Overview
- Researchers from the Massachusetts College of Liberal Arts and Duke University tracked 14 competitive ultrarunners, cyclists and triathletes in a peer-reviewed study published in a Cell Press journal on Oct. 21, 2025.
- Across 30 to 52 weeks, athletes’ average maximum daily energy expenditure stayed near 2.4–2.5 times their basal metabolic rate, defining a long-term metabolic ceiling.
- Short race stretches still produced much higher spikes, with multi-day efforts reaching six to ten times resting metabolism and roughly 7,000–9,000 kilocalories per day, which the authors say is not sustainable over time.
- The team found signs of energy reallocation during extreme efforts, including reduced non-exercise activity and lower testosterone in some ultrarunners.
- The ceiling translates to well over 4,000 kilocalories per day for many athletes and roughly the equivalent of running about 18 kilometers daily for a year, a level the authors caution is impractical for most recreational exercisers and based on a small sample.