Marathon fuel and hydration: How science can help runners avoid hitting the wall

The 2024 TCS London Marathon is almost here with the 26.2-mile distance often feeling like a puzzle for runners to solve. While each training run can provide a different piece, most runners would be wise to consider how they will utilise fuel and hydration on the day, in order to fit the whole thing together.

Dream finishing times are often left agonisingly out of reach when runners neglect this crucial part of the equation, which makes it even more infuriating, given the amount of research and information available to help runners avoid hitting the dreaded wall.

I ran the London Marathon in 2021 and 2022, finishing both times in 3:04, and while proud, there was an element of disappointment after coming agonisingly close to that magic three-hour barrier.

A series of hamstring cramps hit my momentum on both occasions, forcing me to stumble in the closing five to six miles and even stop on several occasions.

So one priority for attempt three this year was to execute a better strategy both before and during the race, which is where Precision Fuel & Hydration came in and a drastically different approach. With experience working alongside elite endurance athletes in running and triathlon, their assistance has proved fascinating. So part of my faith in a sub-three-hour performance is relying on the science for this year’s race, including a sweat test, conducted by Precision’s Stuart Anderson.

Sweat rate can vary up to 15 times between seemingly similar individuals; from 200mg per litre of sweat, to as much as 2,000mg/l, with that rate largely genetically determined.

Fortunately, the test did not involve giving a blood sample, or any variation of the painful bleep test. In fact, it was rather pleasant, sitting down with the assistance of Stuart for approximately 45 minutes. An electric current was delivered to my arm via a strap, much like a watch, with drops of sweat, eventually filling up the space between my skin and a transparent disc. The liquid eventually turned blue before a reading on the monitor flashed up.

This number informed me exactly how much sodium I am losing in my sweat during exercise.

The conclusion from my test? I’m officially a “salty sweater,” which I’m told provides a series of tricky hurdles to negotiate over 26.2 miles, namely ensuring I do not allow my sodium or glycogen stores to dip drastically at any point.

By calculating that I lose 1,146mg of sodium per litre of sweat, I am now aware of exactly what I need to consume on race day to avoid bonking: the point of reaching exhaustion due to the functional depletion of glycogen.

Armed with the science, then, Precision advised me to supplement my marathon, and those hard training runs, with 75g of carbohydrates per hour (high intake), 1,000mg of sodium per litre of sweat (high intake) and drink 450ml of water per hour (moderate intake). Ultimately this has led me to consume, and at times force down, the following: an electrolyte capsule every 30 minutes and then 2.5 PF 30 gels per hour. The advice therefore concludes I need to take on a rather daunting seven to eight gels over the marathon.

But rather than experiment with fuel and hydration over the course of the training block, Precision has now taken the guesswork out of it and delivered an efficient strategy. I know exactly what to gather before each training run and what to carry on race day. The race day plan even starts from the night before and then from 60-90 minutes before the start.

Years previously, I would take on fuel when gasping for water or flagging following a testing part of the course, but this now feels like a more effective and comprehensive approach and ensures a vital factor in endurance performance is not left to chance.

So far there have been good signs, too, including a personal best in a half marathon and successful races over 16 and 20 miles predominantly designed to test my race-day routine, including fuel and hydration.

But ultimately, the proof will be in the race and how far I can go without feeling those nagging cramps at the back of my legs.

Find out more about Precision Fuel & Hydration and the benefits of a sweat test here