Overview

• Researchers followed 14 ultramarathoners, cyclists and triathletes for up to a year during training and multi-day events.
• Peak expenditure during race stretches reached roughly six to seven times basal metabolic rate, equating to about 7,000–8,000 calories per day.
• Averaged across 30–52 weeks, total energy use settled near 2.4–2.5 times BMR, marking a sustainable ceiling observed in this cohort.
• Results were derived from the doubly labeled water method, which calculates free-living energy use from isotope washout in urine.
• Authors report tentative evidence of energy compensation and caution that the small sample may miss rare outliers, adding that most people will never approach these limits.