Debbie Potts Coaching

Fat Oxidation & The Endurance Athlete Review


  1. Lipids as a fuel source for energy supply during submaximal exercise originate from subcutaneous adipose tissue derived fatty acids (FA), intramuscular triacylglycerides (IMTG), cholesterol and dietary fat.
  2. These sources of fat contribute to fatty acid oxidation (FAox) in various ways.
  3. The regulation and utilization of FAs in a maximal capacity occur primarily at exercise intensities between 45 and 65% VO2max, is known as maximal fat oxidation (MFO), and is measured in g/min.
  4. Fatty acid oxidation occurs during submaximal exercise intensities, but is also complimentary to carbohydrate oxidation (CHOox).
  5. Due to limitations within FA transport across the cell and mitochondrial membranes, FAox is limited at higher exercise intensities.
  6. The point at which FAox reaches maximum and begins to decline is referred to as the crossover point.
  7. Exercise intensities that exceed the crossover point (~65% VO2max) utilize CHO as the predominant fuel source for energy supply.
  8. Training status, exercise intensity, exercise duration, sex differences, and nutrition have all been shown to affect cellular expression responsible for FAox rate.
  9. Each stimulus affects the process of FAox differently, resulting in specific adaptions that influence endurance exercise performance.
  10. Endurance training, specifically long duration (>2 h) facilitate adaptations that alter both the origin of FAs and FAox rate.
  11. Additionally, the influence of sex and nutrition on FAox are discussed. Finally, the role of FAox in the improvement of performance during endurance training is discussed.

Nutritional strategies for promoting fat utilization and delaying the onset of fatigue during prolonged exercise


  1. Carbohydrate ingestion before and during endurance exercise delays the onset of fatigue (reduced power output).
  2. Therefore, endurance athletes are recommended to ingest diets high in carbohydrate (70% of total energy) during competition and training.
  3. However, increasing the availability of plasma free fatty acids has been shown to slow the rate of muscle and liver glycogen depletion by promoting the utilization of fat.
  4. Ingested fat, in the form of long-chain (C16-22) triacylglycerols, is largely unavailable during acute exercise, but medium-chain (C8-10) triacylglycerols are rapidly absorbed and oxidized.
  5. We have shown that the ingestion of medium-chain triacylglycerols in combination with carbohydrate spares muscle carbohydrate stores during 2 h of submaximal (< 70% VO2 peak) cycling exercise, and improves 40 km time-trial performance.
  6. These data suggest that by combining carbohydrate and medium-chain triacylglycerols as a pre-exercise supplement and as a nutritional supplement during exercise, fat oxidation will be enhanced, and endogenous carbohydrate will be spared.
  7. We have also examined the chronic metabolic adaptations and effects on substrate utilization and endurance performance when athletes ingest a diet that is high in fat (> 70% by energy).
  8. Dietary fat adaptation for a period of at least 2-4 weeks has resulted in a nearly two-fold increase in resistance to fatigue during prolonged, low- to moderate-intensity cycling (< 70% VO2 peak).
  9. Moreover, preliminary studies suggest that mean cycling 20 km time-trial performance following prolonged submaximal exercise is enhanced by 80 s after dietary fat adaptation and 3 days of carbohydrate loading.
  10. Thus the relative contribution of fuel substrate to prolonged endurance activity may be modified by training, pre-exercise feeding, habitual diet, or by artificially altering the hormonal milieu or the availability of circulating fuels.
  11. The time course and dose-response of these effects on maximizing the oxidative contribution of fat for exercise metabolism and in exercise performance have not been systematically studied during moderate- to high-intensity exercise in humans.

Test and Not Guess your Fat Oxidation Rates

Metabolic Efficiency testing on the treadmill






Pre-exercise carbohydrate and fat ingestion: effects on metabolism and performance


A key goal of pre-exercise nutritional strategies is to maximize carbohydrate stores, thereby minimizing the ergolytic effects of carbohydrate depletion. Increased dietary carbohydrate intake in the days before competition increases muscle glycogen levels and enhances exercise performance in endurance events lasting 90 min or more. Ingestion of carbohydrate 3-4 h before exercise increases liver and muscle glycogen and enhances subsequent endurance exercise performance. The effects of carbohydrate ingestion on blood glucose and free fatty acid concentrations and carbohydrate oxidation during exercise persist for at least 6 h. Although an increase in plasma insulin following carbohydrate ingestion in the hour before exercise inhibits lipolysis and liver glucose output, and can lead to transient hypoglycaemia during subsequent exercise in susceptible individuals, there is no convincing evidence that this is always associated with impaired exercise performance. However, individual experience should inform individual practice. Interventions to increase fat availability before exercise have been shown to reduce carbohydrate utilization during exercise, but do not appear to have ergogenic benefits.

