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  • Jorge Martinez

Training is NOT only about improving FatMax

On this 3rd entry of this blog series, I’m going to focus on why, while improving your Fat utilization at different intensities is very important for endurance sports, like everything in life, there is an optimal balance between better fat oxidation and other important adaptations.

First of all, why is improving your fat utilization so important? As mentioned on the 1st part of this series, focusing on something like FatMax will allow you to increase your mitochondria density of slow twitch fibers which will enable you supply more energy from fat to fulfill the total energy demands for a given power/pace.

In addition, improving your LT1 will allow you to train Slow twitch fibers to clear lactate by shuttling it from fast twitch fibers and re-using it as fuel. Training between LT1 and LT2 will allow you to adapt some of the Fast Twitch IIa to behave as slow twitch fibers (in a general sense).

Finally, by improving your max aerobic capacity (VO2max) you will increase adaptations such blood capillaries to deliver more oxygen rich blood to working muscles to utilized in energy production, stroke volume pumping more blood per heart beat and more.

When we talk about energy demands specifically, to ride at “x” watts or run at “y” pace, your body will utilize mainly Slow twitch and fast twitch IIa fibers for efforts below LT2 and it need to cover those energy demands primarily from 2 main sources of fuel: fat and carbohydrates.

For instance, a 150-pound athlete at 10% body fat can store ~15 pounds or ~ 52,000 kcal. On the other hand, the same athlete may be able to store up to ~3000 kcal of carbs (0.85 pounds) with excellent fitness. Also, since carbs are the preferred source of fuel for the brain, we can only use “a part” of our reservoir, thus, you may be able to utilize about ¾ of that or ~2250 kcal.

Therefore, conserving as much carbs is imperative to be able to complete a 70.3 or and Ironman as fast as possible within your physiological limitations. In other words, is about the balance between: what we have in the carbs tank, what we can intake (carbs per hour) and how much/fast we use energy at a given power/pace distributed among 3 sports.

With that in mind, below is graph for 3 athletes of the similar weight all racing Ironman and all 3 with with different metabolic profiles:

1. One following wanting to Kona qualify. To improve fat oxidation he start following a Keto diet (KT) consuming <40g carbs a day. Since your body burns what it has (i.e. eat more fat = higher fat burning). And he focused mostly on the fat “burning” side of the equation improving fat combustion significantly.

2. A Top Age Grouper (TAG) focused on improving fat utilization while at the same time, improving lactate clearance and aerobic capacity.

3. A Pro Triathlete (PRO) both focused on peak performance.

This first graph shows the blood lactate accumulation at various watts and when each athlete achieve LT1 and LT2.

As quick refresher, LT1 is the point at which your blood lactate concentration increases by 1 mmol/L above your baseline and it should be around the point which your body utilize around the same rate of fat vs carbs (~50%/50%). However, depending on your metabolic profile this is not always the case. And the split may be more like 45%/55% or even 40%/60%. LT1 is an important marker because anything below it indicates the energy required for a given power/pace, will be supplemented from fat sources at a higher rate.

LT2 (often called FTP or Critical Power) is the point when we clear lactate at the same rate it accumulates and anything below it suggests what’s your aerobic “capacity” to efficiently fulfill energy demands for a given duration, power, pace without significantly accumulating waste products like hydrogen ions that may hinder muscle contraction or increase fatigue onset.

Anything above LT2 suggests we could only meet the energy demands for shorter periods of time (i.e. a 5 or 20 min max effort). And, anything below LT2 we can in theory sustain for longer periods of time depending on how close to LT2 we are exercising at (i.e. racing at 70.3 power/pace)

Anyway, the main thing to notice on the graph is how the KT, TAG and PRO have a similar lactate curve up until 225w (LT1 for KT and TAG) and past that, each curve starts diverging from one another. The KT accumulates lactate at a faster rate hitting LT2 at 250w. The TAG accumulates lactate slightly slower hitting LT2 at 280w and finally, the PRO keeps lactate accumulation rather controlled until ~270w and hitting LT2 at 380w.

