Heat study spurs Danish marathon record

How does one break a nearly 40-year-old record? Is it sheer willpower, exceptional talent or perhaps something more? Jacob Sommer Simonsen’s remarkable achievement in breaking Denmark's iconic marathon record reveals how a focused combination of science and dedication can push the boundaries of what was thought possible.

Jacob Sommer Simonsen, both a runner and medical student, completed the Berlin Marathon in 2:07:51, demonstrating how intensive physical training supported by scientific strategies can lead to extraordinary results.

His interest in these new strategies was sparked by an unexpected source.

“I first heard about heat training on a podcast, and it immediately caught my interest. I thought, ‘Could this be the missing piece?’ I started experimenting with a sauna, but it was way too intense and unsustainable. Then I decided to reach out to Lars Nybo. His expertise made all the difference, turning a vague idea into a structured, effective training approach that helped me to elevate my performance to new heights,” Jacob Sommer Simonsen recalls.

Felt like a huge leap

The achievement was bolstered by Lars Nybo’s research on heat adaptation, which played a key role in Simonsen’s preparation.

“At first, the idea of heat training seemed daunting. Even running at moderate intensity in the heat felt overwhelming. But Lars helped me to refine the approach, focusing on controlled sessions and gradually building up. By targeting specific core temperatures and staying consistent, my endurance and performance improved dramatically. It was not about working harder; it was about adapting smarter,” Jacob Sommer Simonsen explains.

“One should never take credit away from the athletes; it is still up to them to pursue this knowledge and integrate it into their daily practice and training. I have always been a bit frustrated about why we have not broken Denmark’s marathon record in 40 years. Jacob had previously reached out, and when I contacted him, we worked to optimise some aspects of his training,” explains Lars Nybo, Professor, Movement and Neuroscience, Department of Nutrition, Exercise and Sports, University of Copenhagen, Denmark.

Jacob Sommer Simonsen’s journey also included an impressive performance at the Copenhagen Marathon as the fastest Dane with a time of 2:11:24 – or about 19.3 km per hour. This race set the stage for his record-breaking attempt, emphasising not only his athletic ability but also his dedication to integrating science with sports.

“When Lars suggested that I aim to run 20 kilometres per hour, I honestly thought it was crazy. My previous personal best at the Copenhagen Marathon was 2:11:24, which is about 19.3 kilometres per hour, so jumping to 20 kilometres per hour felt like a huge leap. But with the heat training and other science-based strategies, I pushed myself further than I thought possible. In the end, I hit 19.78 kilometres per hour in Berlin – not quite 20 but enough to break the Danish marathon record. Now I am motivated to take that next step,” Simonsen elaborates on his goal and the ambitious leap he aimed for.

Integrating science for athletic excellence

Lars Nybo’s previous study on heat adaptation was instrumental in shaping Simonsen’s training. His research demonstrated that heat adaptation, achieved within 4–5 weeks, could significantly boost performance. This finding presented a novel approach, moving away from traditional training models that often emphasise extended periods of high-intensity work.

“We conducted a new heat study, and it shows that the effect can be attained within 4–5 weeks but does not increase even if you train longer,” Lars Nybo describes.

The study’s results encouraged Simonsen to incorporate heat adaptation sessions into his regimen, enabling him to train under conditions that would prepare his body for the physical stress of a marathon, especially in hot environments. Heat adaptation training helps the body to improve its cooling mechanisms and increases blood volume, which can prevent overheating and improve endurance during races.

“We aimed for a target core temperature of 38.5–39.0°C, which he measured rectally and with some wearable devices that, while not entirely accurate, still gave an indication,” Lars Nybo adds.

In addition to heat adaptation, Lars Nybo and his team advised Simonsen on optimising his energy intake to balance performance with weight management. Rather than focusing solely on intense physical training, the strategy included fine-tuning nutrition to maximise endurance and efficiency during the race.

“When we consider the results, I believe Jacob’s four-minute improvement is a mix of factors. But it probably happened because we optimised his heat adaptation and calibrated his energy intake for training – ensuring that he had just enough, but not more than he needed,” Lars Nybo explains.

Challenging conventional training wisdom

Lars Nybo’s research presents an alternative to the traditional interval-heavy approach to marathon training. Instead of pushing Simonsen with excessive power-based intervals, the philosophy emphasised controlled pacing and mileage to build muscle strength and stamina, enabling runners to maintain consistent performance over long distances without risking injury.