What is your maximal fat oxidation rate?Episode #184: The IRONMAN World Championship | The WHOLE Tri-Athlete Podcast by Debbie Potts

Test and not guess.

We have discussed in detail that having a robust capacity to use our fat energy stores to support exercise is important for long-distance triathlons. That is because our stored fat is effectively unlimited in the context of exercise, even very long-duration exercise in very lean athletes. In contrast, our stored carbohydrate reserves are finite and will be depleted to very low concentrations after exercise of sufficient length and intensity. To put this in perspective, a lean, 70-kg triathlete with 10% body fat has at least enough energy in their 7 kg of stored fat to complete more than Ironman triathlons back-to-back (3). Of course, those are theoretical Ironmans; the point I am making here is that whilst depletion of stored carbohydrates can lead to the fatigue we recognise as ‘hitting the wall’, we don’t slow down in an Ironman because we run out of fat energy (4).

As an exercise physiologist working in applied practice, I regularly test the fat oxidation capacities of endurance athletes in the laboratory during routine physiological profiling assessments. These data, particularly the maximal fat oxidation (MFO) rate observed during an incremental exercise test, provide us with useful information about the athlete in front of us. However, I feel that the data generated in these assessments is often misinterpreted. Therefore, this blog aims to explain how fat oxidation data is generated in standard physiological profiling assessments, what it tells us, and what it doesn’t tell us.

How the test works

  1. Fat and carbohydrate oxidation rate data is generated through the collection of expired gases using a technique called indirect calorimetry. I
  2. By collecting the air an athlete is breathing out, we can estimate the rate at which they consume oxygen and produce carbon dioxide at the whole-body level.
  3. As carbohydrates and fat require different amounts of oxygen and produce different amounts of carbon dioxide in their breakdown, we can use these data to estimate the contributions of carbohydrates and fat to overall energy expenditure during the collection period.
  4. We collect expired gases during a standard incremental exercise test, in which the power output or running speed starts at a very easy intensity but progresses to become harder and harder.
  5. We can calculate carbohydrate and fat oxidation rates across a range of different intensities.
  6. It is important that the stages are long enough for the expired gas data to stabilise to produce valid estimates; we prefer to use stages of at least 4 min in length.
  7. We expect to see fat oxidation rates initially rise as exercise intensity increases from low to more moderate intensities, peaking near the first threshold before declining at higher intensities, where we become more dependent on carbohydrate metabolism to fuel exercise (7).
  8. We plot and assess this relationship as a key outcome measure in our routine tests (5).
  9. Fat oxidation profiles can look very different between athletes (see below). 

Figure 1: The graph above shows fat oxidation plotted against power output for two high-level cyclists during an incremental exercise test. You’ll notice that Athlete A (yours truly ;)) oxidises fat at much higher rates than Athlete B.

What the data doesn’t tell you.

  1. We generate and plot carbohydrate and fat oxidation rates for various power outputs or running speeds using indirect calorimetry data collected during routine physiological profiling assessments.
  2. These data are super useful, but it is important first to recognise what the data doesn’t tell you.
  3. The data doesn’t tell you exactly how much carbohydrate and fat you’ll be oxidising at a given intensity during racing or even training; fuel use is impacted by many factors, including recent nutrition, environmental temperature, and exercise duration (8).
  4. These tests are, for example, often conducted after an overnight fast, which results in higher fat oxidation rates than exercise performed after breakfast (1).
  5. They’re usually conducted in air-conditioned laboratories, where the cool temperature will likely result in higher fat oxidation rates than during exercise in hot environments – like Kona (2).
  6. Fat oxidation rates will also progressively increase over time during long-duration exercise as your carbohydrate availability gradually dwindles (9).
  7. The data therefore cannot tell you exactly how much carbohydrate you’ll need to take on in drinks and gels to mitigate your chances of bonking with any real accuracy.
  8. That is, if your carbohydrate oxidation rate is 1.6 grams per minute at 200 W during a physiological profiling assessment, that does not mean you will be metabolising carbohydrates at a rate of exactly 1.6 grams per minute throughout a four-hour long training ride, or during your competition.

What the data does tell you.

