nutritional ketosis; graded exercise test; 30-15 intermittent fitness test; beta-hydroxybutyrate; heart rate variability

Purpose: The aim of this non-randomized parallel group study was to examine the 12 week effects of a very low-carbohydrate high-fat diet (VLCHF) on maximal cardiorespiratory capacity, high-intensity interval training (HIIT) performance, and cardiac autonomic regulation.Methods: Twenty-four recreationally trained participants allocated to either a VLCHF (N = 12) or a habitual diet (HD; N = 12) group completed 12 weeks of a diet and exercise (VLCHF) or an exercise only intervention (HD). Maximal graded exercise tests (GXT) were performed at baseline, after 4, 8, and 12 weeks. A supervised HIIT session and the 30-15 Intermittent Fitness Test (30-15(IFT)) were conducted once a week.Results: Total time to exhaustion (TTE) in both GXT and 30-15(IFT) largely increased in both VLCHF (p = 0.005, BF10 = 11.30 and p = 0.001, BF10 >= 100, respectively) and HD (p = 0.018, BF10 = 3.87 and p = 0.001, BF10 >= 100, respectively) groups after 12 weeks. Absolute maximal oxygen uptake (<(V)over dot>O-2max) was not changed in both groups but relative <(V)over dot>O-2max increased in VLCHF in concert with reductions in body mass (66.7 +/- 10.2-63.1 +/- 8.5 kg). Cardiac autonomic regulation did not reveal any between-roup differences after 12 weeks. VLCHF diet induced an increase in beta-hydroxybutyrate, which tended to normalize during the intervention period.Conclusion: The 12 week VLCHF diet did not impair high-intensity continuous or intermittent exercise lasting up to 25 min, nor did it impair maximal cardiorespiratory performance or autonomic nervous system (ANS) activity.