Carbohydrates During Exercise and Athlete Durability: Our Recent Publication

It’s not what you can do for 5 minutes, it’s what you can do for 5 minutes after five hours. As endurance athletes, we know that our endurance – our physiological and psychological ability to endure, to keep going as the hours tick up in our event – is fundamental to our performance.

‘Durability’ is an emerging concept in exercise physiology, and, more importantly, we think, a critical factor in endurance training and performance. In our 2021 paper, we defined durability as an athlete’s resilience to the effects of prolonged exercise on their physiological profiling characteristics – the thresholds we use to inform pacing strategies, programme training sessions, and monitor training load (1). Subsequently, durability has been proposed as a crucial factor in endurance performance, as it defines, physiologically at least, our ability to stave off the upward creep in effort required at a given speed or power output as exercise progresses (2). As readers of our blogs, or our monthly Training Science Summaries, will know, we are working on durability studies in our lab all the time, and following the durability literature closely.

In this blog, we discuss the results of one of our recent studies – led by our awesome Master’s graduate Harrison Dudley-Rode (3).

A few things we know about durability

Over the last few years, we have learned a lot about durability. Check out our recent blog on durability here.

We’ve now seen, across a number of studies, that power output at the first and second thresholds declines during prolonged exercise (4–10). This loss of power output at the thresholds seems to occur in a non-linear fashion, meaning that threshold power is stable for a while, and then declines slowly, before declining more rapidly after multiple hours of work (5, 8). We’ve also now seen loss of running speed at the first threshold with prolonged running (the initial studies were all conducted in cycling) (11).

This effect – the declines we see in threshold power and speed over time – has a number of important implications for endurance athletes. The first is performance. It seems logical that being resistant to this decline protects you against upward creep in the physiological stress associated with a particular power or speed, helping you to maintain output as you get deeper into the race. We have seen positive relationships between durability of the first threshold and open-ended exercise capacity (5), as well as resilience to the effects of prolonged exercise on 5-min time trial performance; that is, the difference between 5-min time trial performance when fresh, and after 2.5 hours in the saddle (6). There are also a number of studies measuring durability using field data – maximal mean power outputs in training racing data, and heart rate decoupling during marathon running – that suggest durability, measured in these ways at least, distinguishes higher- and lower-performing athletes (12, 13).

The second relates to training and training stress management. We often programme training relative to the thresholds – we want easy, low-stress sessions to focus below the first threshold, and high-stress interval training sessions to focus on bouts above the second threshold, for example. As we often discuss, an effective endurance training is all about stress management. Not enough stress, and you don’t trigger the adaptations that improve performance. Too much stress, and you can push an athlete into excessive fatigue and overreaching, with negative adaptive – or maladaptive – consequences (14, 15).

Durability and carbohydrates

We don’t yet know the mechanisms behind the reductions in thresholds we see with prolonged exercise. One plausible factor is the depletion of carbohydrate energy stores. Our crucial muscle glycogen stores are slowly or rapidly, depending on the athlete and the intensity of the exercise, depleted during prolonged exercise (16–18). We also see depletion of the glycogen stored in our liver (19–21). The breakdown of liver glycogen to glucose, and release into the blood, is important for regulation of blood glucose concentrations. Accordingly, we can also see decreased blood glucose concentrations, and therefore the development of hypoglycaemia, during prolonged exercise (17, 22–24). All these factors could contribute to fatigue and durability responses during prolonged exercise.

A study, published in 2019 by Ida Clark and colleagues at the University of Exeter in the UK, found that carbohydrate ingestion during prolonged cycling mitigated the reduction in second threshold power output – critical power – seen when the cyclists only drank water (8). This positive effect of carbohydrate ingestion on durability is more likely to have been related to effects on liver glycogen and/or blood glucose, given the number of studies suggesting that carbohydrate ingestion during exercise doesn’t slow muscle glycogen use during exercise (17, 20, 25).

Whatever the precise mechanism, the positive effect of carbohydrate ingestion during exercise on durability may well contribute to the positive performance effects associated with carbohydrate ingestion during exercise (26). However, no study had previously looked at the effect of carbohydrate ingestion on durability of the first threshold – which we call the moderate-to-heavy intensity transition. We believe this is an important threshold to consider in day-to-day training, given so much of the training we do as endurance athletes is focused specifically below this threshold, as it is low stress and allows us to accumulate a large overall training volume.

Accordingly, this is the gap Harrison looked to fill in his Master’s thesis.

What happened?

