High-intensity interval training (HIIT) is an effective way to increase cardiorespiratory fitness in athletes and clinical populations. There is growing evidence that HIIT may also promote changes in body composition. In addition to promoting fat loss, HIIT has been shown to effectively stimulate increases in muscle protein synthesis after a single session, but reported increases in lean mass with HIIT are inconsistent. Essential amino acids (EAA) support muscular adaptation through the promotion of muscle protein turnover, stimulation of muscle protein synthesis, and reduction of muscle protein breakdown. HIIT, in combination with EAA, may create a metabolic environment that supports muscular adaptation. Purpose: To compare the independent and combined effects of 8 weeks of HIIT and EAA supplementation on body composition and metabolism in overweight and obese adults. Methods: Sixty-six subjects between the ages of 25-50 years (50% female; Age: 36.7 ± 6.0 years; Weight: 94.5 ± 14.7 kg; %BF: 36.0 ± 7.8%) were randomly assigned to: (a) HIIT, 2 days per week of cycle ergometry; (b) EAA supplementation, 3.6 grams twice daily; (c) HIIT + EAA; or (d) control (CON), no intervention. Measures of body composition, resting metabolic rate (RMR), substrate metabolism (RER), and cardiorespiratory fitness (VO2) were measured at baseline, 4 weeks, and 8 weeks. Results: There were no significant changes in body composition for any group (p > 0.05). Analysis of 95% confidence intervals (CI) showed a significant increase in RMR for HIIT from weeks 0-4 (Adjusted Mean Change ± SE [95% CI]: 71.0 ± 26.5 kcal/d; [18.1-123.96]) and 0-8 (78.4 ± 28.8 kcal/d; [20.7-136.1]); there was also a significant increase for EAA from weeks 0-4 (54.5 ± 25.7 kcal/d; [3.1-105.8]). Resting fat oxidation significantly increased, as indicated by a decrease in RER, for HIIT from weeks 4-8 (-0.04 ± 0.01 a. u.; [-0.07 to -0.02]) and 0-8 (-0.04 ± 0.01 a. u.; [-0.07 to -0.02]); there was also a significant increase in resting fat oxidation for EAA from weeks 0-8 (-0.03 ± 0.01 a. u.; [-0.06 to -0.01]). Cardiorespiratory fitness was higher for both HIIT and HIIT + EAA compared to CON (p < 0.05); VO2 significantly increased for HIIT + EAA in weeks 0-4 (3.7 ± 1.3 ml/kg/min; [1.0-6.4]), for HIIT in weeks 4-8 (5.8 ± 1.3 ml/kg/min; [3.1-8.4]), and for both from weeks 0-8 (HIIT: 5.1 ± 0.9 ml/kg/min; [-3.2 to 2.1]; HIIT + EAA: 4.1 ± 0.9 ml/kg/min; [1.0-6.4]). There was also an increase in VO2 for EAA from weeks 0-8 (2.0 ± 0.9 ml/kg/min; [0.2-3.9]). Ventilatory threshold increased in both HIIT (0.3 ± 0.1 L/min; [0.1-0.4]) and HIIT + EAA groups (0.3 ± 0.1 L/min; [0.1-0.4]) in weeks 4-8. Conclusions: Despite no significant changes in body composition, HIIT and EAA supplementation separately promoted increases in RMR and fat oxidation after 8 weeks. Consistent with previous research, HIIT is an effective form of exercise for improving cardiorespiratory fitness. EAA supplementation, in combination with HIIT, did not appear to provide any additional benefit to the adaptations observed with HIIT. Practical Application: 2 days per week of HIIT training (10 × 1 minute at 90%; 1 minute rest) is an effective and feasible approach to maintaining fitness levels and promoting positive metabolic adaptation in overweight and obese adults, especially for those with limited time to commit to exercise. Supplementation of EAA between meals or before and after exercise sessions may also stimulate positive metabolic adaptations, independent of exercise.
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