World-class coach Dan Plews shares his fueling strategy for 70.3 and Ironman racing in 2025.

Fuelling in 2025

It’s nearly impossible to open an endurance sports article without seeing headlines about athletes pushing “x” amount of carbs per hour for “x” amount of time. And for good reason, over recent years, elite endurance athletes have been shattering records by cranking out more watts, running faster, and recovering from events quicker than ever before. And the secret? It’s all about fuelling.

While carbohydrates remain a cornerstone of endurance sports, research points to a clear key player supporting elite performance: ketones. They’re not here to replace carbs, but to work alongside them, enhancing both performance and recovery. 

Why Add Ketones to the Mix?

We add ketones because they provide something carbohydrates alone cannot: an additional fuel source that can enhance endurance performance, support mental clarity, and accelerate recovery. But timing matters. Research shows that taking 25 g of a ketone ester like Delta G after training, and another 25 g before sleep, is one of the most beneficial strategies—helping athletes sustain higher workloads, recover more effectively, and improve sleep quality.

Peer-reviewed studies back this up. Cox and Clarke (2016) first demonstrated that combining carbohydrates with a ketone monoester could alter fuel use and improve endurance performance. More recently, Poffé and colleagues (2019) showed that ketone ester supplementation during heavy training helped athletes sustain higher workloads, while follow-up work in 2021 reported increases in red blood cell production (EPO up by ~26%), improved muscle capillarization (~44%), and even better sleep quality.

Other research has pointed to benefits in altitude and hypoxic environments, where ketone esters improved oxygen saturation by ~5% during exposure. This makes them particularly useful not only for racing and recovery, but also for athletes traveling to altitude camps or competing in environments where oxygen availability is reduced. 

It’s important to note, however, that not all ketones are created equal. Other forms of ketones, such as ketone salts or precursors like 1,3-butanediol, have not shown the same benefits and, in some cases, come with unwanted side effects like gastrointestinal distress or even mild intoxication. That’s why, when we discuss using ketones in an Ironman or 70.3 fueling strategy, we're really talking about ketone esters and specifically Delta G. This is the most potent and well-researched form of ketone, the one that is constantly used in peer-reviewed studies and the version elite athletes rely on.

When to Use Ketones (and When to Avoid Them)

When it comes to ketones, timing and context are crucial. Their benefits are not universal, and both the intensity and duration of exercise significantly influence their effectiveness.  

Research and practice show that for short, high-intensity efforts—anything under an hour—or sessions that rely heavily on anaerobic power, ketones can actually be detrimental to performance. This is a result of the elevated blood Beta-hydroxybutyrate (BHB) caused by ketone ingestion, which shifts the body toward a more oxygen-efficient but slower energy pathway, suppressing glycolysis, and reducing carbohydrate flux. This trade-off makes sense in long, steady endurance activities, but it works against you when quick bursts of energy are needed, such as in sprints, criteriums, short time trials, or intense interval sessions. 

Where ketones excel is in longer, steady-state events, such as Ironman, 70.3, ultra-endurance races, and self-paced Gran Fondos, as well as in supporting recovery after heavy training blocks, where sleep and adaptation are critical. 

Example Ironman/70.3 Fuel Setup

So, what does this look like in practice? Well, the foundation of your fueling plan should be based around carbohydrates, with the key question being how much you can consistently consume per hour. Research points to three main lanes:

• 60 g/hr - Conservative Fuelling
• Low GI risk; Suitable for newer athletes or sensitive stomachs
• Does not maximize carbohydrate absorption or energy availability
• Ketones:
• 1-2 x 25 g Delta G on the bike (every ~4 hrs)
• Pair with carbs + ~1 tbsp of MCTs + electrolytes
• Provides an additional energy source + cognitive support late in the race
  
  
• 90 g/hr - Sweet Spot
• Near maximal absorption rate for most athletes, uses both SGLT1 (glucose) and GLUT5 (fructose) transporters; reduces GI distress compared to glucose alone
• Ketones
• 1 x 25 g Delta G on the bike (every ~4 hrs)
• Pair with carbs + ~1 tbsp of MCTs + electrolytes
• Sip steadily (avoid large boluses) to prevent BHB spikes and blood glucose drops
• Benefits: Improved cognition, steadier energy, enhanced substrate use

• 120+ g/hr - Elite Fuelling
• Maximum energy availability 
• Only anecdotal evidence in support of its performance enhancement
• Higher GI risk; requires practice in training before attempting in racing
• Ketones
• ~25 g Delta G on the bike with MCTs + carbs + electrolytes
• Benefits: cognitive clarity

Sip Smart: The Right Way to Use Ketones

 

One of the easiest ways to go wrong with ketones is by taking them in one big hit. Early studies that had athletes take 20–25 g all at once caused blood ketone levels to spike rapidly, which in turn triggered an insulin response and drove blood glucose down. The athletes felt flat, dizzy, and their performance dipped. The solution is simple: sip Delta G gradually on the bike and always combine it with carbohydrates. This steadier approach keeps blood glucose stable and holds BHB in the optimal 1–2 mmol range, where the benefits are most likely to show up.

