Within-Day Energy Deficiency and Metabolic Perturbation in Male Endurance Athletes

<jats:p>Endurance athletes are at increased risk of relative energy deficiency associated with metabolic perturbation and impaired health. We aimed to estimate and compare within-day energy balance in male athletes with suppressed and normal resting metabolic rate (RMR) and explore whether within-day energy deficiency is associated with endocrine markers of energy deficiency. A total of 31 male cyclists, triathletes, and long-distance runners recruited from regional competitive sports clubs were included. The protocol comprised measurements of RMR by ventilated hood and energy intake and energy expenditure to predict RMR<jats:sub>ratio</jats:sub> (measured RMR/predicted RMR), energy availability, 24-hr energy balance and within-day energy balance in 1-hr intervals, assessment of body composition by dual-energy X-ray absorptiometry, and blood plasma analysis. Subjects were categorized as having suppressed (RMR<jats:sub>ratio</jats:sub> < 0.90, <jats:italic>n</jats:italic> = 20) or normal (RMR<jats:sub>ratio</jats:sub> > 0.90, <jats:italic>n</jats:italic> = 11) RMR. Despite there being no observed differences in 24-hr energy balance or energy availability between the groups, subjects with suppressed RMR spent more time in an energy deficit exceeding 400 kcal (20.9 [18.8–21.8] hr vs. 10.8 [2.5–16.4], <jats:italic>p</jats:italic> = .023) and had larger single-hour energy deficits compared with subjects with normal RMR (3,265 ± 1,963 kcal vs. −1,340 ± 2,439, <jats:italic>p</jats:italic> = .023). Larger single-hour energy deficits were associated with higher cortisol levels (<jats:italic>r</jats:italic> = −.499, <jats:italic>p</jats:italic> = .004) and a lower testosterone:cortisol ratio (<jats:italic>r</jats:italic> = .431, <jats:italic>p</jats:italic> = .015), but no associations with triiodothyronine or fasting blood glucose were observed. In conclusion, within-day energy deficiency was associated with suppressed RMR and catabolic markers in male endurance athletes.</jats:p>