  1. For me, a controlled assessment in the laboratory, where the athlete conducts the test in a well-rested state after an overnight or > 4-hour fast, gives us a good indication as to whether they have a robust, moderate, or poor capacity for fat oxidation during exercise.
  2. The primary value generated in the test I use to make this assessment is the highest value, or the maximal fat oxidation (MFO). In the graph above, this would be ~1.4 grams per minute for Athlete A and ~0.55 grams per minute for Athlete B.
  3. We previously published normative values for MFO (5), which you can view open-access here.
  4. This assessment – marking an athlete as a robust, moderate, or poor fat burner based on MFO – is fair, as we recently reported a very strong relationship between MFO, measured during a fasted, incremental cycling test, and the average fat oxidation rate during a two-hour long ride taking place after breakfast, with carbohydrate feeding throughout (6).
  5. So, we can say those with the highest MFO will metabolise more fat during exercise in more ecological-valid scenarios reflective of real-world training and competition.
  6. By having a robust capacity to make use of fat as a fuel source during exercise reduces the burden placed on those finite stored carbohydrates and therefore maintains carbohydrate availability and delays fatigue.
  7. The study I mentioned asked the participants to perform a 30-min time-trial after the two-hour long ride, and positive relationships were observed between MFO and time-trial performance.
  8. Perhaps more importantly, adding MFO to traditional endurance performance parameters – V̇O2max, thresholds, and efficiency – improved the fit of these models and better-explained performance.
  9. We took this as evidence that MFO is a useful endurance performance parameter, supporting its consideration in physiological profiling assessments (6).

Alongside the peak value, I also look at the fat oxidation rate at and close to the first threshold – VT1, LT1, or the moderate-to-heavy transition.

  1. That tells us whether the athlete retains that robust/moderate/low-fat oxidation capacity at ‘competitive’ type intensities likely to be undertaken during a long-distance triathlon and most training sessions.
  2. Some athletes display decent peaks at very low intensities but retain little fat oxidation capacity at more steady intensities around that first threshold. I, therefore, take this under advisement when assessing the individual athlete’s profile.

Collectively, these data inform me of a few key takeaway points:

  • How aggressive the athlete might need to be in terms of carbohydrate feeding. While we recommend consuming carbohydrates at decent rates for all athletes during competition – even those with robust MFO of >1 gram per minute – we might need to be more aggressive with poorer fat burners and consider things like glucose-fructose blends.
  • But as discussed before, there are better solutions for consistent performance and better health.
  • How the athlete has responded to diet and training interventions. If we have gone through a block of base work in which we hope to see an increased capacity for fat oxidation during exercise, we can assess this using these data.
  • How the athlete is likely to perform. As indicated above, alongside traditional metrics, MFO helps build models of endurance performance.


Therefore, the main message I am trying to get across in this blog is this: When looking at fat oxidation profiles generated during incremental exercise tests, try not to overinterpret the data; instead, use it to determine if the athlete is a robust, moderate, or poor burner during exercise, and how that has changed over time. You can then use that knowledge to inform your future training objectives.

Peak Fat Oxidation Rates:  What does this tell us?

What is your Fat Ox Rate during running or cycling?

Maximal fat oxidation rate (MFO), and Fat Max are important considerations for Long Distance Triathlon. With the former being, your maximal ability to oxidise and use fat as fuel, and the latter the power/speed at where MFO occurs. Check out our previous blogs here for a little more information.

In the decision tree, we ask questions related to the following factors which have been shown to be key regulators of fat and carbohydrate metabolism during exercise:

  • The carbohydrate content of your diet.
  • Whether you do training sessions in a fasted state and how you fare on them.
  • Your finishing time in an Ironman; and
  • Your sex.


The carbohydrate content of your diet

As the carbohydrate content of your diet is the most critical regulator of substrate metabolism during exercise, this question is extremely important. We ask “Do you deliberately eat a diet low in carbohydrate?” By this, we mean a diet containing less than ~130 grams of carbohydrate per day. This is unlikely to be achieved without concerted effort, 130 grams of carbohydrate are consumed in individuals making a concerted effort to avoid them for example.

Whether you do training sessions in a fasted state and how you fare on them

You are likely to have lower rates of fat oxidation during exercise if you are eating a diet high in carbohydrate, do not perform fasted training sessions, or struggle to finish fasted training sessions without additional fuelling.

Your finishing time in an Ironman

The last question relates to your real or predicted Ironman finishing times, as performance standard or aerobic fitness has a relationship with fat oxidation rates in people eating mixed or high carbohydrate diets.



It is important to note that these values are estimates, and so ranges are provided for all possible answer combinations.


We recommend that you enter a value at the higher and lower end of your predicted range into our online spreadsheet to cover a likely range of individualised metabolic responses during an Ironman Triathlon.

Right Time. Right Fuel. 






