Clock Work Marathon Pacing using fat - and carbohydrate combustion rates


Bernat after his sub-3h Marathon success

Do you want to run a marathon in under three hours? Easy: you only need to run those 42.195 km (26.2 miles) at an average pace of 4:15 per km and smash it in 2 hours and 59 minutes. Of course, it’s not quite that simple: your body needs to be fit to sustain that speed for that amount of time. To get there, you need to put in proper training — and understand how your physiological metrics and parameters cope with those requirements. In most cases, this is a learning process and means you need to learn how your body works.

Bernat Xamena, a professional trumpet player from Mallorca, learned that very well. At the Barcelona Marathon in 2017 he blew up after km 30 and had to pull out of the race. The sustained effort of playing trumpet six-to-eight hours a day couldn’t help him. When you go into the red zone too much, soon you pay the consequences.

“He had nutrition issues and also started too fast. And then he had very bad cramps and couldn’t carry on”, says his triathlon coach Joseph Spindler.

But after a lesson learned in the hardest way, both Spindler and Xamena were ready to do the things properly this year. Xamena was tested with INSCYD three weeks before the race and ran a marathon with the precision of a Swiss clock.

“Based from the test [a simple 4 phase test on the track with lactate sampling] ”, says Spindler, “it was very clear that he had to run 4:08 per km, be fine and have a 2 hours 54 something for the whole marathon.”

After the nutrition issues of last year, Spindler and Xamena also worked on his nutrition strategy: Xamena didn’t take in anything for the first hour (except water) and then 64 grams of carbs per hour for the following two hours.  “We didn’t go crazy with the amount of carbs because we knew that amount would work for him,” explains Spindler. “I just wanted to stay on the safe side because with gel you cannot go as high as with other products.”

The screenshot Bernat sent to his coach Jo after the race.

After the race Xamena sent Spindler a pretty eloquent picture of his pace breakdown. Average pace: 4:08 per km. Total 2:54:19. Job done.

“With INSCYD tests I was able to say at what pace he was able to run without running out of energy and have the best result. We did the test on a track, so the closest possible to a race scenario. And this is the beauty of INSCYD: you can test real-world conditions, you don’t need to go to a lab, you don’t need a treadmill where the athletes often don’t run that hard for the all-out effort and that is actually required. On the track is no problem: you are as close as you can get to real-world conditions.”

But how did Spindler really predicted the result so precisely using INSCYD? He firstly points out that he used a mixed approach:  he firstly looked at what Xamena had run in the past in terms of % of anaerobic threshold and validate this finding with the fat and carbohydrate combustion chart on INSCYD.

“Basically,” he says, “I looked of how many carbs he would burn to sustain a certain speed according to the fat and carbohydrate combustion chart. At the prescribed speed of 4:08min/km (= 4,04m/s) he burns 187g of carbs per hour, so 561 over the entire marathon.”

The Fat  & Carbohydrate combustion curves of Bernat a few days before the Marathon. Data derived from simple field lactate testing

“We planned his race nutrition with a supplement of 60g of carbs per hour after the 1st hour. So that gives him 120g for the marathon (1st hour: 0g / 2nd: 60g / 3rd: 60g). Therefore, to sustain the effort, he needed to have stored in his body about 440g of carbs, which is kind of a realistic amount for a trained athlete. We did a bit of carb-loading during the days leading up to the race to ensure that he had at least that amount stored in his body. We did not stretch the boundaries of carbohydrate loading with an extreme diet, as we wanted to play everything safe.”

Another one of Spindler’s athletes, Carmen Grosse, recently won Ironman New Zealand (Age Group 55-59) and set the new swim record, the new bike record and the new course record for her category. Only the run was not part of the equation. But that was still a pretty mind-blowing result. Last year, Grosse also won the IM 70.3 Saint George, Kona World Champs and IM South Africa on her age group.

When you need to predict the final outcome of a race and a performance (and understand how to get there), INSCYD is the number one tool you have to look at.


Split times like a swiss clock

Profile of the Barcelona Marathon

Bernat happy after the race.

Jo Spindler Coach.jpg

Jo Spindler is a former professional long distance triathlete. After finishing his career he joined the TriSutto group of Brad Sutton. Jo coaches professional and amateur athletes around the globe. Learn more about Jo and his work here:


Nicola Busca