Understanding Fat Oxidation in Endurance Sports I

A very misunderstood topic is fat oxidation. Either it's “keto/fasting/low carb", "trust me bro" advice out there, or a misinterpretation of the role of fat burning in performance with endurance sport. This article is going to explain the basics of fat oxidation. The hope is to provide broader brush strokes around the nuances of fat oxidation, and the definitions of the concepts in the field at the moment, and empower the athlete to new performance levels.

Let's answer the following questions:

Where does Fat Oxidation take place in the muscle cell?
What is the CrossOver Point in fat metabolism?
What are Peak Fat Oxidation and FatMax?
Practical Steps to Calculate Peak Fat Oxidation outside a laboratory setting.

Where does fat oxidation take place in the muscle cell?

Fat is primarily stored in adipose tissue and once acted upon by certain hormone signals, is liberated into free fatty acids and glycerol. In Fig 1 below, the free fatty acid pathway is to be activated into (fatty) acyl-CoA (yes that's the correct spelling) where through a process called Beta Oxidation, the important intermediary called Acetyl- CoA is formed that can then enter the Krebs Cycle (see Fig 2). Acetyl CoA fuels the Krebs cycle and allows the production of ATP (the body's energy currency) via the Electron Transport Chain.

Fig 1. Pathway of a free fatty acid into mitochondrion (Powers and Howley, 2004)

Fig 2. Formation of Acetyl-CoA from fuel sources. Notice Fatty Acids (Powers and Howley, 2004)

Now that we understand energy metabolism to a greater extent, let's move to understand key specific terminology

Crossover Point in Fat Metabolism

Originally a phrase coined by Brooks and Mercier, the Crossover point is a model that shows how the body increasingly shifts from primarily fat metabolism at low intensities to greater carbohydrate usage at higher exercise intensities (see Fig 3 below). As shown, the crossover point typically happens around 65% of VO2max. Training that enhances mitochondrial density, aerobic enzymes, and capillary density, facilitates a state where slowed additional blood flow (capillary density) close to the energy organelle (mitochondria) will mean a greater fatty acid uptake and potentially shift the crossover point further to the right of the graph.

Fig 3. Crossover Point in Exercise Metabolism (Griffen, 2017)

In some cases, athletes do not have a crossover point as seen in Fig 4 below. Increases in catecholamines (epinephrine and norepinephrine) will reduce blow flow to adipose tissue and therefore less fat will be liberated and used in the blood. Another reason is increases in proton production from high-intensity efforts do inhibit carnitine which is the main transporter of fatty acids into the mitochondria.

Fig 4. No crossover point observed (Griffen, 2017)

Peak Fat Oxidation (PFO) and FatMax

Peak Fat Oxidation (PFO) is the maximum amount of fats that the body uses and oxidizes in a minute and is expressed in g/min.

Below is a table showing average g/min of fat oxidation rates (Maunder and Plews, 2018):

Rider Level	g/min AVERAGE
Pro Tour Male	0.67
Trained Male	0.53
Recreationally Active Lean Male	0.46
Recreationally Active Female	0.35
Overweight Male	0.28
Overweight Female	0.16

Fatmax is the intensity at which peak fat oxidation is displayed. This can be a power output where the highest fat oxidation rate occurs. In general terms, the higher the power output at which maximum fat oxidation occurs, this tends to be an advantageous adaptation in endurance athletes especially the longer the event becomes and glycogen stores are challenged. Fat being the primary fuel source helps spare essential muscle glycogen (as will be discussed in blog posts over the next few weeks).

Practical Steps to Calculate Peak Fat Oxidation outside a laboratory setting

FTP

In previous blog posts the discussion around testing beyond one metric, namely the 20-minute test as an example has brought us to a point now where those advanced metrics help to create some broad brush strokes about estimating FatMax. In general as discussed above, fatmax occurs around 60% of an athletes VO2max.

E.g. Max is a 70kg rider and has an FTP of 300w (4.4w/kg) and has a 5min power of 375w (5.5w/kg). 5-minute power is synonymous with VO2 max. Therefore 375 (5min power) x 0.6 (level of Fatmax) = 225w.

Lactate Testing

Nowadays, getting a lactate meter is cost-effective and relatively easy to do for the average athlete which allows for testing outside a laboratory setting. And so bringing back up what was discussed above, increased proton ion production (increased lactate is a surrogate marker) means that the transporters, carnitine, which bus fatty acids into the mitochondria become inhibited or are not able to work as efficiently. This means that the lowest point before lactate begins to appear in the blood can be assumed to be where fatmax is at its highest level

These two ideas give an approximation. If the athlete has access to the Gas Expired Test then it's best to utilize this for the most accurate results.

In the next blog post, building on fat oxidation, examples of why enhanced fat oxidation is important for preserving muscle glycogen and its implications for performance.

Any help with further questions, discussions, consultations, or coaching? Please don't hesitate to reach out to darrin@thethreshold.coach

@darrinjordaan

@wattfarming

Train Hard and Prosper!

Darrin Jordaan

MSc (Med) Biokinetics WITS

HMS (Hons) Sports Science UP

BK 0016934

CSCS

UCI Level 1 Cycle Coach

IronMan certified coach

References

Maunder E, Plews DJ, Kilding AE. Contextualising Maximal Fat Oxidation During Exercise: Determinants and Normative Values. Front Physiology. 2018. May 23;9:599
Powers SK, Howley ET. Exercise Physiology. 5th Edition. McGraw Hill 2003