Fat: the Ultra-Distance Fuel? Part One - Substrate Utilisation.

During physical exercise that is aerobic in nature, energy is primarily derived from fats and carbohydrates. The fat element is procured from Free Fatty Acids (FFA) in blood plasma and from muscle triglycerides (fat stored in skeletal muscle). Carbohydrates are sourced from blood plasma glucose and muscle glycogen.

Substrate utilisation is the term used to denote the pattern by which these fuel sources are used during exercise. There are many factors that influence substrate utilisation, although the two most prominent are exercise intensity and duration (Ramadoss et al., 2022). It is also important to note that fats and carbohydrates are oxidised simultaneously, it is their relative contribution that shifts rather than one completely replacing the other (Muscella et al., 2020).

Exercise Intensity

The reliance on these four primary sources of fuel was measured in a pioneering study involving well-trained young male cyclists at varying exercise intensities by Romjin et al. (1993). They used indirect calorimetry, stable isotope techniques and skeletal muscle biopsies to measure substrate utilisation at 25%, 65% and 85% of VO2 max.

Romjin et al. (1993): Substrate utilisation at 25%, 65% & 85% of VO2 max.

Their findings illustrate that FFA was the dominant fuel at 25% VO2 max, with little carbohydrate used. As intensity was increased to 65% VO2 max, a significant contribution was made from carbohydrates, largely in the form of muscle glycogen, whilst FFA was largely maintained. At this intensity, the total contribution of fat and carbohydrates was roughly equal. When intensity reached 85% VO2 max, fat utilisation decreased and muscle glycogen became the main fuel provider. 

Similar results were found in a study by Van Loon et al. (2001) where eight cyclists were tested during three consecutive 30 min stages of exercise at intensities of 40%, 55% and 75% maximal workload (%Wmax - a correlate to VO2 max but with differing absolute values).

Van Loon et al. (2001): Substrate utilisation at rest, 40%, 55% & 75% Wmax.

Again, they showed that both carbohydrate and fat oxidation rates increased proportionally as intensity increased up to 55% Wmax. When intensity was increased to 75% Wmax, both muscle glycogen and plasma glucose oxidation rates significantly increased and the fat oxidation rate notably decreased.

In summary, at low exercise intensities the energy needed is provided mostly by oxidation of FFAs. As intensity increases, FFA utilisation remains relatively stable and the additional energy required is obtained through the utilisation of muscle glycogen, blood glucose, and muscular triglyceride. At even higher intensities, exercise is fueled mostly by increases in muscle glycogen utilisation and some additional increase in blood plasma glucose oxidation (Coyle, 1995).

Exercise Duration

A study by Watt et al. (2002) reported on a protocol where seven endurance-trained males cycled for 4 hours at 55% of their VO2 max. Muscle samples were taken at rest and after 10, 120 and 240 minutes of exercise respectively, which were examined using indirect calorimetry, and measurements of skeletal muscle triglycerides and glycogen. Substrate utilisation was therefore discerned at each duration as per the below illustrations. It should be noted that, although the participants ate a meal 2 hours beforehand, they took on no additional nutrition during the 4 hour test period. They were permitted to drink water ad libitum throughout were cooled through the use of and a fan.

Watt et al. (2002): Substrate utilisation when exercising at 55% VO2 max for 0-120 mins & 120-240 mins (left) and the relationship between carbohydrate & fat oxidation over the 240 min duration (right).

Carbohydrate oxidation was prevalent in the first two hours, providing slightly more than 60% of the energy requirements and a small bias towards blood glucose over muscle glycogen. At this stage, fat provision was predominantly accounted for through FFA oxidation. However, beyond the two hour mark fat oxidation became the dominant energy source, alongside a steady decrease in carbohydrate utilisation. During this timespan fat was fuelling nearly 60% of the exercise, with almost all provided by FFA. Muscle glycogen significantly decreased for this period, with the majority of carbohydrates provided through blood glucose.

The Romijn et al. (1993) study also looked at substrate utilisation at 25% and 65% VO2 max during an exercise bout lasting 2 hours. The five endurance-trained cyclist were studied after a 10-12 hour fast with no additional fuel during the protocol. Although there was no significant change in the contribution of fats and carbohydrates at 25% VO2 max for this duration, when exercising at 65% VO2 max there was a progressive increase in the reliance on FFA and blood glucose as energy sources.

Romijn et al. (1993): Utilisation of FFA and blood glucose during 120 mins of exercise at 25% VO2 max (A) and 65% VO2 max (B). Triangles denote FFA and circles blood glucose.

Applications to Ultra-Distance

In terms of intensity and duration, ultra-distance cycling events are overwhelmingly ridden at low to medium intensities and, as the name implies, last for ultra-long durations.

