Bone Mineral Density and Body Composition in Ultra-Endurance Running

Earlier this year, Dr Kenna Stephenson explored why bone health is not just an issue for later life, and why building muscle, strength and confidence of movement should be part of a lifelong strategy for reducing fracture risk.

In this follow-up, certified endurance and ultra-running coach Mario Buchs brings that conversation into the world of ultra-endurance sport. After being diagnosed with osteoporosis and sustaining two sacral stress fractures, Mario used serial DEXA scans to track his bone mineral density and body composition across a demanding race season that included UTA100, the Elephant Miler and Coast to Kosciuszko.

'Back in 2020, I suffered my first sacral stress fracture. At the time, I was doing what many endurance athletes do: running high mileage, training hard, chasing goals, and assuming that because I was fit, I was healthy. I recovered, returned to racing, and continued building toward bigger endurance challenges. But in 2024, history repeated itself when I sustained a second sacral stress fracture.

'That second injury changed everything. Instead of simply focusing on getting back to running, I became deeply interested in understanding what was happening beneath the surface. Why was this happening? Was ultra running negatively affecting my bone health? Was recovery enough? Was nutrition enough? Or was I missing something entirely?

That curiosity led me down the path of regular DEXA scanning. A DEXA scan measures bone mineral density (BMD) and body composition, including lean mass, fat mass, and visceral fat. Rather than relying on body weight or how I “felt,” I wanted a clearer picture of how my body was actually responding to training and racing.

'Throughout 2025, I tracked my body composition and bone density across three major ultra-endurance events: UTA100 (100km), Elephant Miler (100 miles), and Coast to Kosciuszko (240km). For each race, I completed a DEXA scan before the event, shortly after finishing, and again around five days later during recovery.

'What I found surprised me. Immediately after racing, my lean mass increased while fat mass and body fat percentage decreased. At first glance, that sounds positive, but the reality is likely far more complex. These short-term changes were probably linked to inflammation, glycogen replenishment, hydration shifts, and the body’s broader stress response following extreme endurance efforts. Interestingly, visceral fat also increased immediately post-race before returning closer to baseline during recovery. It was another reminder of just how much physiological stress ultra-endurance racing places on the body, even when we feel relatively recovered.

'The biggest surprise, however, was my bone mineral density. Despite a hereditary history of osteoporosis and two sacral stress fractures, my bone density remained stable across the season. There were small fluctuations after races, but no long-term decline. That finding challenged many of the assumptions I previously held about ultra running and bone health.

'But there is an important catch: I was not just running. Alongside my endurance training, I was strength training consistently three times per week. Recovery was also structured and intentional, including daily cold-water immersion, relative rest after races, and a nutrient-dense whole-food plant-based diet. Looking back, I believe those factors played a major role in helping maintain my bone health throughout such a demanding season.

'This experience reinforced something I now strongly believe as both an athlete and coach: running alone is rarely enough for long-term durability. Ultra runners often pride themselves on accumulating miles, but bone health, recovery, and longevity require a much broader approach. Strength training, proper fuelling, sleep, and recovery strategies all matter.

'It also reminded me that fitness and health are not always the same thing. You can be aerobically fit while still under-fuelled, under-recovered, or at risk of injury. For me, those two sacral stress fractures became the catalyst for learning more about my body rather than simply pushing harder.

'Somewhere along the way, what started as a personal recovery journey evolved into a full scientific paper exploring bone density, recovery, and ultra-endurance running. The biggest takeaway was simple: longevity in endurance sport is not just about how much training you can tolerate, but how well you support the training you do.

Link to Full Paper

Mario Buchs is a certified sport endurance coach, ultrarunning coach, and the founder of Healthetica Coaching, where he takes a human-first approach to performance, resilience, and wellbeing. Specialising in ultra-endurance running, leadership, and holistic athlete development, Mario works with runners and professionals to build sustainable performance both on and off the trail/road/track.
Through his coaching, podcast Strides & Vibes, and involvement in projects such as the humanitarian IRAQ100, Mario combines practical experience with evidence-based strategies in endurance, recovery, mindset, and nutrition. His philosophy is simple: consistent, sustainable habits create long-term performance and healthier humans.
mario@healtheticacoaching.com
healtheticacoaching.com