Improving nitrogen use efficiency (NUE) in dairy cows remains a priority for sustainable milk production to achieve sustainability goals of reducing nitrogen losses to the environment and maintaining or increasing productivity. Amino acid (AA) supplementation offers a strategy to meet animal protein and AA requirements while reducing dietary CP and improving NUE. This thesis used a meta-analysis approach to evaluate the effects of AA supplementation on productive performance in lactating dairy cows, focusing particularly on the effects of metabolizable protein (MP), metabolizable energy (ME), and the supply of key amino acids such as lysine and methionine. A total of 11 peer-reviewed studies were included in this analysis following rigorous inclusion criteria. From each study, treatment means and standard errors were extracted for milk yield (MY), energy-corrected milk (ECM), and milk fat, protein, and lactose concentrations and yields. Dietary composition variables such as ME and MP, and AA supply were also recorded. Random-effects meta-regression models were developed to identify the dietary factors associated with changes in MY and ECM. Results showed that Met supplementation improved MY and ECM above the basal diet. The effect of lysine supplementation was consistently estimated as a negative value, indicating a reduction in MY and ECM when compared to the basal diet. The supply of ME and MP significantly affected the response to AA supplementation, with the ratio of MP:ME in particular causing a significant positive effect.