Tackling duration first, over time the body switches to favouring fat as a fuel source and particularly that procured from FFA in blood plasma. The carbohydrate used is also derived more and more from exogenous sources as muscle glycogen stores are depleted. These mechanisms underscore the requirement to replenish energy stores as the duration of an event increases.

The durations studied to date only extend to four hours, which is below any commonly accepted definition of ultra-distance. We cannot therefore be certain whether the discrepancy between fat and carbohydrate substrate utilisation continues to grow, whether there is a point at which it becomes stable, or even whether it starts to reverse. The evidence however points to fat supplying the majority of energy requirements over longer durations. An important caveat however is that the studies examining duration involved no addition of exogenous fuel throughout the protocols. It would be instructive to understand more about how the ingestion of fats and carbohydrates affects substrate utilisation over ultra-long durations, particularly muscle glycogen which is easily depleted without replenishment.

Returning to intensity, it has been demonstrated that even in ‘shorter’ ultra-distance events the vast majority of exercise is performed at a moderate intensity and lower. For example, in a case study of a 460km / 11,000m cycling event, Neumayr et al. (2002) found that exercise intensity was below 70% VO2 max during 87% of the race and below 60% VO2 max during 73%. At these intensities and below, fat will again be the primary energy source and increasingly so as the intensity drops.

One final observation is that each of these experiments involved trained male cyclists. Romjin et al. (2000) did however conduct a similar protocol with young well-trained women where substrate utilisation at 25%, 65% & 85% VO2 max were almost essentially identical to those of the men in the Romjin et al., (1993) study. A comparison between endurance-trained and untrained subjects was made by Coggan et al. (1995) who found that those who are untrained had a higher reliance on carbohydrates at moderate and higher power outputs, even when the exercise was performed at the same relative intensity. They also noted that exercise at power outputs of around 60% VO2 max and above could not be sustained for as long as trained subjects, potentially as a result of this increased reliance on carbohydrates as fuel. Training status therefore appears to have a significant influence on ones ability to utilise fat as an energy source for exercise.

Summary

In brief, the evidence we have leads to the understanding that, when cycling at a low relative intensity for an ultra-long distance, the body fuels primarily on fat. This raises a number of questions. Should we thereby be primarily fueling on fat? How much fat do I need to fuel my ultra-distance event? How can I improve my ability to oxidise fat as a fuel? The subsequent articles in this series will attempt to address these issues and more.

Part two will examine more closely how much energy is required to fuel an ultra-distance endeavour and what we know about how athletes are currently going about plugging this calorific gap.

References

Coggan, A.R., C.A. Raguso, B.D. Williams, L.S. Sidossis, and A. Gastaldelli (1995). Glucose kinetics during high-intensity exercise in endurance-trained and untrained humans. J. Appl. Physiol. 78:1203-1207.

Muscella A, Stefàno E, Lunetti P, Capobianco L, Marsigliante S. The Regulation of Fat Metabolism During Aerobic Exercise. Biomolecules. 2020 Dec 21;10(12):1699. doi: 10.3390/biom10121699. PMID: 33371437; PMCID: PMC7767423.

Neumayr G, Gänzer H, Sturm W, Pfister R, Mitterbauer G & Hörtnagl H. 2002. Physiological effects of an ultra-cycle ride in an amateur athlete - a case report. J Sports Sci Med. https://pubmed.ncbi.nlm.nih.gov/24672268/

Ramadoss R, Stanzione JR, Volpe SL. A Comparison of Substrate Utilization Profiles During Maximal and Submaximal Exercise Tests in Athletes. Front Psychol. 2022 Apr 8;13:854451. doi: 10.3389/fpsyg.2022.854451. PMID: 35465548; PMCID: PMC9024409.

Romijn, J.A., E.F. Coyle, L.S. Sidossis, A. Gastaldelli, J.F. Horowitz, E. Endert, and R.R. Wolfe (1993). Regulation of endogenous fat and carbohydrate metabolism in relation to exercise intensity and duration. Am. J. Physiol. 265:E380-391.

Romijn, J.A., E.F. Coyle, L.S. Sidossis, J. Rosenblatt, and R.R. Wolfe (2000). Substrate metabolism during different exercise intensities in endurance-trained women. J. Appl. Physiol. 88:1707-1714.

van Loon, L.J., P.L. Greenhaff, D. Constantin-Teodosiu, W.H. Saris and A.J. Wagenmakers (2001). The effects of increasing exercise intensity on muscle fuel utilisation in humans. J. Physiol. 536: 295-304.

Watt, M.J., G.J.F. Heigenhauser, D.J. Dyck, and L.L. Spriet (2002). Intramuscular triacylglycerol, glycogen and acetyl group metabolism during 4 h of moderate exercise in man. J. Physiol. 541:969-978.