BACKGROUND: This pilot study examined the effect of a 2-wk ketogenic diet (KD) compared with a carbohydrate (CHO) diet in military personnel on cognitive performance, mood, sleep, and heart rate variability (HRV).METHODS: A randomized-controlled, cross-over trial was conducted with eight male military personnel (age, 36 +/- 7 yr; body mass, 83.7 +/- 9.2 kg; BMI, 26.0 +/- 2.3 kg . m(-2)). Subjects ingested their habitual diet for 7 d (baseline), then an iso-energetic KD ( approximately 25 g CHO/d) or CHO diet ( approximately 285 g CHO/d) for 14 d (adaptation), separated by a 12-d washout. HRV, fasting capillary blood D-betaHB, and glucose concentration, mood, and sleep were measured daily. Cognitive performance was measured on the 7(th) day of baseline and the 7(th) and 14(th) days of adaptation. Data were analyzed using a series of linear mixed models.RESULTS: Mean weekly D-betaHB was higher (95% CI, +0.34 to +2.38 mmol . L(-1)) and glucose was lower (-0.45 to -0.21 mmol . L(-1)) in the KD compared with the CHO diet. Cognitive performance (Psychomotor Vigilance Task, 2-choice reaction time, and running memory continuous performance test) and mean weekly fatigue, vigor, and sleep (sleep duration, sleep efficiency, and sleep onset latency) were similar between diets. A diet x week interaction for HRV approached significance, with exploratory analyses suggesting HRV was lower compared with baseline during week-2 adapt (-27 to +4 ms) in the KD.DISCUSSION: A 2-wk KD induction in military personnel does not appear to affect cognitive performance, mood, or sleep, but may lower HRV, indicating increased physiological stress.Shaw DM, Henderson L, van den Berg M. Cognitive, sleep, and autonomic responses to induction of a ketogenic diet in military personnel: a pilot study. Aerosp Med Hum Perform. 2022; 93(6):507-516.