want to get that base fitness as high as possible so let it be the center of your training outside of competition. Efficiency is easily gained in a few weeks and then reaches a plateau so it doesn’t make sense to continuously invest in it when you don’t need it.


And lastly a few words on nutrition.

Nutrition after the base workout is important.  Not before.  Base training has nothing to do with performance, it is all about staying perfectly in that zone.  There’s no competition aspect to it, it doesn’t matter how hard you ride, what your wattage is, no comparison to others etc.. There’s only you and the flat steady monotone effort that keeps you in the designated heart rate zone.

The goal is to get to that muscle glycogen depleted. Next your body will want to adapt. It needs to build material.  Yes indeed, we all know that means protein are important.  But it is not just protein, muscle cells need to increase mitochondrial mass, nucleuses are donated from smooth muscle cells, hypertrophy requires expanding cell membranes, DNA needs to be duplicated for cell proliferation etc.. 

This means that in addition to dietary protein you also need fat, cholesterol and a whole bunch of relevant vitamins and minerals that are used in these adaptation processes. 

One crucial element I would like to highlight is zinc.  Insufficiency may lead to stunted adaptation.  Endurance athletes do risk having low levels due to the many hours sweating while food sources may not be optimal. 

Caution though, only take it if you suspect insufficiency (check with your doctor) because adding more when you are already adequately equipped will not lead to faster/more adaptation. But look for signs of deficiency rather than only relying on blood measurements. It reflects what is in the blood and not what is in the cells. There is a complex mechanism that causes other cells to release and distribute zinc via the blood stream when there is a shortage elsewhere so deficiency is not easy to spot with apparent sufficient levels but low levels in the blood are certainly to be corrected.


I hope this has generated enough insight to understand base training, no matter what type of endurance sport you do.

Knowing what to do and be consistent about doing it is key I guess in achieving goals no matter what those goals are.

– – – T H E – E N D – – –


5 responses to “Endurance training”

  1. I think the 2 and 4 mmol/L are arbitrary thresholds. They may hold for some people, but certainly not for everyone.
    For the aerobic threshold, the first deviation (+- 0.3mmol/L) from baseline is more accurate I think.
    For the second threshold, the ‘modified-D-max’ is proposed as a good way to determine LT2.

    Liked by 1 person

    1. It is a good point to raise because each endurance sport will have different thresholds as the muscle groups used are different and the intensity at which they are used are also different.

      For cycling, the 2 mmol is fairly accurate though. I have seen lots of data that fit this approach but there certainly can be exceptions. Below 2 mmol it tends to fluctuate so that it is unclear when exactly we reach this point. As you can see from the GCN guy, he started at 1.6 and it still dropped to 1.

      The 4 mmol is indeed much more of an approximation and probably will vary with fitness. It is a good ballpark figure though but if you are measuring lactate then you might as well go for the modified D-max method. In my case it puts me at 5 mmol which better fits my HR at 1 hour races going max effort. Thanks for bringing that up.


      1. “As you can see from the GCN guy, he started at 1.6 and it still dropped to 1.”

        Yes, so 1 is his baseline. And once he reaches +- 1.3, there is already a distinct shift in his physiology (fuel use, fiber type recruitment, …).
        So for this person, riding at 2mmol/L is an intensity that probably puts him well into the tempo zone.
        And for building your base / aerobic engine, that’s not where you want to be.

        You have to do a proper warm up (long enough + very low intensity), to reach your true baseline. And then start comparing your results relative to your true baseline.

        Lactate varies way to much from person to person (due to fitness level, stress, last food intake,…) to just use a fixed threshold. There are diabetic people that probably have 2mmol/L at rest. Whereas a very fit person, might have 0.5 at rest. 2mmol/L is something entirely different for these two examples.


      2. Do you have some data sources that show lactate levels before and after that warmup? I’m greatly interested in that because my examples do not really involve such a long enough warmup up.


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