Accordingly, we recruited twelve trained cyclists and triathletes to our lab for four trials. After the first trial, which was for initial testing and familiarisation, the cyclists completed three demanding trials, all involving an incremental cycling test to determine power output at the first threshold and a 5-min performance test, where the performance measure was average power output. These three trials were completed:

• When completely fresh
• After 150 min of moderate-intensity cycling, with water consumption
• After 150 min of moderate-intensity cycling, with carbohydrate ingestion at 60 grams per hour

These trials were conducted in random order, and therefore allowed us to assess the impact of prolonged exercise with and without carbohydrate ingestion on durability of the first threshold and 5-min time trial performance.

In line with our previous studies (4–6), when the cyclists drank water, their power output at the first threshold declined by ~6% after 150 minutes in the saddle. When they consumed carbohydrates, this reduction in threshold power was reduced, but not completely removed – power at the threshold fell by ~3% compared to the fresh trial. The differences between trials were statistically significant, meaning we can conclude with some confidence that carbohydrate ingestion during prolonged cycling has a positive effect on durability of the first threshold, or moderate-to-heavy intensity transition.

Our performance data were interesting, too. Unsurprisingly, 5-min time trial performance was worse in the water-only trial, by ~10% vs. the fresh trial. This difference was ~4% in the carbohydrate ingestion trial, which looks as though carbs helped preserve performance, too. However, as 5-min time trial performance was not statistically different between the carbohydrate and water trials, we can’t make this conclusion with confidence. We reckon that this lack of difference between the carbohydrate and water trials was due to the low-ish sample size, and because in a few of the more-durable cyclists, the effect of prolonged exercise on performance was actually minimal – some fell of a cliff, some saw only small reductions in performance – which may have masked the trial’s ability to show a positive effect of carbohydrate ingestion. You can’t repair what’s already fixed. Perhaps a more consistently-demanding preload exercise protocol would have uncovered a positive effect.

One thing I’d like to add is that I think it’s highly-unlikely that a greater rate of carbohydrate ingestion would have yielded larger positive effects on performance. That’s because many studies don’t see a dose-response effect of carbohydrate ingestion on performance or relevant metabolic responses (27–31). In our study, our carbohydrate ingestion regimen was sufficient to offset the declining blood glucose concentrations seen in the water-only trial, and to maintain blood glucose concentrations throughout exercise. Figure 2 (E) of the manuscript illustrates this beautifully. During the 150-minute carbohydrate trial, blood glucose remained impressively stable when carbohydrate was consumed at a rate of 60 g per hour.

Figure 1: Changes in blood glucose recorded every 30 minutes during the 150 min pre-load. There were significant differences in blood glucose levels between CON and CHO at 120 and 150 minutes.

It’s clear that simply increasing carbohydrate intake won’t necessarily do a better job of preventing hypoglycaemia or reducing muscle glycogen depletion (17, 20, 25). If higher doses of carbs (like 90 g, 120 g, or even up to 180 g per hour) seem to be helping, it’s likely working through a different mechanism. One possibility if it is helping? These higher doses could be having a direct stimulatory effect on the brain, helping to maintain focus and drive during prolonged efforts.

What it means

So, what does this all mean? Well, I reckon a few useful things:

1. Carbs help. These data add to the literature showing that carbohydrate ingestion can have a positive effect on durability and performance. So, if the goal of your nutrition regimen is to promote performance during a race or training session, ingesting carbohydrates is probably going to help.
2. Durability is probably mechanistically related to carbohydrate availability. Given that carbohydrate ingestion during exercise doesn’t seem to have a convincing muscle glycogen sparing effect, this may be something to do with liver glycogen and/or blood glucose.
3. When regulating intensity according to thresholds, carbohydrate ingestion during exercise will allow a higher workload – power or speed – to be sustained without crossing the first threshold from the moderate- to the heavy-intensity domain. Therefore, when programming training sessions in accordance with thresholds, or monitoring training load, carbohydrate ingestion needs to be considered.

This latter point is an important one, as programming training and monitoring load is the business of physiologically-based endurance training. To do this well, we will eventually need a non-invasive wearable tool that allows us to estimate thresholds – and our proximity to them – in real-time during training sessions. This tool needs to be responsive to the effects of things like carbohydrate ingestion on durability.

Development of this tool is a key opportunity for scientists and developers working in the endurance world!

Summary

In summary, then – durability is an athlete’s resilience to the effects of prolonged exercise on their thresholds. We have seen that durability is quite variable between athletes, and that it is probably a crucial determinant of performance. It has previously been shown that carbohydrate ingestion during exercise improves durability of the second threshold – the heavy-to-severe intensity transition – and our new study, led by Harrison Dudley-Rode, shows that it improves durability of the first threshold – the moderate-to-heavy intensity transition – too.