Since lactate accumulation can be a proxy for carb utilization (in a generalized way), we can begin to see how the metabolic picture for each athlete is different. Although the KT athlete may have a great fat utilization similar to the pro up until ~200w, the TAG seems to be able to sustain a degree of fat utilization for a longer period (up until ~250w), while the PRO can sustain this the longest.

Below are the actual fat burning rates per minute for each athlete at various intensities. You notice the KT athlete has the highest fat burning rate of all peaking at 8.1 kcal/min at 205w while the TAG has the lowest at the same intensity. Yet, at 225w (LT1 for both KT and TAG athletes), the KT fat burning rate drops significantly while the TAG continues to burn almost as much fat as he/she did at 205w. And the PRO burns slightly more at 280w

On the next graph you can see how the KT athlete have a significant higher fat utilization peaking 205w but thereafter, the fat utilization drops faster out of the 3 athletes. The TAG “only” achieves a fat utilization peak of ~6.5kcal/min at the same power, but, can continue to sustain a similar fat burning at 225w. Finally, the PRO also hits a max fat utilization of ~7.3kcal/min but he/she sustain a greater fat burning rate for a greater range (from 205w to 280w)

This example provides a clear visual representation as to how FatMax training is just one piece of the metabolic puzzle and the goal is not simply about “burning” more kcal/min of fat. Instead, it’s about striving for the optimal energy expenditure curve in which you can burn a bit more fat AND sustain such utilization rate for a wider range. In other words, your energy utilization curve shouldn’t just be about having the highest fat burning peak, but also, is to be able to sustain fat utilization at various intensities resulting in a wider curve.

And bringing this back to your training. On the 1st blog I mentioned that for TAG athletes racing Ironman tend to have a fat utilization rate of ~6-7kcal/min and PROs 7> kcal/min. At the same rate, TAG tend to have a total energy utilization of around 12-14 kcal/min and race around 3-3.5 w/kg while PRO utilize ~15+ kcal/min and race ~4-4.5 w/kg.

Based on our 3 athletes, the KT would need to ride at ~225w to get to the 12kcal/min range (~3.25 w/kg competitive range) however due to the inability to sustain higher fat burning rates as the intensity increases & the inability to efficiently use carbs suppling a greater percentage of that from carbs, this would be unrealistic.

Considering our limited carb reservoir, and the fact he/she will require energy to cover the swim, bike, run and transitions, well, the KT athlete in spite of a superior “fat burning” peak would struggle at this power (in spite of proper fuel intake) and therefore 205w would be a more realistic goal.

The TAG on the other hand could confidently race around 225w as he/she could sustain a higher fat burning rate at said intensity to properly fulfil the energy demands for the race (with proper fuel intake). Finally, the PRO with their superior metabolic profile can manage to race at extraordinary intensities (4-4.5 w/kg) despite a higher energy expenditure (~15kcal/min)


  1. If your goal is to optimize your metabolic profile for a faster 70.3 or Ironman, you must determine what are the areas that you need to target in order to improve said profile as a whole (i.e. improve your limiters, maintain your strengths)

  2. No training approach should focus only on a single marker whether it’s FatMax or FTP or other as this will limit your overall fitness gains.

  3. Like most things in life, a balance training approach may be needed to achieve your best results. That includes doing long/mid/short/easy/moderate/hard sessions with a focus on limiters early on and switching to specific race needs later on.

  4. In general, a Keto diet (consuming <50g of carb of less per day) may not be sustainable for an athlete *if* peak performance is the primarily goal. In this case, the KT athlete switched to a LCHF diet (more below), increased carb intake and switched focus to improve LT1/LT2 more while maintaining a solid (yet lower) fat burning rate and in fact, he/she ended up achieving the Kona goal. I'll share the info in a future post.

  5. An alternative diet of value for some athletes for various reasons (i.e. health) is the Low Carb Higher Fat diet (LCHF). In general, this means consuming ~10-30% of daily kcal from carbs. However, if your goal is also optimal endurance performance, you'll need to adjust it based on energy demands of each training cycle. i.e. less carbs during FatMax focus, and add more carbs for a LT1/LT2 focus.

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