D-Aspartic Acid is a popular supplement that many believe can boost testosterone levels. This effect has only been seen in rats or during in vitro studies – human studies show no effect in younger populations. One study does show mild benefits for older men. As far as training gains go, some studies do show increased strength in subjects taking D-Aspartic Acid while others don’t show any benefits. No studies have found any increase in muscle size, however, most studies were only performed for 2-4 weeks, so effects on size remain inconclusive.
Reference: LaMacchia, Z., William, B., Furst, T., & Horvath, P. (2017). Acute D-Aspartic Acid Supplementation does not have an Effect on Serum Testosterone but does have an Effect on Strength Measures in College Aged Male Athletes. European Journal of Sports & Exercise Science, 5(3), 34-41.