Background/Objectives: This study aimed to evaluate the effects of four weeks of combined Acetyl-l-Carnitine and alpha-lipoic acid (ALA) supplementation on anaerobic and aerobic performance and fatigue resistance in trained cyclists, hypothesizing improvements in maximal aerobic power (MAP), Wingate test performance, and reduced lactate accumulation. Methods: In a double-blind, randomized trial, 41 male trained cyclists (age: 36 ± 12 years; MAP: 4.35 ± 0.60 W·kg-1) were assigned to a supplement group (SUP, n = 19; 1200 mg/day Acetyl-l-Carnitine, 300 mg/day ALA, 1.1 mg Vitamin B1, 2.5 µg Vitamin B12) or placebo group (PLA, n = 22) for four weeks. Performance was assessed pre- and post-intervention via counter-movement jumps (CMJs), Wingate tests (WG1, WG2), and a graded exercise test (GXT). Blood lactate ([La-]) was measured post-Wingate. A three-way mixed ANOVA analyzed Wingate performance (session, order, and group), and a two-way ANOVA assessed MAP and fatigue effects. Results: MAP increased by 3.4% (314 ± 32 W to 324 ± 37 W; p = 0.005) with no group interaction (p = 0.457). Wingate peak power showed main effects for order (p < 0.001) and session (p = 0.011) but no group interaction (p = 0.676). SUP reduced [La-] by 1.5 mmol·L-1 post-WG2 in POST (p = 0.049). No significant group differences were found for CMJ or fatigue metrics. Conclusions: Four weeks of Acetyl-l-Carnitine and ALA supplementation did not enhance aerobic or anaerobic performance in trained cyclists, despite reducing blood lactate after high-intensity exercise, suggesting no ergogenic benefits.
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