A metabolism expert on how to fuel up properly for a marathon

The London Marathon is little over a month away, and more than 56,000 entrees – including running legend Eliud Kipchoge and Olympian Alex Yee – are expected to wind through the capital’s tasking route. Those set for the event, which takes place on 27 April, have likely sorted their gear and have (hopefully) clocked up a fair few miles already. Nutrition is also an integral factor when prepping for the distance, yet it’s an area a fair few amateurs may not have much insight on.

With this in mind, Ulrike Kuehl, the head of nutrition at Lumen, the creator of a premium at-home metabolic tracker, provides us with a succinct rundown on what she suggests you include in your training diet, whether or not you should carb-load leading up to the race, and what things you should eat just before the 26.2 miles.

Athletic performance, including for endurance sports, is closely tied to metabolic flexibility – or the ability of your mitochondria (the cells’ powerhouses) to efficiently switch between burning fats and carbs to best support your body’s energy needs.

When mitochondria aren’t in optimal condition, metabolic flexibility declines, muscles tire faster, injuries are more frequent and recovery drags on. Mitochondria are impacted by the four key lifestyle pillars – nutrition, movement, sleep and stress – and when these are in balance, your metabolic mediators and mitochondria remain healthy, improving sleep and recovery, reducing injuries and increasing endurance and strength.

Generally speaking, in metabolically healthy individuals, mitochondria prefer to burn fat during prolonged low- or medium-intensity exercise, saving limited carb reserves for bursts of intensity. During a race, this can allow for steady progress, followed by bursts of speed when they are needed most – as long as your mitochondria can effectively make the switch.

A balanced diet is essential for supporting mitochondrial health and can look a bit different for everyone. However, an approximate macronutrient breakdown to support endurance training would likely include: 

• Carbs (50–60 per cent) – the primary fuel source for running that’s essential for maintaining endurance and preventing early fatigue. Whole grains, starchy vegetables and fruits should form the base of daily meals
• Protein (15–20 per cent) – supports muscle repair and recovery. Lean meats, eggs, dairy and legumes
• Fats (20–30 per cent) – provide long-lasting energy and support overall health. Sources like avocados, nuts, seeds and olive oil are beneficial

Beyond macronutrients, specific micronutrients play a critical role in endurance performance:

• Magnesium – supports the key metabolic mediator of muscle mass, helping to promote mitochondrial health. It is crucial for muscle function, preventing cramps and supporting recovery. Endurance runners deplete magnesium through sweat, making foods such as nuts, seeds, dark leafy greens and whole grains important
• Iron – essential for oxygen transport and energy production, and is particularly important for female athletes who are at higher risk of deficiency. Found in lean meats, lentils, spinach and fortified cereals. Low iron levels can lead to fatigue and reduced endurance

To optimise performance and recovery, certain foods that can have a negative impact on your metabolic mediators, or on comfort levels, should be limited or avoided:

• Highly processed foods and refined sugars – these can cause blood-sugar fluctuations, leading to inconsistent energy levels
• High-fat, greasy foods – these can slow digestion and cause gastrointestinal discomfort during runs
• Excess fibre before long runs – while fibre is essential for digestion, consuming too much before a workout can lead to bloating and stomach discomfort
• Excessive caffeine and alcohol – these can contribute to dehydration and impair recovery

Pre-workout-fuelling needs vary based on the individual and the type of workout. Some runners perform well with fasted runs, particularly during low-intensity or short training sessions. However, for a half-marathon or longer runs, most athletes benefit from pre-workout nutrition to sustain endurance and prevent early fatigue. Individual experimentation during training will help determine the best strategy.

Yes, carb-loading is beneficial for a marathon as it helps maximise muscle glycogen stores, which serve as the primary fuel source during prolonged endurance exercise.

Carb-loading is particularly effective for runners expecting to be on their feet for 90 minutes or longer. For shorter races, regular carbohydrate intake should be sufficient.

Carbohydrate intake should be gradually increased two to three days before the race, making up about 65 per cent of total daily calories during this period. High-fat should be limited to prevent digestive issues. And hydration should be maintained, as glycogen storage also retains water, which can enhance performance.

The ideal pre-race meal should be high in carbohydrates, moderate in protein and low in fat and fibre to ensure easy digestion and sustained energy. Good options include:

• Oatmeal with honey and a handful of berries
• A small bowl of rice with grilled chicken
• A sports drink and a low-fibre energy bar

This meal should be consumed two to three hours before the race to allow for digestion. If additional fuel is needed closer to the start, a light snack – such as a banana or an energy gel – 30–60 minutes before running can be beneficial.

• www.lumen.me

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