Male Marathoners and Carbohydrate Periodisation: A Study

Rationale for the study

Over the past decade, marathon running has surged in popularity around the world. Today, more than 800 marathons are held each year globally, and the demand shows no sign of slowing. A record 840,318 people applied to run the 2025 London Marathon. We are, without question, in the midst of a modern running boom. What makes the marathon so special is its unique inclusivity: it’s one of the few sporting events where runners of all levels can stand shoulder to shoulder with Olympic and World Champions, racing the same course, on the same day, and often under the very same conditions.

With this surge in running’s popularity has come a growing fascination with how to train smarter and perform better. Runners everywhere are searching for the optimal training programme to help them peak on race day, the most effective running shoes and clothing to gain those all-important marginal improvements, and the best nutrition strategies to fuel their efforts. After all, no one wants to experience the dreaded bonking, that sudden, energy-draining crash that can strike during a long run.

However, to truly understand what drives marathon performance, it helps to take a step back and look at the key physiological determinants that underpin success over 26.2 miles:

1. Maximal Oxygen Uptake (VO2max) – the maximum amount of oxygen your body can utilise during exercise.
2. Fractional utilisation of VO2max – the highest sustainable fraction of VO2max that can be maintained for a given distance, which is, in turn, related to the transition between domains of exercise, or so-called physiological thresholds. For example, elite runners can hold around 80-85% of their VO2max for the entire distance, whereas less well-trained runners are more likely to run a marathon at 60-75% VO2max.
3. Running economy – the amount of oxygen our bodies use when we run at a given speed or intensity. While two individuals may possess similar VO2max values, performance is not solely determined by maximal oxygen uptake; efficiency in utilising that oxygen is critical, particularly at race pace. For example, if two runners with comparable fitness levels run at the same speed, but Runner A has a running economy of 200 ml/kg/km at that speed, and Runner B’s running economy at that speed is 185 ml/kg/km, then Runner B demonstrates superior running economy. Despite running at the same speed, Runner B requires 7.5% less oxygen to cover the same distance, allowing them to sustain the effort longer and with less metabolic strain.
4. Durability or fatigue resistance – the ability to preserve physiological function throughout a marathon. It specifically reflects an individual’s ability to resist or minimise fatigue-induced deteriorations in key determinants of marathon performance, including increases in oxygen cost, reductions in running economy, heart rate drift, and progressive neuromuscular fatigue, that would otherwise impair the ability to sustain target race pace.

While developing these physiological attributes is crucial, achieving optimal marathon performance also requires an individualised and meticulous fuelling strategy. When the goal is to set a personal best, maintaining a carefully devised and strategized target race pace is key. The energy store that fuels this targeted pace (which is often associated with moderate to high intensity) is predominantly fuelled by carbohydrate metabolism. Unfortunately, the body’s carbohydrate stores are limited, primarily in the form of glycogen in the muscles and liver, along with a small amount of circulating blood glucose. During prolonged endurance exercise, like a marathon, these finite stores become progressively depleted. As this happens, the rate of carbohydrate use declines, and the body increasingly relies on fat oxidation for energy.

While fat is a predominant long-term fuel source, it cannot produce energy at the same rate as carbohydrates. This may not be a problem if it is low intensity, but at race pace, when you are already working hard, this means that we will have to consume more oxygen to maintain the same speed, and our running economy gets worse. Eventually, a point is reached where the required energy can no longer be sustained, resulting in fatigue and slowing down, the infamous “hitting the wall” or “bonking”. This reflects a significant reduction in whole-body carbohydrate oxidation, leading to an unavoidable drop in pace.

To maintain whole-body rates of carbohydrate oxidation and support efficient energy production, runners can ingest carbohydrates during exercise. These ingested carbohydrates contribute to exogenous carbohydrate oxidation, supplementing the body’s endogenous (stored) carbohydrate reserves. This, in turn, means that the body can then start to use these as fuel and prevent the switch over to predominantly using fat as fuel or at least delay the time point at which this occurs.

Whole body carbohydrate oxidation = endogenous carbohydrate oxidation (stored glycogen) + exogenous carbohydrate oxidation (ingested fuel).

As stored glycogen becomes depleted, you will have to ingest more fuel, but importantly, be able to digest, absorb, and utilise it effectively.

Current sport nutrition guidelines recommend consuming 30-60g of carbohydrate per hour for exercise lasting 1-2.5 hours, and up to 90 g/h of multiple transportable carbohydrates (i.e., maltodextrin/glucose and fructose blends) for exercise lasting longer than 2.5 hours.

Interestingly, a recent modelling study estimated that to sustain a sub-2-hour marathon, elite male runners would require 93±26 g/h of ingested carbohydrates. Further practitioner insights and real-world observations suggest that carbohydrate intakes may need to exceed 100 g/h, provided gastrointestinal tolerance allows.