My PhD student Jeff Rothschild and I recently performed a meta-regression, a statistical analysis of all the studies in this area, and found that diet around training was more important than carbohydrate intake during exercise and, interestingly, that the duration of exercise had a particularly large and positive effect on fat oxidation (10).

This shows how a lower carb and higher fat diet around training will help with fat oxidation; indeed, habitual fat intake was a big driver of fat oxidation during exercise (10).

Such data is one of the main incentives around the “Life” range at SFuels, making everyday lower-carb living easier for athletes.

…. given that carbohydrate intake during exercise can extend how long we can stick out there (11), we might recommend restricting carbohydrate intake during the early stages of endurance training.

if this restriction begins to limit our ability to “go long”, then some carbohydrate supplementation in the later stages should be recommended.

exercise duration is the most potent stimulant for fat oxidation; this is above specific macro-nutrient manipulation.

In other sessions, such as high-intensity interval training sessions targeted at producing the highest possible power output and increasing our V̇O2max, we are less worried about fat oxidation.

In these sessions, we want to provide the muscle with the carbohydrates it needs to maximize high-intensity performance, so we might add some to our pre-exercise meal to ensure we are properly topped up.

This approach is supported by another of our recent meta-regressions, which found that the activation of the protein AMPK – a key signal for training adaptation – doesn’t appear to be blunted by extra carbohydrate availability around very high-intensity sessions; it appeared that other signals, like lactate accumulation and the degree of acidosis generated by the exercise, overrode those signals and are the main driving factors (9).

High Carb or Low Carb Diet

As a coach, I have a reputation for being a low-carb promoter. But, in reality, this is not always the case. My approach is very specific to the athletes (pro vs age group), their personal interest in the low-carb diet, and the amount of training volume they do habitually or acutely.

For example, I coach four high level pros, and only one of these is specifically low carb, with the rest taking a Right Fuel, Right Time approach.

As we now know, improving fat oxidation is the most important outcome here, and the Right Fuel, Right Time approach can be very effective at producing the desired result.

The data below shows the fat vs carbohydrate oxidation of Chelsea Sodaro in the time we have been working together. Notice the big rightward shift in the cross-over point. This shows how she can now use fat oxidation much more at higher power outputs. This was achieved by only taking the Right Fuel, Right Time approach.

Figure 1: Changes in % of fat vs carbohydrate oxidation (y-axis) at increasing power output (x-axis) pre vs post Right Fuel Right Time. Power output pre and post are identical, but data is protected.

When it comes to the carbohydrate periodization, the research mainly focuses on high carbohydrates diets, with specific carbohydrate restrictions around certain training sessions. Personally however, I believe this is fundamentally wrong approach for the majority of athletes. The better approach is to maintain a lower-carb diet, with specific carbohydrate injections around certain training sessions. After all, following polarized or pyramidal training intensity distribution means most of our training is performed at a low “fat-burning” intensity; and maintaining a lower carb diet habitually means dietary fat content will be increased (increasing fat oxidation (9)). Last this is also far superior for overall athlete health, wellbeing and body weight maintenance (5).

How to implement carbohydrate periodization according to your training 

Let me now distil that down into a few key practical recommendations, summarised by session type (Endurance, Lower Tempo, Upper Tempo, Threshold, VO2max, and Anaerobic Capacity) in the Table below:

  1. Restrict carbohydrate intake around and during lower intensity activity with low glycolytic demand.
  2. Don’t be afraid to ingest carbohydrates during very long duration sessions (> around 90-120 min) targeted at improving fat oxidation, as this will help extend the duration of those sessions.
  3. During longer tempo-based session when intensity is clamped (e.g., between the first and second threshold), there may be more adaptation to be gained by restriction carbohydrate beforehand. Starting the session with lower muscle glycogen
  4. Fuel more intense sessions with a few extra carbohydrates to support training quality and maximise the stimulus for adaptation.
  5. Ensure adequate overall calories are always ingested, even and perhaps particularly on days when you are having lower carbohydrate intake.

So, give it try, and check out SFuels for all your Right Fuel, Right Time nutritional needs!


Learn more about fuels to match your training and racing plans at SFuels Go Longer here


Endurance racing: What is Top-End Intensity?
Racing intensity for most endurance athletes’ (even some pros) for a full Ironman is typically in the 77-82% (FTP) range, and upto 85% for 70.3 distances., suggests that full Ironman racing is typically performed at ~70% of Vo2Max, 75% for 70.3 distances, upto 85% for Olympic distances, and likely higher for 5000m track racing.

So the first point is if the race duration is over 4 hours (i.e. Ironman, ultras, and 70.3s etc.) then realize, that your race intensity is mostly supported by the aerobic system, of which fat is the pre-dominant fuel source.

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