Making Sense of Incremental Exercise Test Fat Oxidation Data

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Fat oxidation, and fat oxidation rates get talked about a lot in the world of endurance sports and triathlon. Of course, being able to break down fats for use as fuel during prolonged exercise is an important determinant of performance in ultra-endurance events like Ironman. Our other fuel source – carbohydrates – are relatively limited and are depleted during prolonged or intense exercise. Being able to use fat effectively reduces the rate at which we burn through our carbohydrate stores, and thus delays the fatigue associated with depleted carbohydrate stores, or ‘hitting the wall’.

Given that having a robust capacity to make use of fat stores during exercise is important, I quantify it in the lab with the athletes I work with. We can measure fat oxidation rates during exercise using a technique called indirect calorimetry, which involves the collection of expired air, and we typically do this across a range of exercise intensities in what’s called an incremental exercise test. We usually do these tests in well-rested athletes, who come to the laboratory after an overnight fast; this is purely so the measurements are standardised and can be compared against normative values (3). These tests generate a curve that shows fat oxidation rates at different exercise workloads. We blogged about interpreting peak fat oxidation rates previously.

However, such data can also often be used out of context. Even to prescribe carbohydrate feeding regimes during racing. But how should such data be used? In this blog, I am going to discuss how I interpret the fat oxidation data generated in these tests effectively.

The curve above shows fat oxidation data generated in an incremental exercise test. Notice how the fat oxidation rate rises to a peak at 270 W, and then declines, with very little fat oxidation occurring at 375 W.

So, how do we interpret these curves?

The first thing that I do is look for the peak and the shape of the curve. By peak, I mean the highest rate of fat oxidation observed during the test, which is sometimes abbreviated as PFO or MFO to indicate peak or maximum fat oxidation rate (3, 4). By shape, I mean:

1. The intensity at which the peak – or near peak – rates occur, which is sometimes called Fat_max (1, 5)
2. The intensity at which fat oxidation really starts to drop off
3. The fat oxidation rates observed at and around the first threshold (VT₁/LT₁)

Collectively, the peak and shape of the curve tells us not only about an individual’s capacity for fat oxidation during exercise, but also how effectively they are able to oxidise fat at power outputs and speeds relevant to competition and training scenarios. Both are important parts of the metabolic picture we are looking to build during profiling assessments.

More specifically, fat oxidation data measured during incremental tests tells you if an athlete is likely to metabolise fat at low, moderate, or high rates during real-world training scenarios. It does not tell you exactly what rate you are going to burn fat during training sessions, and therefore the exact rate of fat and carbohydrate use at a specific workload. There are two main reasons for this:

• An incremental test lasts ~25 min, training sessions usually last much longer; fat oxidation tends to increase over time during prolonged exercise, even at a consistent power output (6).
• Like I said, we tend to perform these tests after an overnight fast for standardisation; pre-exercise feeding impacts fat oxidation rates (2). Most training and competition takes place not only after breakfast, but also with some feeding during the session itself.

We can say that fat oxidation data derived from an incremental test performed after an overnight fast tells us if an athlete is likely to burn fat at low, moderate, or high rates during real-world sessions based on a study we published in 2022 (4). We found that PFO had a strong relationship with fat oxidation rates during a long ride at 80% of VT₁, even though the ride took place after breakfast and with carbohydrate ingestion during exercise. The fat oxidation rates were much lower during the long ride with carbohydrate feeding compared to the peak of the fasted test, but we found that those with higher PFO had higher fat oxidation rates during the long ride. Therefore, the incremental test data distinguished the low, moderate, and high fat burners during prolonged exercise.

Let’s look at a couple of example curves, and how I would interpret them.

Here is incremental test fat oxidation data from two athletes. The two curves have very similar shapes, but Athlete A has a much higher peak, and higher rates at all intensities, including at typical training intensities (200-300 W). We can say that Athlete A is a high fat burner, and that Athlete B is a moderate fat burner. We can also say that Athlete A is likely to make use of fat at much higher rates than Athlete B during real-world training sessions. Here is more incremental test fat oxidation data from two athletes. The curves have similar peaks, but different shapes. We can say that the athletes have similar capacities for fat oxidation during exercise, but that Athlete A is likely to make use of fat at higher rates during real-world training sessions, as both athletes do the bulk of their training between 200 and 300 W.

In summary, fat oxidation rates measured during incremental tests performed after an overnight fast…

1. …tell you about an athlete’s overall capacity to metabolise fat during exercise
2. …tell you if an athlete is likely to burn fat at low, moderate, or high rates during real-world training and competition
3. …don’t tell you exactly what rate an athlete is going to burn fat during real-world training and competition

EndureOn!

References

1. Achten J, Gleeson M, Jeukendrup AE. Determination of exercise intensity that elicits maximal fat oxidation. Med Sci Sport Exerc 34: 92–97, 2002.
2. Achten J, Jeukendrup AE. The effect of pre-exercise carbohydrate feedings on the intensity that elicits maximal fat oxidation. J Sports Sci 21: 1017–1024, 2003. doi: 10.1080/02640410310001641403.
3. Maunder E, Plews DJ, Kilding AE. Contextualising maximal fat oxidation during exercise: Determinants and normative values. Front Physiol 9: 1–13, 2018. doi: 10.3389/fphys.2018.00599.
4. Maunder E, Plews DJ, Wallis GA, Brick MJ, Leigh WB, Chang WL, Stewart T, Watkins CM, Kilding AE. Peak fat oxidation is positively associated with vastus lateralis CD36 content, fed‑state exercise fat oxidation, and endurance performance in trained males. Eur J Appl Physiol 122: 93–102, 2022. doi: 10.1007/s00421-021-04820-3.
5. Randell RK, Rollo I, Roberts TJ, Dalrymple KJ, Jeukendrup AE, Carter JM. Maximal fat oxidation rates in an athletic population. Med Sci Sports Exerc 49: 133–140, 2017. doi: 10.1249/MSS.0000000000001084.
6. Watt MJ, Heigenhauser GJF, Dyck DJ, Spriet LL. Intramuscular triacylglycerol, glycogen and acetyl group metabolism during 4 h of moderate exercise in man. J Physiol 541: 969–978, 2002. doi: 10.1113/jphysiol.2002.018820.