However, one key question remains: how much carbohydrate can runners actually digest, absorb, and utilise as fuel during marathon pace running? Most research in this area has used cycling as the mode of exercise for evaluating the upper limits of intra-exercise carbohydrate ingestion, a discipline with distinct biomechanical and physiological differences from running. Understanding this in runners themselves remains a critical area.

Aim of the study

Therefore, through a partnership between Science in Sport x England Athletics Endurance Program x Liverpool John Moores University, this revolutionary study evaluated the effects of 60 g (maltodextrin), 90 g (60 g maltodextrin + 30 g fructose) and 120 g per hour (60 g maltodextrin + 60 g fructose) on whole body and exogenous carbohydrate metabolism, running economy and gastrointestinal symptoms during 2 h running in an elite running cohort, within a controlled laboratory setting.

What was done (the methods):

To evaluate the aims of the study, eight elite marathon runners participated in the study who whom all had a marathon personal best of <2 h 30 min and had completed a certified race within the 12 months before the study.

Each runner was required to undertake three separate experimental trials during which they were provided with either (maltodextrin only), 90 (2:1 maltodextrin-fructose), or 120 g.h-1 (1:1 maltodextrin-fructose) of a carbohydrate in a random order. Each trial consisted of a 120-minute run on the treadmill, consisting of an initial 15-minute period at 95% of lactate threshold, a subsequent 90-minute period at 94% of lactate turn point (i.e., an intensity close to race pace), and a final 15-minute period at 95% of lactate threshold.

Each of the drinks was labelled with 13C tracers, which allows for carbohydrate metabolism to be traced, such that when the drinks are consumed during exercise, the labelled carbohydrates are digested and absorbed by the body. The movement of the 13C labelled molecules is followed through the body by collecting breath samples throughout the running period. A sophisticated analysis is then undertaken to measure the ratio of 13C to 12C in the samples; this ratio can indicate how much of the actual consumed carbohydrate has been metabolised and utilised as energy.

In addition to these measurements, further respiratory gas exchange measurements were taken at different points throughout the run to measure whole body carbohydrate and fat oxidation, and running economy. Additionally, a subjective measure of gastrointestinal symptoms was recorded throughout.

What did the study find:

1. For the first time within the published literature, a clear dose-response effect of carbohydrate ingestion in trained male runners on both whole-body and exogenous carbohydrate oxidation during simulated marathon running was observed.
2. The highest rates of exogenous carbohydrate oxidation yet reported in runners in the scientific literature.
3. Consuming higher rates of carbohydrate during running allows you to maintain your ability to rely on carbohydrates as a fuel and reduce your reliance on fat as a fuel, with 120 g/h reporting no crossover to fat oxidation as the predominant fuel source during the 2 hours of running.
4. A 3% improvement in running economy with 120 g/h compared with 60 g/h.
5. Despite the improved metabolic advantages, moderate symptoms of GI discomfort across all doses of carbohydrate, and peak symptoms for nausea, stomach fullness, and abdominal cramps were greatest in the 120 g/h trial.

What does this all mean for you?

Ultimately, fuelling effectively during a marathon can make the difference between simply finishing and truly performing at your best. The insights from this research study highlight just how crucial it is to consume sufficient carbohydrates during the race to sustain your body’s ability to rely on them as its primary energy source. Doing so helps you maintain target race pace for longer.

However, every runner is different; what works seamlessly for one person might cause discomfort for another. That is why individualised nutrition strategies and practicing your nutrition strategy are so important. Just as you train your legs, heart, and lungs, you also need to train your gut to tolerate and efficiently absorb carbohydrates during prolonged runs. This process allows your digestive system to adapt to fuelling on the run, reducing the risk of dreaded stomach issues that can derail a race.

By developing, testing, and refining your fuelling plan, you can optimise carbohydrate availability, delay fatigue, and maintain pace all the way to the finish line. In the end, smart fuelling isn’t just about avoiding the dreaded bonk; it is about unlocking your full marathon potential.

In summary:

• Consuming 120 g per hour improves running economy by 2.6% compared with 60 g per hour.
• 120 g per hour maintains your whole-body rates of carbohydrate use, reduces the reliance on fat as a fuel, and reduces how much oxygen you need to consume to maintain your race pace. The result is a 2.6% improvement in running economy compared to traditional sport nutrition guidelines.
• SiS redefine fuelling guidelines for runners.

Read the Full Paper: https://journals.physiology.org/doi/abs/10.1152/japplphysiol.00665.2025
Ravikanti, S., Silang, K.G., Martyn, H.J., Johnson, K.O., Louis, J.B., Bampouras, T.M., Owens, D.J., Jones, A.M., Morton, J.P., & Pugh, J.N. (2025). 13C labelled glucose-fructose shows greater exogenous and whole-body CHO oxidation and lower O2 cost of running at 120 vs 60 & 90 g/h in elite male marathoners. Journal of Applied Physiology, published ahead of print.