Another issue is that ketones naturally lower blood pH, adding an extra strain during long races. That’s why it’s best to stack them with electrolytes and a touch of MCTs. The electrolytes buffer acidity, while MCTs slow the rise of blood ketones, smoothing out their absorption. Framed this way, ketones aren’t a standalone energy source but an enhancer—working with carbs to provide steadier energy, sharper focus, and better decision-making when it matters most.

 

Figure 1 illustrates how the addition of MCT and electrolytes slows the absorption and rise of BHB, producing a more favourable pharmacological profile. The curve shifts to the right, resulting in lower, more stable BHB concentrations.

 

 

 

Figure: Pharmacological profile of Ketone ingestion under three different situations. 

Key Takeaways

During long, steady-state races like Ironman or 70.3, carbohydrates remain the cornerstone of a solid fuelling plan, with athletes typically opting for 60-120 g/hr of a glucose-fructose mix to improve absorption and minimize gastrointestinal issues. Ketone esters like Delta G should be layered on top, not used in isolation, and only in the right context: longer efforts where consistency, cognition, and durability matter, not short, high-intensity events where they may blunt performance. The most effective strategy is to sip Delta G gradually, preferably on the bike, alongside carbs, electrolytes, and a small amount of MCTs to smooth absorption and buffer acidity. This helps to keep blood BHB in the performance-relevant range, providing steadier energy and clearer focus deep into the race.

References

 

Cox, P. J., Kirk, T., Ashmore, T., Willerton, K., Evans, R., Smith, A., Murray, A. J., Stubbs, B., West, J., McLure, S. W., King, M. T., Dodd, M. S., Holloway, C., Neubauer, S., Drawer, S., Veech, R. L., Griffin, J. L., & Clarke, K. (2016). Nutritional Ketosis Alters Fuel Preference and Thereby Endurance Performance in Athletes. Cell Metabolism, 24(2), 256–268. https://doi.org/10.1016/j.cmet.2016.07.010

Hearris, M. A., Pugh, J. N., Langan-Evans, C., Mann, S. J., Burke, L., Stellingwerff, T., Gonzalez, J. T., & Morton, J. P. (2022). 13 C-glucose-fructose labeling reveals comparable exogenous CHO oxidation during exercise when consuming 120 g/h in fluid, gel, jelly chew, or coingestion. Journal of Applied Physiology, 132(6), 1394–1406. https://doi.org/10.1152/japplphysiol.00091.2022

Jeukendrup, A. (2014). A Step Towards Personalized Sports Nutrition: Carbohydrate Intake During Exercise. Sports Medicine, 44(S1), 25–33. https://doi.org/10.1007/s40279-014-0148-z

Poffé, C., Ramaekers, M., Van Thienen, R., & Hespel, P. (2019). Ketone ester supplementation blunts overreaching symptoms during endurance training overload. The Journal of Physiology, 597(12), 3009–3027. https://doi.org/10.1113/JP277831

Poffé, C., Robberechts, R., Podlogar, T., Kusters, M., Debevec, T., & Hespel, P. (2021). Exogenous ketosis increases blood and muscle oxygenation but not performance during exercise in hypoxia. American Journal of Physiology-Regulatory, Integrative and Comparative Physiology, 321(6), R844–R857. https://doi.org/10.1152/ajpregu.00198.2021

Poffé, C., Robberechts, R., Stalmans, M., Vanderroost, J., Bogaerts, S., & Hespel, P. (2023). Exogenous ketosis increases circulating dopamine concentration and maintains mental alertness in ultra-endurance exercise. Journal of Applied Physiology, 134(6), 1456–1469. https://doi.org/10.1152/japplphysiol.00791.2022

Poffé, C., Robberechts, R., Van Thienen, R., & Hespel, P. (2023). Exogenous ketosis elevates circulating erythropoietin and stimulates muscular angiogenesis during endurance training overload. The Journal of Physiology, 601(12), 2345–2358. https://doi.org/10.1113/JP284346

Robberechts, R., Albouy, G., Hespel, P., & Poffé, C. (2023). Exogenous Ketosis Improves Sleep Efficiency and Counteracts the Decline in REM Sleep after Strenuous Exercise. Medicine & Science in Sports & Exercise, 55(11), 2064–2074. https://doi.org/10.1249/MSS.0000000000003231