This is more evidence that carbohydrate ingestion is likely to improve performance in long-duration events, and also that whether carbohydrates are ingested or not during training should be factored into training programming and load monitoring. We look forward to the development of practical, non-invasive tools that allow us to assess our thresholds in real-time during exercise to allow us to do this.

Endure On!

References

1. Maunder E, Seiler S, Mildenhall MJ, Kilding AE, Plews DJ. The importance of ‘durability’ in the physiological profiling of endurance athletes. Sports Medicine 51: 1619–1628, 2021. doi: 10.1007/s40279-021-01459-0.
2. Jones AM. The fourth dimension: physiological resilience as an independent determinant of endurance exercise performance. Journal of Physiology 0, 2023. doi: 10.1113/JP284205.
3. Dudley-Rode H, Zinn C, Plews DJ, Charoensap T, Maunder E. Carbohydrate ingestion during prolonged exercise blunts the reduction in power output at the moderate-to-heavy intensity transition. Eur J Appl Physiol in press.
4. Stevenson JD, Kilding AE, Plews DJ, Maunder E. Prolonged cycling reduces power output at the moderate-to-heavy intensity transition. Eur J Appl Physiol 122: 2673–2682, 2022. doi: 10.1007/s00421-022-05036-9.
5. Gallo G, Faelli EL, Ruggeri P, Filipas L, Codella R, Plews DJ, Maunder E. Power output at the moderate-to-heavy intensity transition decreases in a non-linear fashion during prolonged exercise. Eur J Appl Physiol 124: 2353–2364, 2024. doi: 10.1007/s00421-024-05440-3.
6. Hamilton K, Kilding AE, Plews DJ, Mildenhall MJ, Waldron M, Charoensap T, Cox TH, Brick MJ, Leigh WB, Maunder E. Durability of the moderate-to-heavy-intensity transition is related to the effects of prolonged exercise on severe-intensity performance. Eur J Appl Physiol 124: 23427–2438, 2024. doi: 10.1007/s00421-024-05459-6.
7. Clark IE, Vanhatalo A, Bailey SJ, Wylie LJ, Kirby BS, Wilkins BW, Jones AM. Effects of two hours of heavy-intensity exercise on the power-duration relationship. Med Sci Sports Exerc 50: 1658–1668, 2018. doi: 10.1249/MSS.0000000000001601.
8. 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. J Appl Physiol 127: 726–736, 2019. doi: 10.1152/japplphysiol.00207.2019.
9. Clark IE, Vanhatalo A, Thompson C, Wylie LJ, Bailey SJ, Kirby BS, Wilkins BW, Jones AM. Changes in the power-duration relationship following prolonged exercise: estimation using conventional and all-out protocols and relationship with muscle glycogen. Am J Physiol Regul Integr Comp Physiol 317: R59–R67, 2019. doi: 10.1152/ajpregu.00031.2019.
10. Spragg J, Leo P, Swart J. The relationship between physiological characteristics and durability in male professional cyclists. Med Sci Sports Exerc 55: 133–140, 2023. doi: 10.1249/mss.0000000000003024.
11. Nuuttila OP, Laatikainen-Raussi V, Vohlakari K, Laatikainen-Raussi I, Ihalainen JK. Durability in recreational runners: effects of 90-min low-intensity exercise on the running speed at the lactate threshold. Eur J Appl Physiol in press.
12. Smyth B, Maunder E, Meyler S, Hunter B, Muniz-Pumares D. Decoupling of internal and external workload during a marathon: An analysis of durability in 82,303 recreational runners. Sports Medicine 52: 2283–2295, 2022. doi: 10.1007/s40279-022-01680-5.
13. Mateo-March M, Valenzuela PL, Muriel X, Gandia-Soriano A, Zabala M, Lucia A, Pallares JG, Barranco-Gil D. The record power profile of male professional cyclists: Fatigue matters. Int J Sports Physiol Perform 17: 926–931, 2022. doi: 10.1123/ijspp.2021-0403.
14. Cardinale DA, Gejl KD, Petersen KG, Nielsen J, Ørtenblad N, Larsen FJ. Short term intensified training temporarily impairs mitochondrial respiratory capacity in elite endurance athletes. J Appl Physiol 131: 388–400, 2021. doi: 10.1152/japplphysiol.00829.2020.