“The philosophy was to run the mileage to build muscular robustness. Many marathon runners push intense intervals, thinking that they need power training. But we approached it differently,” Lars Nybo describes.

Like any rigorous training programme, preventing injury is crucial. For runners, Achilles tendons and connective tissues are particularly susceptible to strain. To address this, the programme incorporated alternative methods such as cycling. Cycling simulated heat adaptation with low impact, helping Simonsen to avoid the joint strain running can cause while conditioning his body for hot environments.

“For most runners, Achilles tendons are a weak spot, since they often develop tendon and connective tissue injuries. We must balance training intensity and injury prevention to optimise their long-term performance,” Lars Nybo explains.

Gains through haemoglobin and endurance

The culmination of this scientific approach was not only evident in Simonsen’s performance but also measurable in physiological markers such as haemoglobin mass, which increased through his heat training. Haemoglobin is a protein in red blood cells that carries oxygen to muscles. Increased haemoglobin mass improves the body’s ability to deliver oxygen during intense, prolonged activity, which is essential for endurance sports such as marathon running.

“This time, we decided to focus solely on heat training, partly to keep the approach simple, and we are quite confident about the performance results. On average, we achieved about 2–3% improvement in performance and a similar increase in haemoglobin mass,” explains Lars Nybo.

He elaborates on the practical implications of the findings: “What is interesting about applying this in practice is that I could have simply done a study with Jacob Sommer Simonsen and observed that he gained 40 grams of haemoglobin. However, as a scientist, I could not definitively say that this was caused by the heat training alone.”

Lars Nybo continues, “But since I had conducted other studies with similar results – showing athletes gaining about 40 grams of haemoglobin – the pieces of evidence align reasonably well.”

A final push before race day

The final phase of Simonsen’s preparation was critical, involving carefully tapering heat training sessions to retain physiological adaptations without overextending. As race day neared, his team reduced the frequency of these sessions, helping him to arrive at the Berlin Marathon in peak condition.

“We know that three sessions per week is enough to maintain adaptation. In the final week leading up to Berlin, we therefore stuck with three sessions per week to enable what is commonly called tapering,” Lars Nybo elaborates.

Lars Nybo emphasises that athletic training in Denmark benefits from a unique model of direct collaboration between athletes and researchers. This close relationship enables real-time adjustments based on cutting-edge research, ensuring that athletes such as Simonsen can apply the most current scientific insights to their training.

“Denmark’s model is quite unique since athletes have direct access to the latest research knowledge and engage in dialogue with researchers. This new knowledge is then combined or mixed with existing knowledge. Researchers often focus on one thing, losing sight of the holistic picture,” Lars Nybo describes.

“This journey has taught me the power of combining science with hard work. Whether it was refining heat training, adjusting my pacing or rethinking my recovery, every change added up. Working with Lars Nybo helped me to realise that science is not just theory – it is something you can actively apply to push your limits and achieve results you never thought were possible,” Jacob Sommer Simonsen adds.

A unique approach to athletic training

Altitude training has shown benefits in endurance sports, but the heat study focused on adaptations that last longer and can be more consistently maintained, making it ideal for marathon preparation.

“Heat and endurance training increase the volume of blood plasma and, even more importantly, the amount of haemoglobin and thus also improve maximal oxygen uptake at high altitude. In studies, we sometimes say that they just did ‘extra’ work. But if you are already running close to 200 kilometres per week, you cannot just add more,” Lars Nybo clarifies.

Jacob Sommer Simonsen’s record-breaking journey showcases how science and dedication can combine to achieve extraordinary athletic feats. His success underscores the value of integrating scientific research into elite training, paving the way for more athletes to adopt similar strategies. As sports science continues to advance, the principles demonstrated in this heat adaptation study provide a promising future for endurance athletes aiming to push their limits and set new records.

“What is intriguing here is that highly adapted athletes can also achieve these improvements. For untrained individuals, almost any type of training will yield improvements. But for highly trained athletes, our optimisation procedures need to be effective. So, it is exciting to see that this approach also works with well-adapted athletes,” Lars Nybo explains.

“Right now, I am training in South Africa, preparing for the spring season. I believe that I can take another leap in performance by building on what we have already achieved. The heat training was just the start – there is so much more to explore in optimising every part of my preparation,” Jacob Sommer Simonsen concludes.