Understanding Fat Oxidation III: Achieving Higher Fat Oxidation

Previously, in two blog posts (https://www.thethreshold.coach/single-post/optimizing-fat-oxidation-for-peak-cycling-performance-a-comprehensive-guide) and (https://www.thethreshold.coach/single-post/understanding-fat-oxidation-and-athletic-performance-ii-practical-insights-and-recent-studies) we explored fat oxidation and its importance when it comes to endurance sport

Here is a summary so far:

The need to preserve muscle glycogen

It is crucial to preserve muscle glycogen levels because depletion can lead to fatigue and a decrease in performance. Athletes should aim to boost their fat oxidation capacity to better conserve this finite energy source, with muscle stores totaling around 2400kcal and the liver holding a mere 375kcal. By maintaining glycogen levels, athletes can guarantee they have enough of this essential fuel for critical race moments. Additionally, enhancing fat oxidation helps athletes reduce the amount of glucose they need to carry and may even help prevent gastrointestinal issues.

Cross Over Point

Brooks and Mercier originally introduced The term Crossover point to illustrate the transition in the body's energy metabolism from predominantly utilizing fat at lower exercise intensities to relying more on carbohydrates at higher intensities (refer to Fig 1 above). Typically occurring around 65% of VO2 max, the crossover point can be shifted to the right of the graph through training methods that boost mitochondrial density, aerobic enzymes, and capillary density. This enhancement enables improved blood flow (due to increased capillary density) near the mitochondria, leading to greater uptake of fatty acids and a potential displacement of the crossover point towards higher exercise intensities.

Peak Fat Oxidation and FATmax

Peak Fat Oxidation (PFO) is the maximum amount of fats the body uses, 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 refers to the intensity level where peak fat oxidation is observed. It represents the power output at which the highest rate of fat oxidation takes place. Typically, a higher power output at which maximum fat oxidation happens is considered beneficial for endurance athletes, particularly in longer events when glycogen stores are depleted. By using fat as the main fuel source, muscle glycogen is preserved.

In general, Fatmax occurs around 60% of an athlete's 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.

Fatigue Resistance/Durability

Fig 2. Power Duration Curve Showing Power Outputs after different levels of energy burn

Fatigue resistance/durability refers to the ability to maintain muscle glycogen and perform well in the later stages of an endurance event, ensuring consistent performance over time. The data in Figure 2 above shows a cyclist maintaining a mean maximal power of 444w at 5 minutes, sustaining 443w after 500kj, and maintaining power output even after expending 2000kj. While demonstrating remarkable durability up to 2000kj, there is an 80w decline in performance beyond 3000kj, indicating a need for attention. To address this decline, it is important to consider both physiological factors (such as VO2 max, FTP, etc.) and nutritional aspects

To assess impact of nutritional factors on performance, Cark et al. 2019 published work with 16 competitive athletes and had them perform 3 min all-out cycle test under the following protocols:

(Read about Critical Power (CP) and W'prime test here:https://www.thethreshold.coach/single-post/unlocking-cycling-performance-understanding-critical-power-and-wprime-for-optimal-training)

Only warm-up

2 hours of riding with NO carbs

2 hours of riding with carbs (60g/h)

Before and after the muscle biopsy was used to measure muscle glycogen depletion

Results from the testing (taking into account the muscle biopsy)

	Warm-up and Test	2-hour ride with NO CARBS and test	2-hour ride with Carbs and test
W Prime	17.9 KJ	13.8 KJ * (significant decrease)	13.5 KJ* (significant decrease)
Critical Power	260w	236w* (significant decrease)	254w

the MORE glycogen decreased (as seen through the biopsy after testing), the greater the W Prime decreased
muscle glycogen is a key determining factor in fatigue resistance/durability for maintaining W Prime (FRC or anaerobic power) which is above threshold power and key to race-winning moves

Mechanisms enabling increased rates of fat oxidation

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. Acetyl CoA fuels the Krebs cycle and allows the production of ATP (the body's energy currency) via the Electron Transport Chain

Fig 3. Fatty Acid activation (Hawley, 2003)

To reach the mitochondria and participate in energy production (as ATP), fatty acids must pass through the muscle membrane when they come from adipose tissue as well as both the outer and inner mitochondrial membranes (see Fig 3 above).

Carntine — Fig 4. Carnitine Transporter of Inner Membrane of the Mitochondria

Figure 4 displays a close-up image of the inner membrane of the mitochondria depicted in Figure 3. It's important to recall that mitochondria serve as the cell's energy powerhouse, generating ATP, the currency of energy. The greater the ATP production, the more muscle contractions and overall performance.

Notice two of the fatty acid transporters in the form carnitine or the more scientific names of:

CPT1: Carnitine palmitoyltransferase 1
CPT2: Carnitine palmitoyltransferase 2

Other notable transporters involved in fatty acid metabolism:

FATP1: Fatty Acid Transport Protein No.1
FATP4: Fatty Acid Transport Protein No. 4
CD36: Differentiation Cluster 36/SR-B3
FABPpm: Fatty Acid Binding Protein to the Cell Membrane (Maunder et al. 2023)

Does the presence of more transporters in the muscle and mitochondrial membrane lead to higher rates of fat oxidation, as previously mentioned?

Maunder et al 2023 wrote a paper that looked at fat oxidation rate and the fatty acid transporters:

17 Cyclists endurance-trained cyclists and triathletes performed Peak Fat Oxidation (PFO)	Measurement of the impact of two previous testing days on transporters. The higher the %, the more likely the fatty acid transporters were responsible for the increased fat oxidation

Fasted (Warm up and testing)	87% increase
2 hours at 90% of the FIRST threshold (Zone 2 intensity) with 60g/h of carbs	65% increase

Summary to add to our series so far:

Cark et al. 2019 revealed that muscle glycogen preservation impacts W Prime (anaerobic repeatability) so increasing fat oxidation and using exogenous carbs (gels etc) would have positive benefits to performance, especially as kilojoule burn increases
The latest study we have looked at with our repeat friend of the blog Maunder et al 2023 shows increased muscle and mitochondrial fatty acid transporters and increased fat oxidation. In other words, more transporters, more fat oxidation which is another potential mechanism to help spare muscle glycogen.
Training in a fasted state could be a beneficial practice to upregulate fatty acid transporters.

References

Maunder E, Plews DJ, Kilding AE. Contextualising Maximal Fat Oxidation During Exercise: Determinants and Normative Values. Front Physiology. 2018. May 23;9:599

Maunder E, Rothschild JA, Fritzen AM, Jordy AB, Kiens B, Brick MJ, Leigh WB, Chang WL, Kilding AE. Skeletal muscle proteins involved in fatty acid transport influence fatty acid oxidation rates observed during exercise. Pflugers Arch. 2023 Sep;475(9):1061-1072.

Clark IE, Vanhatalo A, Thompson C, Joseph C, Black MI, Blackwell JR, Wylie LJ, Tan R, Bailey SJ, Wilkins BW, Kirby BS, Jones AM. Dynamics of the power-duration relationship during prolonged endurance exercise and influence of carbohydrate ingestion. Journal of Applied Physiology. 2019. Sep 1;127(3):726-736

Powers SK, Howley ET. Exercise Physiology. 5th Edition. McGraw Hill 2003

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