15. Flockhart M, Nilsson LC, Tais S, Ekblom B, Apró W, Larsen FJ. Excessive exercise training causes mitochondrial functional impairment and decreases glucose tolerance in healthy volunteers. Cell Metab 33: 957–970, 2021. doi: 10.1016/j.cmet.2021.02.017.
16. Bergström J, Hultman E. A study of the glycogen metabolism during exercise in man. Scand J Clin Lab Invest 19: 218–228, 1967.
17. Coyle EF, Coggan AR, Hemmert K, Ivy JL. Muscle glycogen utilization during prolonged strenuous exercise when fed carbohydrate. J Appl Physiol 61: 165–172, 1986.
18. Watt MJ, Heigenhauser GJF, Dyck DJ, Spriet LL. Intramuscular triacylglycerol, glycogen and acetyl group metabolism during 4 h of moderate exercise in man. Journal of Physiology 541: 969–978, 2002. doi: 10.1113/jphysiol.2002.018820.
19. Casey A, Mann R, Banister K, Fox J, Morris PG, MacDonald IA, Greenhaff PL. Effect of carbohydrate ingestion on glycogen resynthesis in human liver and skeletal muscle, measured by 13C MRS. Am J Physiol Endocrinol Metab 278: E65–E75, 2000.
20. Gonzalez JT, Fuchs CJ, Smith FE, Thelwall PE, Taylor R, Stevenson EJ, Trenell MI, Cermak NM, van Loon LJC. Ingestion of glucose or sucrose prevents liver but not muscle glycogen depletion during prolonged endurance-type exercise in trained cyclists. Am J Physiol Endocrinol Metab 309: E1032–E1039, 2015. doi: 10.1152/ajpendo.00376.2015.
21. Stevenson EJ, Thelwall PE, Thomas K, Smith F, Brand-Miller J, Trenell MI. Dietary glycemic index influences lipid oxidation but not muscle or liver glycogen oxidation during exercise. Am J Physiol Endocrinol Metab 296: E1140–E1147, 2009. doi: 10.1152/ajpendo.90788.2008.
22. Coyle EF, Hagberg JM, Hurley BF, Martin WH, Ehsani AA, Holloszy JO. Carbohydrate feeding during prolonged strenuous exercise can delay fatigue. J Appl Physiol 55: 230–235, 1983. doi: 10.1152/jappl.1983.55.1.230.
23. Coyle EF, Coggan AR, Hemmert K, Lowe RC, Walters TJ. Substrate usage during prolonged exercise following a preexercise meal. J Appl Physiol 59: 429–433, 1985.
24. Coggan AR, Coyle EF. Reversal of fatigue during prolonged exercise by carbohydrate infusion or ingestion. J Appl Physiol 63: 2388–2395, 1987. www.physiology.org/journal/jappl.
25. Jeukendrup AE, Wagenmakers AJM, Stegen JHCH, Gijsen AP, Brouns F, Saris WHM. Carbohydrate ingestion can completely suppress endogenous glucose production during exercise. Am J Physiol Endocrinol Metab 276: E672–E683, 1999.
26. Stellingwerff T, Cox GR. Systematic review: Carbohydrate supplementation on exercise performance or capacity of varying durations. Applied Physiology, Nutrition, and Metabolism 39: 998–1011, 2014. doi: 10.1139/apnm-2014-0027.
27. Smith JW, Zachwieja JJ, Péronnet F, Passe DH, Massicotte D, Lavoie C, Pascoe DD. Fuel selection and cycling endurance performance with ingestion of [13C]glucose: evidence for a carbohydrate dose response. J Appl Physiol 108: 1520–1529, 2010. doi: 10.1152/japplphysiol.91394.2008.
28. Smith JW, Pascoe DD, Passe DH, Ruby BC, Stewart LK, Baker LB, Zachwieja JJ. Curvilinear dose-response relationship of carbohydrate (0-120 g·h-1) and performance. Med Sci Sports Exerc 45: 336–341, 2013. doi: 10.1249/MSS.0b013e31827205d1.
29. Baur DA, Schroer AB, Luden ND, Womack CJ, Smyth SA, Saunders MJ. Glucose-fructose enhances performance versus isocaloric, but not moderate, glucose. Med Sci Sports Exerc 46: 1778–1786, 2014. doi: 10.1249/MSS.0000000000000284.
30. King AJ, O’Hara JP, Morrison DJ, Preston T, King RFGJ. Carbohydrate dose influences liver and muscle glycogen oxidation and performance during prolonged exercise. Physiol Rep 6: e13555, 2018. doi: 10.14814/phy2.13555.
31. King AJ, O’Hara JP, Arjomandkhah NC, Rowe J, Morrison DJ, Preston T, King RFGJ. Liver and muscle glycogen oxidation and performance with dose variation of glucose–fructose ingestion during prolonged (3 h) exercise. Eur J Appl Physiol 119: 1157–1169, 2019. doi: 10.1007/s00421-019-04106-9.