Purpose: Considerable interindividual heterogeneity has been observed in endurance performance responses following induction of a ketogenic diet (KD). It is plausible that a physiological stress response in the period following the dramatic dietary shift associated with transition to a KD may explain this heterogeneity. Methods: In a randomized, crossover study design, 8 trained male runners completed an incremental exercise test and ran to exhaustion at 70%VO2max before and after a 31-day rigorously controlled habitual diet or KD intervention, and recorded heart rate variability (root mean square of the sum of successive differences in R-R intervals [rMSSD]) upon waking each morning along with the recovery-stress questionnaire for athletes each week. Data were analyzed using linear mixed models. Results: A significant reduction in rMSSD was observed in the KD (-9.77 [4.03] ms, P = .02), along with an increase in day-to-day variability in rMSSD (2.1% [1.0%], P = .03). The reduction in rMSSD in the KD for the subgroup of individuals exhibiting impaired exercise capacity following induction of the KD approached significance (Delta -22 [15] ms, P = .06, N = 4); whereas no effect was observed in those who exhibited unchanged exercise capacity (Delta 5 [18] ms, P = .61, N = 4). No main effects were observed for recovery-stress questionnaire for athletes. Conclusions: Our data suggest those working with endurance athletes transitioning onto a KD may consider using noninvasive, inexpensive resting heart rate variability measures to gain individual-level insights into the likely short-term effects on exercise capacity.
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