The effects of d-aspartic acid supplementation in resistance-trained men over a three month training period: A randomised controlled trial

CONTEXT: Research on d-aspartic acid (DAA) has demonstrated increases in total testosterone levels in untrained men, however research in resistance-trained men demonstrated no changes, and reductions in testosterone levels. The long-term consequences of DAA in a resistance trained population are currently unknown. OBJECTIVE: To evaluate the effectiveness of DAA to alter basal testosterone levels over 3 months of resistance training in resistance-trained men. DESIGN: Randomised, double-blind, placebo controlled trial in healthy resistance-trained men, aged 18–36, had been performing regular resistance training exercise for at least 3 d.w(-1) for the previous 2 years. Randomised participants were 22 men (d-aspartic acid n = 11; placebo n = 11) (age, 23.8±4.9 y, training age, 3.2±1.5 y). INTERVENTION: D-aspartic acid (6 g.d(-1), DAA) versus equal-weight, visually-matched placebo (PLA). All participants performed 12 weeks of supervised, periodised resistance training (4 d.w(-1)), with a program focusing on all muscle groups. MEASURES: Basal hormones, total testosterone (TT), free testosterone (FT), estradiol (E(2)), sex-hormone-binding globulin (SHBG) and albumin (ALB); isometric strength; calf muscle cross-sectional area (CSA); calf muscle thickness; quadriceps muscle CSA; quadriceps muscle thickness; evoked V-wave and H-reflexes, were assessed at weeks zero (T1), after six weeks (T2) and after 12 weeks (T3). RESULTS: No change in basal TT or FT were observed after the intervention. DAA supplementation (n = 10) led to a 16%, 95% CI [-27%, -5%] reduction in E(2) from T1-T3 (p<0.01). The placebo group (n = 9) demonstrated improvements in spinal responsiveness (gastrocnemius) at the level of the alpha motoneuron. Both groups exhibited increases in isometric strength of the plantar flexors by 17%, 95% CI [7%, 28%] (p<0.05) as well as similar increases in hypertrophy in the quadriceps and calf muscles. CONCLUSIONS: The results of this paper indicate that DAA supplementation is ineffective at changing testosterone levels, or positively affecting training outcomes. Reductions in estradiol and the blunting of peripheral excitability appear unrelated to improvements from resistance training. TRIAL REGISTRATION: Australian New Zealand Clinical Trials Registry ACTRN12617000041358