The measurement of total energy expenditure (TEE) outside of the laboratory enables a more comprehensive understanding of the human ceiling for extended work and competition. Doubly labeled water remains the gold standard for quantifying the extended (several hours to days) TEE for humans in free-living environments without the need for additional body-worn sensors or expired-air collection, thereby optimizing both accuracy and portability concerns. High rates of TEE have been demonstrated during multistage/day cycling events including the Giro d`Italia (32.3 [3.4] MJ·d-1, 7719 [813] kcal·d-1)1 and the Tour de France (24.8 [1.7] MJ·d-1, 5927 [411] kcal·d-1).2 Doubly labeled water has also been used for shorter measurements (12-24 h) including the Kona Ironman Triathlon (9.8 [1.3] MJ, 9040 [1390] kcal) and the Western States 100 ultramarathon (68.2 [12.4] MJ, 16,310 [2960] kcal).3,4 The physiological responses and self-selected feeding strategies of cross-country ultraendurance mountain bikers have not been explored simultaneously with measures of TEE. The purpose of this case study was to quantify the ad libitum nutrient and fluid intake in parallel with stable isotopic turnover and derived measures of TEE in a competitive male cross-country cyclist during a sanctioned 24-hour event.
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